Normal Skin Histology – Explained by a Dermatopathologist
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Normal Skin Histology – Explained by a Dermatopathologist

Alright I’m Jerad Gardner and today we’re
going to talk about normal skin histology. So this is the skin and what we’re looking
at up top the purple layer up here that’s the epidermis the top part of your skin that
protects you from the outside world. Underneath that all of this pink that’s the
dermis made mostly of collagen and its job is to support the epidermis and there are
blood vessels here and other things that support and make sure the epidermis stays happy. And then when you go down below, deeper down
below. Well there’s not on this slide but there should
be fat down in the subcutis. We will look at that in a minute on another
slide. So let’s go and take a closer look at the
epidermis. A lot of skin diseases involve the epidermis. Rashes, dermatitis, all these things and also
lots of different tumors come from epidermis. If you want to be good at skin pathology,
dermatopathology, you have to be a master of normal skin histology. If you don’t know what normal looks like it’s
really difficult to know what abnormal looks like. Alright, so there’s little color changes in
light changes there as I’m changing my objectives and that’s why that’s happening. Alright so let’s look at will go at one closer
than this. Let’s look at the layers of the epidermis
okay. There are in the skin cells begin their life
down here at the bottom this is called the basal layer of the epidermis it’s the bottom
layer or the stratum basale if you like the Latin names for things. In the basal layer there are occasional little
stem cells in here that give rise and reproduce and dividing give rise to squamous cells. So the epidermis is a stratified squamous
epithelium it means it’s stratified means multiple layers you can see there’s layer
after layer of cells here so it’s stratified and it’s squamous that’s the type of cells
these are kind of they’re round or some people say polygonal shape meaning they look
kind of a little bit like a maybe a hexagon or a pentagon or something if you have a really
good imagination. And they’re pink because their cytoplasm
is filled up with cytokeratin which is an intermediate filament and that filament acts
like strong kind of you know steel cables basically going back and forth across the
cytoplasm hooking up to desmosomes around the cell membrane which then hook up to neighboring
desmosomes and all of that together makes that the epidermis a really strong network
that resists damage from friction or pressure. So that’s why when you rub on your skin it
doesn’t fall off. Okay so anyway back to what we were talking
about here the basal layer of the epidermis the cells there they begin their life and
then they start moving up there pushed upwards by the production of new cells and as they
move up they go into a new layer this is called the spinous layer or the stratum spinosum. And the reason it’s called that is because
there are actually little tiny spines which are desmosomes in between each of the cells. You can barely see them here we’ll look
at another a slide in a minute that shows that much more clearly. And you can see the nice round nuclei of the
cell and then as the cells get up towards the top they begin to pick up these purple
dark purple granules in their cytoplasm and so this is called the granular layer. It makes sense right because they have dark
granules in the cytoplasm these are these are keratohyalin granules. Keratohyalin granules contain a variety of
different substances like loricrin and involucrin, kind of fancy things like that that dermatology
residents need for their board exam but probably medical students don’t need to know. But the job of those granules is to kind of
help the keratinocytes come to the end of their life and die and lose their nucleus
and the skeleton of proteins left over from inside the cells gets transferred up here
and becomes the corneal layer or the stratum corneum and the granular layer kind of helps
seal these cells together, kind of cements them together like with glue. And the reason that we do that is because
you don’t want, you know, water things that are outside this is like an extra barrier
against them. And then a water things like that getting
past your epidermis. So that when you rub your skin and those little
dead cells flake away from the skin that’s what’s flaking away there. That’s the corneal layer. That layers actually dead it’s not living
any longer and so that eventually will fall off over time. The time it takes for a squamous cell to get
from the basal layer, where it’s born all the way up to the top where it kind of dies
and becomes the stratum corneum is roughly 28 days depending on what source you ask. Now in some diseases the epidermis starts
growing really quickly. For example, psoriasis it takes about 7 days
from when a squamous cell goes from the basal layer up to the top. And then this little pink line here that you
see this is known as the stratum lucidum. And even though that that name implies that
it’s clear or easy to see through, it’s actually usually more pink than clear. And there’s only two situations that you actually
see a stratum lucidum in the human body. Number one is on acral skin, the skin of the
palms and soles. And number two is any skin that’s been rubbed
or scratched chronically begins to get a thicker granular layer and it begins to get a stratum
lucidum. And eventually if you rub and scratch skin
long enough the whole epidermis will thicken and begin to look like the skin of the palms
and soles. And that’s a protective mechanism to avoid
injury of the skin. Now a couple of things we can point out about
the epidermis while we’re here. Number one there other cells that live in
the epidermis, not just keratinocytes. So these squamous cells are keratinocytes,
those names are interchangeable. They make up the bulk of epidermis, but we
have a couple other neighbors here that that play a role in the epidermis. So this one guy right here you can see he’s
kind of gray, it’s a little hard to get him in focus, but he’s a little bit gray and not
quite as pink. And he has a little vacuole around the outside,
that little white space, that’s an artifact of a tissue processing when we soak the tissue. You know tissue doesn’t come out your body
pink and purple like this, we have to actually soak in formaldehyde or formalin and run it
through a series of different chemicals to make sure that the tissue doesn’t break down. And then we dip it in a series of stains. And so this pink and purple stain that we
see here is called hematoxylin and eosin, or H&E. So the H&E stain is probably over 100 years
old and we’ve been doing it relatively the same way for about 100 years. It’s still it’s that the bread and butter
of what we in pathology is look at tissue under the microscope on H&E. So a lot of people think pathologists just
do autopsies all day long, some of us do, but I haven’t done an autopsy in a long
time and what I do is look at biopsies from living people mostly from their skin and decide
if they have cancer or not. So that’s the kind of job pathologist do. But anyway back to this little cell here,
this little gray cell does have a vacuole around the outside and that that’s an artifact
in the reason he has that vacuole is because if the cell shrinks during processing and
its cytoplasm clumps up to the nucleus and the vacuole forms are on the outside because
it doesn’t have any desmosomes to hang onto its neighbor. So that’s a melanocyte. Melanocytes are not hooked up to their neighbors. In contrast look at these nuclei are over
here these are keratinocyte nuclei and they actually about little halo, a little vacuole
around their nucleus. The nucleus is kind of naked floating in the
middle of that little space, that’s because when they shrink up all of their cytoplasm
is hooked up to the desmosomes of outside of the cell and it can’t go anywhere. It’s attached to all of the neighbors. So that poor little nucleus shrinks up it
is naked and alone. So when you see vacuoles in the epidermis
and you’re trying to decide is it a keratinocyte or a melanocyte. If they have the little halo right around
the nucleus, it’s probably keratinocyte. If the nucleus as a little blob of spidery
cytoplasm around it, like that guy, it’s probably a melanocyte. So these two guys here are melanocytes. And normal melanocytes live along the basal
layer. Here’s another one there. You can kind of tell because you would think
melanocytes their job is what it’s to make melanin pigments that makes your skin turn
colors when you when you tan your skin gets darker people that are have brown or black
skin darker skin patients they have more melanin production not more melanocytes. But they’re melanocytes are just more active
and make more melanin pigment. But you’d say well why are the melanocytes
gray and not brown. That’s because the melanocyte has an unusual
job. It makes melanin pigment and then instead
of keeping it and hoarding it all to itself, it actually shares it with all of the keratinocytes
around it. So these little finger like process called
dendrites. And it sends these all dendrites out to the
keratinocytes it live directly adjacent to it. And the keratinocytes actually kind of eat
the little fingers off of those little dendritic branches and that’s how they ingest on the
pigment into their cytoplasm. So you can actually see this here it’s a little
easier in some places. Here we go. If you look at these cells right here let’s
see if we can get maybe one higher. One higher power, a little closer view. So this is 600 times magnification. So we don’t usually use this in real practice
but for a video this’ll help. What you can see is that these cells. Oh hold on, I’ve lost my arrow, there it is. If you can see the cells right here, see how
they have a brown pigment in the cytoplasm. That pigment is kind of over top of the nucleus. You can see it up here too. It’s kind of like a little umbrella or a cap. And if you look, if you go up this way, that’s
where the outside world is. That’s where the sun is. So the sun would come down and would normally
hit these little keratinocyte nuclei. And you know sunlight contains ultraviolet
rays and ultraviolet rays damage DNA, and the DNA of cells lives inside the nucleus. And if you damage that DNA you get skin cancer
and things like that. So the job of melanin is to kind of try to
protect your skin to absorb some of that light and protect your skin from the damaging rays
the sun. So I like to think of it is that the melanocyte
makes the pigments. So here’s another melanocyte. You know how I know? Look it’s got a vacuole on the outside and
then a little blob of but gray cytoplasm stuck to the nucleus. Okay. So he feeds the melanin to all these keratinocytes
around him. I don’t know that the cells are actually all
boys, but I just like to talk to them like the cells are people. If you’re a pathologist too long that’s all
happened to you too. So I’m just kidding. Okay, anyway, the pigment up there, that little
it’s like a little hat or a little umbrella I think to protect the nucleus of the keratinocyte
from the sun. So when you see a brown pigment cell in the
epidermis, it’s actually probably a keratinocyte, not a melanocyte. You would think that it should be melanocyte. So you know with a little practice you can
do this. That little gray guy out there, that’s a melanocytes. All these guys appear that a brown, those
are keratinocytes. Okay so we said there are several cells that
live in the epidermis, there’s another one too. Let’s see if we can find one of them. And we might have to hunt around a little
bit. In the mid-level of the epidermis near the
spinous layer, we have a type of cell, that’s probably one right there is a little hard
to see him. But this little cell here that has a little
bit of a kind of bean shaped nucleus, that’s called a Langerhans cell. So Langerhans cells are basically, they’re
kind of related to histiocytes and macrophages. And they live in the mid-level of the spinous
layer. And the reason for that is that if the skin
barrier function breaks down antigens, things from bacteria, fungus, stuff from the outside
world, will come through the skin. And the Langerhans cell, their job is to cut
it eat that stuff up and break down the proteins. And then they come out of the epidermis and
go into the dermis, into lymphatic channels, and go all the way back to lymph nodes, and
then alert the immune systems and basically say, “Hey here’s some stuff we found. Should we, should the body fight against this
or not?” And then the immune system decides whether
or not it’s a bad thing or a good thing. And so that’s why the Langerhans cells live
in the epidermis because of the first line of defense against an incoming infection or
allergen or something like that. So they are antigen presenting cells, and
they are basically related to histiocytes, and they live in the mid-level of the epidermis. And they are a lot easier to see on immunostains. You can the stains for S-100 protein or CD1a,
which will highlight those Langerhans cells. In some diseases, like some rashes, like contact
dermatitis, you tend to see lots of Langerhans cells. And we will look at that in a minute. So that’s the epidermis. Now let’s move down into the dermis and see
what’s down here. The dermis has, we’ll go one lower power,
the dermis has two different parts. The first part is called the papillary dermis,
and the second part is called a reticular dermis. And the way to tell them apart is this… Well we actually didn’t talk about one thing
about the epidermis I forgot to mention. You can notice that all of the epidermis has
these little fingers that point down. There is one. There’s one. There’s one. Here’s one. There’s one. There. There. There. Those are called rete ridges and most areas
of the skin have these, little fingers that kind of undulate down into the dermis. Those are rete ridges. In between the rete ridges, the pink stuff,
is part of the dermis. And that part of the dermis is called the
papillary dermis. And each one of those little pink fingers
pointing up in between the rete ridges, each one of those is called a dermal papilla. And the dermal papilla has an important job. Look what’s inside of each papilla. This is kinda hard to see, but this little
tiny area in the middle, that’s a little tiny blood vessel or little capillaries. And the reason capillaries are right here,
right next to the skin, is because inside of the epithelium, in the epidermis that is,
you don’t have any blood vessels. So the way that the epidermis is nourished
is by absorbing oxygen and nutrients from blood vessels that are located just below
the epidermis. And so these blood vessels here basically
bring oxygen and nutrients, and that soaks out across this little space here and gets
up in the epidermis and feeds those cells. And then it also takes away waste products
from the cells. And that’s true of all epithelia lining layers
in the body. They don’t have blood vessels within the epidermis,
I’m sorry, within the epithelial layers. They always have in the stroma or the underlying
supporting collagenous tissue that basically provides a supporting structure for the epithelium
set to live and grow on. So the collagen, this little pink, the little
pink fibers here, that you’re seeing this is collagen. Okay collagen is one of the most common proteins
in the human body. It’s all of the soft tissue, and your bone
too. All of the supporting tissues in your body
are mostly made up of collagen, which are really strong kind of protein fibers. And you can see that they’re kind of, these
fibers in the papillary dermis, the fibers are very small and thin and fine. And as you go deeper into the dermis, all
of a sudden the fibers become much bigger, darker, pink in color, much larger. So this is that the reticular dermis has really
thick fat collagen fibers, and the papillary dermis has really fine collagen fibers. So you can tell the papillary dermis apart
because it’s actually up it’s the layer that’s right up here in between the rete ridges. But you can also tell it apart because it
has really fine, delicate collagen, whereas the reticular dermis has really thick, fat,
big collagen bundles. And the other thing is a little layer of larger,
not larger but larger than these capillaries, a little bit slightly larger vessels appeared. There’s a vessel there. Here’s a blood vessel there. Blood vessels here. So this layer of blood vessels runs along
the whole area of the skin right underneath that separates the papillary dermis from the
reticular dermis, and this is called the superficial vascular plexus. It’s a branching network of blood vessels
that runs right underneath there. Its job is to kind of feed blood up into the
dermal papilla a so that the epidermis can get nourishment. Okay. So that is the papillary dermis. And then down here we have the superficial
vascular plexus. And then underneath that we have the reticular
dermis. Okay, the reticular dermis is kind of the
main supporting structure of your skin. It’s the thickest layer until you get down
to the fat, and what you have is some larger blood vessels in here. Let’s take a look at one of these vessels. So this empty space is a blood vessel. So whenever you see a white space on a histology
slide, what you’re seeing is nothing basically. White is basically clear in histology. It means the white light for the microscope
is coming straight through with nothing blocking it. So if you have a white space on a tissue slide,
it’s always one of 3 things. Either number one, it’s a vascular channel,
lymphatic or vein or artery, a lymphovascular channel, and that’s what we have here. And we know that that’s the case because the
space is lined by a thin single layer of endothelial cells on the outside. That’s the lining cells of blood vessels and
lymphatics are called endothelial cells. And you can even see that little red guy in
there, that’s a red blood cell. So normally these are filled with red blood
cells, but during processing some of that blood washes away and then oftentimes they
leave empty space instead. So that’s one thing an empty space can be,
a vascular space. Number 2, we’re going to move to a different
part of the slide to show you. Well hopefully we have one… Yeah. Number 2 empty spaces can be lined instead
of by endothelium, they can be line by epithelium. And we can tell that this is epithelium because
it’s a couple layers thick and has a lot of extra pink cytoplasm around it. And actually in the middle of this little
space is also some little bluish goo. That’s actually sweat. These are actually sweat glands. We will come back to in a minute. So the other times when you see a space that’s
not lymphovascular, that space would be a glandular or cystic space that’s line by,
not endothelium, but by epithelium. And then the third time that you see a white
space is what we call artifact. That means that there’s something that used
to be there in the tissue that either dissolved during our tissue processing procedure in
the pathology lab, or the tissue tore or ripped when we tried to cut it because they make
these slides, we cut thin little slices, just like you take a slice off of a piece of our
lunch meat or salami, only instead of being that thick it’s actually, these are four micron
thick sections. Four micrometers ,very, very thin. That should have to be a topic of another
video for how we do that. But see these cells are here, these are fat
cells. These inside the human body, in real life,
these are filled with fat, with lipid. But during processing, the chemicals we use
dissolve the lipid and leave an empty space there. So even though this looks like white space,
this wasn’t white space inside. Wasn’t a hole or a space inside the patient,
there was actually fat filling the space, but now it’s artifact we washed out. So sometimes artifacts are a sign of a certain
type of cell, like fat cells, and other times I think we had a piece up here, like that,
that white space right there. It’s not line by any layer. This is a break, a little break, or tear in
the tissue. When we cut the tissue, it folded and ripped
a little bit and it left a space there, and that’s just an artifactual space. So now you know if you see a white space and
tissue is one of three things: a lymphovascular type space, a glandular or cystic type space,
or an artifactual space, that’s an artifact of our processing. So let’s get back to blood vessels. Here are vessels here. And in fact, we have kind of a little cluster
of things here. We have a small thin walled blood vessel,
and then this vessel is actually a little thicker wall. This is probably an arterial of the tiniest
branch out of an artery. This is probably a little venule or a small
vein. So that arterials are bringing blood from
the heart out to the tissue and then the venules are bringing blood back to the veins, which
will drain back into the circulatory system. Sometimes when they’re small, it’s hard to
tell arteries and veins apart. When they’re larger, it’s easier. But these are both vessels of some sort, and
you can see that they’ve got the little luminal lining of endothelial cells, and then this
little structure right next to this, is a nerve, a tiny little nerve. These are Schwann cells, and around the outside
of the nerve is a thin layer called the perineurium. And the nerves, there’s another little tiny,
tiny baby nerve right there, it’s hard to appreciate, but nerves run next the blood
vessel. And so together that’s called a neurovascular
bundle. And kind of we see this throughout the body,
for all the way from the really large vessels all the way down to the tiny ones, that nerves
and arteries and veins, they have a tendency to want to run together in little, in a little
group together, they kind of travel together throughout the body. Okay, so we talked about the dermal reticular
collagen, we talked about vessels, and we talked about a little bit about nerves. Here’s that vessel we started on before I
got distracted and look great there is another up above it you can see this vessel, the cells
are little more plump, but it’s just a single layer lining cells. That’s a good sign that it’s going to be a
blood vessel space okay. Let’s see. Okay then we have another type of bundle of
pink stuff here, this is some pink bundle that looks a different shade of pink than
the collagen. The dermal collagen is right there, and then
this bundle of kind of organized like lighter pink color, and it’s made of these spindled
cells, these elongated cells that are kind of running together in the same direction. This is smooth muscle. And in the skin, smooth muscle is in one of
two places. It’s either in the blood vessel walls or it’s
in these little bundles called arrector pili. And that’s true of most skin, there’s a couple
exceptions and a few other places have extra types of muscle like the nipple, the scrotum,
and the labia of the female genitalia, those all have extra smooth muscle bundles. But elsewhere in the body, you just have these
bundles which are arrector pili muscles, or you have smooth muscle that are in the vessel
wall. So arrector pili, you can kind of see here,
it’s getting close to hooking up to this sebaceous gland, which is hooked up to a hair follicle. So these muscles, when they contract, they
make your hair stand up straight and that’s why you get goose bumps or why the hair stands
up on the back of your neck when you’re scared or something like that. So those are those three pink bundles here,
those are called arrector pili muscles. Right, this we talked about. I mentioned it briefly. This is a sebaceous gland. So this is a lobule, a cluster of cells together
and these cells have a very different look than anything we’ve looked at yet. Each individual cell has a tiny little white
space in it. Those little white spaces are also artifact. This is because there used to be lipid or
fat in here, so that each cell, here’s one whole cell, this one right here is one cell. It’s got a nucleus, the little purple thing. And then all the cytoplasm is completely filled
up with these tiny white little bubbles. And these are perfectly round because these
were fat in the human body is made up of water, and when you mix oil or fat with water, what
does it do, it makes perfectly round little droplets, little spheres. So anytime you see a perfectly round little
white circle inside the body, it’s usually a good sign that you might be dealing with
something that was fat that’s been dissolved during processing. Because fat only can make little perfectly
round droplets. So these cells are called sebaceous cells
or sebocytes, and their cytoplasm is filled with little tiny vacuoles of fat. And when these cells, we will look at it another
section, when these cells all break down, they come, they squeeze out of the gland into
the hair follicle and they make a type a secretion called sebum or sebaceous secretion. And so this kind of secretions are more common
in the groin and the axilla and the kind of, you know, the kind of place of the body that
don’t smell quite as nice. And the reason that happens is that the sebaceous
secretion when it comes out on to the skin surface, certain types of bacteria break it
down and make it smell bad because it makes bad smelling compounds and so that’s where
body odor comes from. It might be kind of gross but it’s important
to understand where all these things, why all these things happen. So anyway, these are sebocytes and this is
a sebaceous gland. Around the outside of the gland, the cells
are a little bit more blue and what we call basaloid, so that’s kind of the what we call
the germinative layer there, the layer of cells that give rise to the more mature sebocytes. And then as the cells mature, they pick up
lipid in their cytoplasm, become more white and bubbly looking. Alright so that’s a sebaceous gland. Over here you can see, oh this is nice, here
is a hair follicle. Let’s go to a lower power, and you can see
that this is a follicle, but it doesn’t look to connect the surface. And that’s one thing that’s a little hard
for some people to learn about pathologies, you have to think a little bit. We’re looking at a 2 dimensional slice of
3 dimensional tissue. So that follicle definitely connects up to
the surface, you can kind of see the top of it up here, it just kind of is out of the
plane that we slice through. So if you sliced a little deeper, you might
see where it actually starts to connect up to the top there. So you have to get kind of used to looking
at pieces of things that are always perfectly showing the entire detail. But here you can see it kind of the base of
one of these hair follicles, and in the center, this little piece right here, that if you
kind of put your finger underneath the light you can kind of see it kind of move in and
out, that is actually the hair shaft. That’s a hair shaft, that’s what your hair
looks like inside your skin, and the follicles, this layer of epithelium surrounding the hair
shaft. And you can see the sebaceous glands, one
here and one here. Their job is to drain straight into and kind
of coat the outside of the hair shaft. And so that sebaceous secretion will drain
into the follicle and it come out of the top of that the follicle on the skin surface right
around where the hair shaft exits. And see here’s the opening of the follicle. So we will look again at follicles in a minute. We covered many of the structures. I think we have one more, one I wanted to
show you in here. Oh yes, these. So this, I said earlier, I showed you that
this is a sweat gland. This is an eccrine sweat gland. The kind of liquid-like watery sweat that
you get when you get sweaty palms, if you’re nervous, that comes from these glands. And these are called eccrine sweat glands,
or the word we often use the eccrine coil because what it is a simple little gland,
a tubular gland and that’s all tangled up into a coil and it makes the sweat. And then the sweat is secreted out through
this little tube called the duct, the sweat duct. And again, you can’t see all the sweat ducts
travels all over the surface, it kind of seems to disappear. But it goes all the way up to the surface
and empties out. And if you look around we may be able to find
another one where another one empties out. Or maybe we won’t. Maybe this piece of tissue doesn’t have one. You don’t always see every structure that
you’re looking for on every piece of tissue. Alright, well we will have to see one in another
piece of tissue. So here, these eccrine coils they have a unique
kind of lining. They’re lined by a double layer of cuboidal
epithelium. So cuboidal cells have round nuclei, and kind
of are slightly square or cuboidal in shape. Now I know you have use your imagination to
think that these are square but they’re kind of, they’re not taller than they are wide,
like a columnar cell would be. So you can see that they have these rounds
nuclei, and they’re kind of 2 layers of them. I think you can best appreciate the double
layer here in the duct. So you can see the double layer here, the
inner layer, is this layer here, that’s more pink, and this outer layer, which is actually
made of a special cell type called myoepithelial cells. They kind of have a little bit of contraction
properties supposedly. I don’t know how much of that it’s actually
used here but they do have some contractile filaments in them, so they have a little bit
of muscle like quality. So anyway that’s the myoepithelial layer on
the outside or the basal layer, some people might call it. Then the inside is the apical layer, these
larger nuclei with pink cytoplasm. So it’s double layer cuboidal cells, and really
there’s only, I think, there’s only two places you really see that in the body that I can
think of. I’m not a histology expert, but I do look
at a lot of tissue. You see them in the ducts of sweat glands,
and you also see them in the ducts of the salivary glands. You can see kind of a double layer of cuboidal
epithelium and that’s I think because the sweat glands and salivary glands have a lot
of things in common. And when you get tumors in your salivary glands,
they tend to have a lot of overlapping features with tumors that grow on the skin from sweat
gland origin. And this is a good example of, here is lumen,
the center part of a gland or center part of a vessel, so that anti space we call lumen. So this is a lumen and then here is the sweat
ducts and out here is a comparison. This is a blood vessel space, see it’s got
a red cell in it, and it’s got one layer lining it. You might say, “Well what about this out
layer here?” That’s actually, those are modified smooth
muscle cells called pericytes. And that’s a little thin layer of muscle around
the outside of the vessel. So if you did different special stains, immunostains,
you can show that this is made of epithelial cells and this is actually made of endothelial
cells and muscle. And then right up above we have a nerve. There’s a tiny little nerve right there. And another you can compare, this is nerve. And this is smooth muscle. And actually another thing one of my professors,
Dr. Jae Ro, taught me lots and lots of histology. He liked to teach triads. And he said that whenever you see pink bundles
of spindle cells in the human body, they’re one of three things. Number one, they’re dense regular connected
tissue, they’re made up of dense collagen. And so even if this isn’t really a bundle,
you can see this dense pink here, that’s collagen, like the dermis is made of. Number two, they’re nerve. This is nerve right here and you can see it’s
nerve lined by this little layer perineurium on the outside and it’s a little hard to tell
nerves when they’re small. When they’re bigger, it gets easy to see. We will look at a bigger one in a minute. And then over here, this is smooth muscle,
arrector pili. The cells have more of a kind of cigar-shaped
nucleus, the cytoplasm is a little bit different color. So you can see right here, collagen, nerve,
smooth muscle all in one picture. And there’s a little blood vessel up there. See you guys are probably already pros. You starting to probably recognize all the
structures, it just takes practice. But looking at the stuff in real life. So there’s that eccrine coil. And that eccrine coil usually resides deep
in the dermis, right at the junction between the dermis and the subcutis. And you can see there’s a little bit of fat. This stuff here, these are adipocytes, and
this is a little pouch, outpouching of the subcutis, where the fat kind of pushes up
and surrounds the eccrine coil. But usually, when you’re down to the level
of eccrine coils, that means you’re almost at the bottom of the dermis and you’re about
to enter into the subcutis. And when this skin excision was done, there
was cut at the level of the deep dermis, and so we don’t actually see any subcutis here. That black stuff is ink that we add in our
laboratory to help us see where the edge of the tissue really is, the true edge the surgeon
cut. Or the dermatologist in this case cut. So that is enough from this piece. Let’s move to another piece of tissue. So this is an example of acral skin, skin
on the palm or sole. And you can tell that because it’s got a very
thick corneal layer here at the top. It’s got that stratum lucidum that we talked
about previously. And the reason I’m showing this, this is actually
not normal skin. This is skin that’s involved by contact dermatitis,
like you get if you touch poison ivy. And there’s some extra white space up in the
surface here, that’s making a little blister. And there’s some of this stuff that’s called
parakeratosis. We can go closer and look. So normally you don’t have nuclei left in
the corneal layer, but when skin gets irritated, and starts growing quickly, the nuclei get
retained. We talked a little bit about that earlier. The granular layer kind of goes away, or gets
diminished, and that parakeratosis, the presence of nuclei in the corneal layer, shows up. And so that’s a sign that the skin’s abnormal,
that’s not a normal finding. To have parakeratosis means the skin’s been
irritated from dermatitis or rash. Or it’s a tumor or a pre-cancerous, growth
something like that. Right. And then here in the dermis, you can see that
around the vessel, see here’s a little blood vessel, right here little thin one, but rather
they’re all these blue cells. Those blue cells are lymphocytes and maybe
some histiocytes, also macrophages if you like. The reason I show these is that even though
these are, when we have a lot of them it is kind of an abnormal finding, almost all skin
that we see biopsied has at least some degree of lymphocytes around the vessel. So a little bit of lymphocytes is actually
normal, and it’s not anything to get too worked up about. So we see these blue cells around vessels,
they’re probably just lymphocytes. Don’t be too concerned. And they’re also the other reason I show this… Ah here. See we will go higher power and see if we
can see them better. These bright little guys here, it’s really
hard to pick them up because their granules don’t show up, but they’re bright orange. They’re like brighter than, those are red
blood cells. These bright orange cells, these are eosinophils
and they have two lobes of their nuclei, looks like little pair of like aviator sunglasses
or something. And then they have these bright orange granules. So eosinophils are involved in a variety of
processes, including allergic processes. So in this case, this is allergic contact
dermatitis, that’s why they’re here and also they’re involved in fighting off parasites
and other things like that. And then, sometimes you talk about neutrophils. So in contrast, these dark guys that are almost
purple and black in color, those are lymphocytes. See that guy, there, that has multiple nuclei,
that’s actually a neutrophil. And he’s just there in a blood vessel. That’s normal. When you see a lot of them that usually means
something else. Alright, so the reason I want to show this
slide is that when you go over here, there’s this space and looking next to the space,
you can see there’s a lot of extra white space between each of the keratinocytes in the epidermis. And this is called edema, or when edema is
present in the epidermis, it’s called spongiosis. And so spongiotic dermatitis is things like
eczema or eczematous dermatitis, atopic dermatitis, contact dermatitis, there’s a large number
of different rashes that give you this finding microscopically. The reason that I’m showing you in a normal
histology video is that when you look at this on higher power, what you can see, it’s hard
to get a focus, but between each keratinocyte, see those little lines. Those are the spines. That’s why we call this the spinous layer
because those spines, those are desmosomes. And you can see them much more clearly, when
they’re stretched out by all that edema fluid in between each individual cells. So that space there is because of all the
fluid that’s pushing apart, but it lets you really see how prominent those spines are
there. Usually kind of difficult to see on normal
skin, but here when you have spongiosis, it’s a nice way to showcase just how dramatic the
spines are. And they’re really strong because even though
they’re being pulled apart by all this fluid, they’re holding on for dear life to their
neighbor. And sometimes when they get pulled too hard,
they lose connection and that’s when you get this little blister here. That’s when you get these little bumps. And those little cells inside there, these
are Langerhans cells. And so we talked about Langerhans cells living
in the on mid-level part of the epidermis, and the reason they live there is because
when you get contact with, when you touch an antigen, those cells go in and find that
antigen and eat some of it and then take it back to the lymph node and shows the immune
system to see if you should mount an immune response. So these little guys here, those are Langerhans
cells. And they’re kind of collecting together, and
in contact dermatitis, you get that. Again look at these beautiful spines. There’s really, really very nice example. And a look that’s a mitosis too while we’re
here. See that’s a little mitosis that’s probably
in anaphase. It’s on its way to becoming two new daughter
cells. So this skin is revved up and irritated, and
so it’s dividing and making new cells to try to repair itself. Alright, so those are the spinous processes. Now let’s look at some more acral skin. And this is normal acral skin, look for low
power. And again looked the really massively thick
and a corneal layer, you can see this pale line here that’s the stratum lucidum again. Only place that’s really present in the body
normally is acral skin on the palms and soles. You also get if you scratch or rub your skin
and even from this power, where this is magnified 20 times normal, so 20 times a normal eye
view, and you can see those open spaces are blood vessels. Here, up here, you have your epidermis. You have your papillary dermis, the little
pink stuff in between. This is the reticular dermis down here. And even from here you can see the difference
in the collagen: the fine collagen of the papillary dermis, and in the thick, chunky
collagen, and the thick bundles of collagen in the reticular dermis. And you can see down here that they’re at
the bottom, we’re starting to get into fatty layer, the subcutis. Right. And then here between the dermis and the subcutis,
we have all these little round structures those are all the eccrine glands that we talked
about before that live at that level of the dermal subcutaneous junction. So even from very low power, you can begin
to see all the structures once you know what you’re looking for. So let’s go in a little closer because they’re
a couple unique structures in acral skin that are worth knowing about. I really like acral skin. It’s very pretty under the microscope and
it has a lot of cool little things to see. So let’s look. Number one, if we can find the structures
up here that I really like and of course, they probably won’t be here now that I’m looking
for them. That’s disappointing. Well, we’ll have to go down here instead said
this structure right here look, it looks a blood vessel. It’s got kind of a little muscular pink wall. And in the middle, it’s got a space that’s
called the lumen, and that lumen is lined by a single layer of little kind of flattened
cells, those are endothelial cells. And there’s little couple red blood cells
in the middle. But look, this is kind of different from other
blood vessels because look at the small round cells, very nicely and neatly organized around
the outside of this vessel. So this is called a glomus apparatus, or glomus
body. And these mostly exist at the tips of the
fingers, near the nail beds, and they’re arteriovenous anastomosis. And the theory is that they play some role
in temperature regulation and those little cells around called, they’re kind of modified
to muscle cells, pericytes, we talked about earlier, they’re just very prominent here
and they kind of have contractile properties that allow this vessel to kind of open and
close, and allow for blood through. So that’s a glomus apparatus. And then look over hear, there’s another little
tubular shape, but this is not vessel. It’s got too layers of cuboidal cells, so
this is actually an eccrine duct. Alright, so you can begin to tell these things
apart once you look at them. A lot. And the other thing we’ll look at here, we
have this structure which is a pink bundle, and remember we talked about pink bundles
can be nerve, smooth muscle, or dense regular connective tissue like you’d see in the tendon
or fascia. And this in this case, it’s nerve. And in nerves it’s kind of a little bit
wavy, it kind of undulates back and forth. And when you look closer, you can see right
there, this little line in the middle, that’s actually an axon, and those little bubbles
around that are where the myelin normally resides. Let’s see if we can get that in the better
focus. So I only see it just very focally. So that little pink line there is the axon. And then there’s little bubble, and in that
little space is either this probably the node of Ranvier or there’s also this thing called
an incisure (of Schmidt-Lanterman), I can’t repeat the name of the person, but someone
named it so it must be important. So anyway, that little space of white is where
myelin was that’s now washed out. So only in like kind of larger, deeper nerves,
do you really see much myelin. Up near the surface of the skin, we don’t
usually see myelinated nerves, we only see unmyelinated most of the time microscopically. Alright, and then let’s look at the adipocytes
while we have them here. So this is what that the fat looks like in
the subcutis. The fat cells are large and white large white
circles and they have nuclei, you just don’t get to see them all the time because it depends
what cut you’re at. Each of these big huge fat cells only has
one nucleus. So the nuclei are these little dark guys at
the very edge of the fat call, and they’re actually kind of disk shaped, but they look
like little spindle stretched out cells because they’re squished out by all this fat. And so it’s kind of hard to see what their
nuclei look like in a normal fat. But when you get them cut at an angle, you
can kind of see their nuclear features, where they have a little bubble of little fat droplets
in their nucleus. And we’ll see if we can find that here. I don’t… Oh there this. So here this little adipocyte nucleus is kind
of folded over, so we can see it, we can see it cuts across. And that little hole in the center is called
a “lockhern”, that means “nuclear lake”, it’s a little lake of lipid/fat in the middle
of the nucleus. And it’s always there in adipocytes nuclei,
you just can’t usually see it. So when the fat is kind of atrophic or damaged,
you can kind of see it. We cut through the adipocyte nucleus at an
angle and then you can see this little bubble. It’s always there, but people sometimes get
confused about that because they’re not used to seeing it. And there’s another little tiny glomus apparatus. See the little vessel in the middle, and the
little clump of cells around it. Alright. Now let’s look at the different part of the
body. Oops, we’ll turn it over. Now this is beautiful. This is the scalp. And here we are even at the lowest possible
power on my scope, try to get the focus there, you can’t even see all of it in one field
and that’s because these giant hairs that go all the way down. So these are called anagen hairs, they’re
the normal hair that make up most the hairs on your scalp. And you can see that these blue kind of nodules
that here they almost look like little flames in the end of a candle, if you have a good
imagination. Those little purple things, those are the
roots of the hair. That’s what’s giving rise to the hair. It’s growing the hair from that place there. And those in the scalp and near the scalp,
the hair roots are situated down here in the subcutaneous fat. So most other places on the body, your hair
follicles kind of arise in the dermis, but here on the scalp, particularly you have lots
of these really large anagen hairs. And they have their roots down in the subcutaneous
fat. And you can see we can’t, you know, unless
we get really lucky with the way we slice the tissue, usually don’t get to see the whole
hair all the way up. So you have to see it kind of has multiple
sections that hair connects to the surface. We just can’t see it because it’s not on the
plane there were sectioned through tissue. So and then up here, you can see where the
hair shafts come out or where the follicles open to the surface and the hair shaft would
come out through there. You can see these white sebaceous glands. Right. And those make the sebaceous secretion we
talked about that earlier. So let’s look a little closer. I think that hair follicle anatomy is one
of the more confusing aspects of histology, I think there’s a lot of different structures
in there. So here, we’ll go down the higher power. And I’m sorry for the line across the screen,
that’s a scratch on the surface of the slide. And it’s at the diagonal line, it’s not real
it’s just on the surface of the glass. So here we are. This is hair root or the hair bulb, and it’s
made up that these dark blue cells. This is called the germinative epithelium. These cells are actively dividing, you can
see mitosis right there. And their job is to grow quickly and that’s
why our hair grows because of these cells way down here at the very root. And then this little pink, this little pink
papilla invagination into the bulb of the hair roots, or the hair bulb, this is called
the hair papilla. And it’s made a kind of specialized mesenchymal
spindle cells, like modified fibroblasts. There’s little blood vessels in there and
that’s what gives the support and nourishment to the developing a hair bulb. Okay and as you go up, you can see that these
germinative or also we call the blue cells matrical cells, these matrical cells are germinitive
cells. So they begin to change and they begin to
look less blue or purple and a little bit more pink, and that’s because they’re developing
more keratin in their cytoplasm. And what’s going to happen is that these cells
will eventually die, the nuclei will go away, and what you’ll be left with is this compact
like kind of cable made out of dead keratin compacted together, and that’s what the hair
shaft is made of. It’s made all these cells that are dead and
compact together. But we can also see that around the outside
of these matrical cells that are developing in the hair, we have this bright red beautiful
layer here. And this is called the inner root sheath,
and it’s got a couple layers generally I don’t think are really important to know that that
eventually will develop a cuticle, a Henle layer, and Huxley layer. I don’t think that those are really that important
for most medical students to know at least and honestly probably not that important for
most pathologists to know. And then here on the outside, you can see
this outer layer looks very very different. It’s very clear. It kind of has these very columnar, like tall
cells that almost look, some people imagine they look like piano keys, and sometimes when
we have a better cut they do. So that’s called outer root sheath, and it’s
clear because the cells are filled with glycogen and that glycogen washes away during tissue
processing and leaves these clear spaces. And then as we travel up, these layers continue
to change and look different. So here, this is a different hair follicle,
but a little bit further up, and you can see this is very different now. And we’re cutting kind of an angle, that’s
why it looks like a little oval rather than a tube. This is the outer root sheath, and you could
see those nice piano keys along the basal layer. You can see this clear cytoplasm. The inner root sheath now doesn’t have those
bright red granules, those are called, and that we looked at, those are called trichohyaline
granules by the way. Those bright red granules, those are trichohyaline
granules. Oh and look, this is nice. This is a different hair shaft, but you can
see that the cells, those blue matrical cells we talked about earlier, they’ve now like
gotten to be very pink, and they have docked their nuclei. They are on their way toward towards dying,
and so this central structure here is eventually going to become the whole hair shaft. So again that’s outer root sheath. These three layers here are the inner root
sheath. And then this is the developing hair shaft. And so as we go further up, now you can see
it’s kind of fragmented but let’s look closer. The inner root sheath is now just a single
pink dense layer. The hair shaft is kind of broken here but
that it has no nuclei retained anymore. That’s the hair shaft in the middle. This is the inner root sheath right here. This is the outer sheath that’s like glycogenated
and has those little piano keys. And then there’s this little pink layer on
the outside that we call the adventitia, it’s kind of modified fibroblasts that are surrounding
the hair shafts. Let’s go up a little further on the hair. So here’s a nice long cut. The hairs actually missing, it’s fallen out
of the center during processing. Hair is a little bit fragile, so sometimes
it does that. But as you go up, you can see that the outer
root sheath doesn’t look quite so clear anymore. Now it’s looking much more like the kind of
epidermis does. It’s kind of pink and has a basal layer, and
then a more pink layer on the inside. It doesn’t have that really prominent clearing
anymore. Now the hair follicle keeps going up and eventually,
even though see the white space here, this is kind of a chunk of tissue that was missing
from when we cut that the tissue, but you can see that what happens is the sebaceous
gland drains into the hair follicle. And so that’s called the sebaceous duct. This little area here, where the sebaceous
gland drains directly into the follicle. And the follicle opens up on the surface. And on the surface you can see that this portion
of the follicle is called the infundibulum, the portion where that the sebaceous duct
drains in, that’s called the isthmus, and in this portion appears the infundibulum. And the infundibular portion of the hair follicle
looks just like the epidermis. And we talk a lot in pathology about epidermal
inclusion cysts or epidermoid cysts. The surgeons call them sebaceous cyst even
though they’re not actually sebaceous. What they’re lined with is this lining right
here. So they’re really actually follicular cysts
of the infundibulum. They’re growing from infundibular portion
of the hair follicle, and they have a lining just like this, and that flaky loose keratin
in the middle. So this basically at the very opening of the
hair follicle looks just like the surface of the epidermis. And you can see this little detached fragment
here, this is a hair shaft. It’s kind of refractile, you can kind of,
it kind of floats above the rest of the tissue. It’s hard to see because it’s fragmented. And it’s got little bits of brown and that’s
melanin. And that’s where hair has color because we
have melanocytes down in that root of the hair follicle. They give the hair its pigments. Let’s see if we get another, a better look
at a hair shaft. They’re hard to find sometimes. The little one there, but maybe there’s a
bigger one somewhere. That’s pretty good. See here’s a hair shafts, so the follicle
is that that outside epithelial layer here that kind of a surrounds the hair shaft. And the shaft is the hair itself that comes
out atop your head. And again you can see these little streaks
of pigment there, that’s melanin. That’s what would make your hair brown or
different colors. And then the rest of the pink of the hair
shaft is keratin. It’s from those dead matrical cells down the
root that gave rise to this. And that’s when you get a haircut, that’s
the stuff that’s out on the surface that gets cut. Here’s another picture of hair root cut a
little bit differently. There’s the hair bulb, and look at the brown
pigment in there, those are actually melanocytes. Try to get it in focus here. There’s little brown pigmented cells, those
are melanocytes and they have these are branching dendritic processes, these little tiny branches
and they’re feeding melanin to all of the matrical cells. And that’s why when the matrical cells go
up towards the top and die and turned the hair shaft, the hair shaft has pigment included. And again that’s why we have pigmented hair
of different colors. So that’s how hair works. It’s a little bit complicated, and I still
think it’s like one of the last great histologic mysteries for me. And I still hair pretty challenging. I think we have one other view, maybe of a
hair follicle here. Let’s take a look and see. That’s about the same as what we were showing
before. You can see the bulb down there at the bottom,
it may have matrical cells. You can see the developing inner root sheath
of those bright red granules. And then moving up, you can see the outer
sheath that’s made of pale, glycogenated cells. And in the middle here, you can see the matrical
cells are now turning more pink and elongated. And so that’s actually that’s called the cortex
of the developing hair shaft and then that’s the medulla. So that the bulb is turning into a cortex,
an outer layer and a medulla middle layer, and then eventually that will all squish together
and die and turn into air shaft. And again we have nice adipose tissue out
here. Oh and that’s pretty good too. Here’s a little bit thicker blood vessel. And you see the blood vessels have that inner
layer of endothelial cells that are flat, and then on the outside as you get in the
larger more muscular vessels, you actually have two layers of smooth muscle. You have kind of an inner layer and then an
outer layer. And the outer layer is kind of wrapping around
in a circle, we’re cutting across that’s why the cells instead of being spindly they look
like little tiny round dots, because we’re cutting them in cross section. And if you get in here, just a little bit,
you can see that if we cut it a little bit different angle, that they actually spindle
cells, they are elongated. And this cell and this cell, those of the
same cell, we’re just cutting them 90 degrees to one another. So when you cut long thin spindled cells in
half, kind of like if you imagined a hot dog, you got a long ways versus a cross section
half, it looks totally different. You have to think of cells in 3 dimensions
just like that. And then right here we got a nerve. And there’s those little pink dots are axons. The little purple dots are Schwann cell nuclei. And then on the outside this layer is called
the perineurem. It wraps and supports the nerve. This is a section of adipose tissue, fatty
tissue from near the hand of the wrist. And the reason I’m showing you this there’s
no actual epidermis on this piece but you can see where near the dermal subcutaneous
junction because look, in the fat we have these. Eccrine coils, eccrine glands. So you know we’re near the skin once you
see those you know that the skin has got to be… Sorry. Skin has got to be nearby. There they are, the eccrine sweat glands. And then these structures over here are one
of my favorite structures. One of my favorite normal structures in the
human body. There they are. The onion skin beauty of the Pacinian corpuscle
and so these structures are basically attached to a nerve. And they’re made of multiple swirled layers
around the outside, and their deep down in the tissue. And their job is to receive deep touch or
pressure. So that’s why they’re not near the surface,
they’re down deep. I kind of think of them as having all these
multiple rings so that they can be kind of spongy and they can squish down when someone
pushes on your hand. And so you tend to have these most prominently
near the acral surfaces, and they’re down usually in the fat or the deep dermis. So I think they’re just really pretty to look
at and I never get tired of seeing them, no matter how many times I’ve seen them. See, here’s another one. A big huge kind of one, cut at kind of different
angle. Looks like an onion, right? More than anything else, that’s called onion
skin, this looks like an onion. Pacinian corpuscles. Plus they have a cool name, so I really like
the Pacinian corpuscles. All right. Here, let’s take a minute to look at a big
nerve. And this is not actually from skin, this if
from deep soft tissue, but just demonstrate what nerves look like. This is a large nerve made of multiple nerve
bundles. You can see each of these pink bundles is
a nerve bundle all packed together. And they’re made of these spindle cells that
are running at together in parallel. And again those are Schwann cells. They’re kind of protecting and wrapping around
each individual nerve fiber. Nerves tend to get a little bit undulating
and wavy, but be aware that that they’re not the only thing that’s wavy. Actually tendon and other fibrous tissue tends
to be wavy as well. But that’s a large, kind of thick nerve. We don’t usually see nerves like this in normal
skin. This is from deeper and soft tissue. Alright. Think we already looked at this tissue actually. And then I think one last a structure to show. We didn’t really cover this is fat, subcutaneous
fat. And here in the fat is a very large tumor. This is sarcoma actually. We’re not gonna talk about that in this video,
but the reason I’m showing this is that down at the very bottom of the fat where the fat
of the subcutis runs into the muscle underneath, you have this thick layer of pink stuff this
is dense regular connective tissue, dense collagen and this is called the fascia. Tendon, fascia, ligament all look more or
less identical microscopically just depends on where exactly it’s from. But what it’s made of is really thick and
dense compact bundles of collagen and those dark stripes there, that’s artifact, that’s
folding from when we cut the tissue. Sometimes the tissue cuts don’t fold, don’t
lay out perfectly flat, and we get little folds in it so. I’m sorry, we try to avoid that but it’s impossible
sometimes on big specimens. And here if you look at higher power, this
is what dense, regular connective tissue looks like. If you cut cross sectionally, the fibroblasts
look like they’re little tiny round nuclei. But if you cut it like here, in longitudinal
section, you can see that the fibroblasts actually stretch out and are kind of elongated. And there’s dense pink stuff in between, and
that’s the collagen, collagen type one. So a lot of people confuse this with smooth
muscle but I think that a key is that if you recognize, it’s a little hard to get on video,
but if you recognize that the pink bundles are actually bundles of collagen outside of
the cell. And under a microscope, you can kind of see
they were refractile a little bit, they kind of move in and out of the tissue. If you kind of move your condenser up and
down. Maybe we can try that and see if it works. Not really. Oh there. I guess there you can kind of see that they
kind of move in and out of focus. And at these cracks in between them, so those
are collagen in bundles and collagen is an extracellular matrix protein. So it’s outside of the cell. So these are fibroblast, they’re making all
this pink collagens and pushing them outside of the cell, as opposed to smooth muscle which
looks pink because it’s filled with contractile proteins. So those are intracytoplasmic proteins. Takes a little practice, but eventually you
can kind of learn to tell them apart. But I think that’s important. I find that even that at the resident, and
sometimes higher levels, people still sometimes struggle with telling apart tendon or fascia
from muscle. So if you struggle with that as a medical
student, don’t feel bad you’re not alone. Alright. Now let’s look at a couple of immunostains
before we finish up. And immune stains again are antibodies that
are targeted at certain proteins. And they are then tagged with a colored molecule. And that lets us see what kind of proteins
we are actually dealing with here. So let’s start with cytokeratin. So we said cytokeratin is an intermediate
filament. Oops, we will turn it around. It’s intermediate filament that fills up epithelial
cells of all sorts. All epithelial cells in the whole human body
should have cytokeratin, and they sometimes have different types of keratin. But this is a marker, this one right here,
is a pancytokeratin. Those six stains multiple different types
of keratin, so it will stain pretty much every type of keratin ideally that’s out there. So if we see that positive, we know that what
we’re dealing with is an epithelial cell. So here, let’s get it in focus. So there’s the normal epidermis, and you can
see the epidermis is bright, this dark brown color because it’s filled with keratin filaments. Where’s the dermis, which is made of collagen,
is totally negative for keratin. So keratin highlights the normal epidermis. And then when we looked down at the structures
in the epidermis, you can also see… I’m sorry, the structures down in the dermis,
you can see that these things we talked about earlier. The hair follicle so made of epithelium, so
it’s keratin positive. The sweat glands also made of epithelium,
so their keratin positive. So all these cells that are brown are some
sort of epithelial cells. So here we have sweat glands, hair follicle,
and then we already talked about that the surface in this skin, the epidermal surface
is made of keratin as well. And so we can use, the reason we use these
immunostains is to help us when we have tumors if we’re not sure what type of cell tumors
coming from, we can use the immunostains to help us determine if the cell is epithelial
or its muscle or nerve or melanocyte origin. And that makes a big difference because those
tumors all have different properties that we pay attention to. So the next thing we’re look at is S-100
protein. And the reason I’m showing S-100 protein is
that it does, it stains nerve and some other things. It stains melanocytes in the epidermis. So it will stain melanocytes along the basal
layer, so these little guys down at the bottom here. Let’s find them. These little cell sitting on the bottom basal
layer that are dark brown, those are melanocytes. You can see they have these little branches,
those that we call them dendrites. Those are the branches that feed melanin to
the neighboring keratinocytes. So melanocytes are S-100 positive. Alright, but they’re not the only cell in
the epidermis that’s S-100 positive. You can see another branching little cell
up here in the mid-layer of epidermis, those are Langerhans cell. The only reason I can tell them apart is because
of where they’re located in this skin, which is actually a piece of normal epidermis. So you have S-100 positive melanocytes down
at the basal layer, and then also Langerhans cells, which again, are antigen processing
cells up there in the mid portion of the epidermis. And then also fat, adipocytes tend to stain
with S-100 as well. So you can see S-100 staining these fat cells
here in the subcutis. S-100 is not a very useful stain for fatty
tumors, but it does stain normal fat. And it will stain nerves as well, but I didn’t
have a good nerve to show in that piece. And then here is a stain for a muscle marker
called desmin. Desmin is a muscle protein. And you can see actually that the epidermis
is dead negative, right, completely negative for this marker. But down in the dermis, you can see these
bundles that are dark brown, those of the smooth muscle bundles, the arrector pili muscles
that we talked about earlier in the video. Those are the muscles that give you kind of
goosebumps, right, and they hook up the hair follicles. And then also look at these round little donut
shaped rings around here. Those are blood vessels. Alright, so as opposed to desmin around vessels
highlighting the wall, let’s look at this immunostain. This is an immunostain called CD 31. And so you can see it’s actually staining
the central lining of the vessel, the lining of the lumen, which the endothelial cells,
but the muscle wall on the outside is actually completely negative. So again, you can tell that the inner lining
and the wall of the vessel are actually made of different components. You can see all these little vessels around
here, the smaller capillaries, they all have the lining cells, the endothelial cells, are
staining with CD 31. And CD 34 is another marker that does the
same thing. It stains the lining cells of the vessels
and it will stain both lymphatics and arteries and veins. Endothelium of any kind will stain with this
marker. So again, one more look. There that’s the lumen being lined by endothelial
cells which a positive on a CD 31 immunostain. Alright, and one other immunostain to show
is a stain called Sox10, and it’s a nuclear marker. It’s a protein that’s in the nucleus of melanocytes. And you can see this is, I’m just showing
this to highlight that melanocytes normally are present on the basal layer of the epidermis,
and they’re kind of spaced out. And this is a patient with a lot of sun damage,
and so the number of melanocytes kinds of increases a little bit in sun damaged skin. But they’re down there on the basal later
for the most part. They’re spread out with that kind of melanocyte
for every 7 to 10 keratinocytes, depending on who you ask and where exactly you are on
the body. And look a little closer. You see this is highlighting the nucleus of
a cell. So we should the S-100 earlier they showed
both the cytoplasm and the nucleus. Sox10 mostly highlights just the nucleus. So it just helps you see very nicely how many
cells are there. It will also stain other things too, like
it stains Schwann cells and nerves. So it’s not a perfectly specific marker but
it stains both benign and malignant melanocytes, and it will also stain most nerves. I’m not sure if we have a nerve down here
but we can look and see. If there’s any nerve in the dermis. I’m not seeing one. So anyway, that’s a Sox10 immunostain and
that’s just to highlight kind of the normal pattern of how melanocytes are situated in
the basal layer of the epidermis. Alright. And now let’s go. There’s a couple things I realize that I
skipped over and did not show you. And I wanted to demonstrate those. So one is a structure that I was looking for
in acral skin. And this is not the most perfect example,
but it will have to work. For now. Right here beneath the epidermis, so in the
papillary dermis, is this round, or kind of oval shaped pink structure. And it has a kind of little layers, little
lines that run across and little spindle cells inside when sometimes these are like perfectly
oval, and they somehow remind me of a striped pink Easter egg. I’m probably the only person the world that’s
reminds me of but it that’s what I think of when I see them. But these are called Meissner corpuscles. And Meissner corpuscles or Meissner bodies
live right here in the superficial dermis, right the papillary dermis, so they’re right
next to the epidermis. And the reason is that unlike the Pacinian
corpuscles, that big onion skin looking guys that are down deep, their job is to detect
that kind of firm pressure or deep pressure or touch or vibration sensation, Meissner
bodies, these little guys up here, are right next to the epidermis because their job is
to detect fine touch. Basically, so they need to be right under
the epidermis so that they can detect the most minute of movements and they’re mostly
located on acral skin, particularly near the fingers. And that’s why we’re able to have the most
discreet fine touch with our fingers because of all these little Meissner corpuscles. You don’t see them much elsewhere except on
acral skin. Even on acral skin, they usually really have
a predilection towards the tips of the fingers, on the finger pads, where you have the most
fine touch ability. So that’s a Meissner corpuscle. We’ll look a little closer. And for a normal structure. And then a nerve kind of hooks up to this
thing. You can’t always see the nerve in there, but
basically this is made a kind of a layered bundle of nerve like material and structures
that then connect up to a nerve and go back to the nervous system and give fine touch
information to the brain eventually. So that’s a Meissner corpuscle. And I also wanted to show a couple of interesting
slide that we don’t get to see very often. Now this unfortunately, this slide is too
big to even entirely fit on one view, but this is acral skin. You can see that thick corneal layer. And there’s dermis, subcutis, and this is
bone. And that’s the bone marrow space which is
filled with fat and a little tiny bit of bone marrow. And this is actually an amputation specimen
from a, I can’t remember, I think this is from a toe. And the reason is because of this massive
tumor here, this is a melanoma growing underneath the toenail. So this is a big huge melanoma, that’s not
really the point of the video, but the point is just to show that this is what a toe looks
like. You’ve got the acral skin, and underneath
it that you’ve got the dermis. You got sweat glands down here. There are large dilated blood vessels. I think underneath here we can see some very
nice glomus structures. Glomus apparatus. So you see this kind of tangled coil of vessels
here, each of those central spaces is a lumen lined by endothelial cells. You even see some blood cells in there. And in those little perfectly round guys kind
of organized around the outside, those as modified muscle cells, the pericytes, that
help kind of constrict these little vascular channels. So that’s a glomus apparatus or a canal of
Sucquet and Hoyer if you want to be fancy and impress your friends, you can tell them
that. And then down here is bone, this is a cortical
bone, lamellar bone, you can see that the lines laid down here in organized fashion. And in these central spaces, those are called
lacunar spaces, and in there you have osteocytes that have become entrapped. As they started as osteoblasts and then as
they built the bone around themselves, they made a little tiny space where the osteocytes
are entrapped inside and so that’s cortical bone. So this is the bone of the phalanx of the
great toe. And let’s go around to the other side here. The other thing that’s kind of interesting
here is that you can see, coming right off of the bone, is dense pink layer. This is dense regular connective tissue, and
again I told you earlier, that dense regular connective tissue is either fascia, tendon,
or ligament. And here you can see that it’s actually connecting
this big band of it right here is connecting broadly to the bone. So if that on the other side, which we can’t
see here, if it hooks up to a muscle then this would be a tendon. If on the other side it hooks up to a bone,
it would be a ligament. So ligament and tendon and fascia look very
similar at high power. And really it’s kind of the context that you
find them in that you can tell them apart. So that’s a nice example of probably a tendon
in this case. It’s hooking up to one of the muscles that
allow you to contract or extend your fingers, to flex or extend fingers. Oh and another nice structure that we have
here, this is an artery and how we can tell it’s an artery is that we got this nice
thick smooth muscle wall around the outside and then look what’s happening here right
outside the lumen, there’s the lumen. The lumen is lined by a layer of thin flat
endothelial cells, and then out here, let’s see if we can get it to show up. We’re going to flip the condenser. See that squiggly line there that comes in
and out of focus? That’s called the internal elastic lamina
and it’s this little band of squiggly elastic tissue that’s right underneath the intima. And the intima is this inner layer and then
the media is this outer layer that’s made of muscle. And there’s also a little tiny layer outside
called the adventitia. But when you see a presence of the internal
elastic lamina, that means it’s an artery actually rather than a vein. So veins can have thick muscular walls just
like arteries can, but finding that elastic lamina that’s a pretty good sign that you’re
dealing with an artery. So there’s a nice example of a big artery. We don’t see those in most skin biopsies,
but when you have larger specimens like this, you can see them sometimes. So we’ve got artery, we’ve got some probably
tendon, then we got nice bone down here, and then we’ve also got a big melanoma arising
under the nail. And I think somewhere I also saw… Over here. Oh yeah. It’s kind of fragmented, but you can see part
of that onion skin. So there it is again, the Pacinian corpuscle. It’s kind of broken in this case artifactually
during processing, but that’s a Pacinian corpuscle. Alright I think I’ve got another amputated
toe here for a melanoma. So fortunately, these are rare melanomas on
the toe, but they do happen. And actually famous musician Bob Marley died
from a subungual melanoma of the great toe, unfortunately. So this is when dark skinned patients get
melanoma, this is the kind of melanoma they tend to get. You don’t usually get melanoma if you’re black
or brown skinned, but if you do get melanoma, it’s usually going to be under the toenail/fingernail
or on the palms or soles. So the reason I’m showing this one is here,
this is the top of the toe, and when we come out, this is what we call the nail fold going
over the top of the nail. And this dense pink structure right here,
this is nail, the nail plate. The nail plate is the part of your nail that
you clip when you clip your nails. And where the nail plate grows from deep down
here underneath that little area at the top of your nail when you look down at it this
is called the nail matrix. So these are the dividing cells. They look more or less like keratinocytes,
they’re basically modify keratinocytes. And these cells are growing and giving rise
to the nail plate. The nail plate just like your hair is made
of dead keratin. It’s just a bunch of keratin stuck together. And you can see that the keratin develops
into this nice plate and that plate comes out and exits from underneath the fold and
then goes along the top surface of the toe. And again here, it’s unfortunately underlined
by this huge massive melanoma, all of this purple stuff down here. This whole huge thing unfortunately is melanoma,
which is very bad. But this is that’s a nice example, though
aside from the melanoma, is an example of the matrix and the nail plate. And the little layer of epithelium underneath
the nail plate, we call that the nail bed epithelium. And again, it’s not very normal here. It’s kind of destroyed by the melanoma underneath
it, but it is a good idea to get from low power to see that this is what your nail fold
up there, the nail plate, the matrix of the nail. And then out here is the tip of the toe, lined
by acral skin. It’s kind of torn up from processing. Unfortunately to get the cuts of these sections
with bone in them, we have to soak them in a type of acid to dissolve the calcium. And that kind of damages the quality of the
histology, and makes the tissue not look quite as good, but that otherwise we’re not able
to cut through the bone. So and again you got your nice acral skin
here. And what other structure to show while we’re
on acral skin. Let’s see if we can find a good. A good one here. Ah, there it is. So right here, on acral skin, when the eccrine
sweat duct, that’s the eccrine sweat duct, see the double cuboidal layer with the little
lumen lined by pink cuticle, it eventually comes up and empties into the surface. And when it does, you can see it kind of spirals,
you get this, this is all one duct that’s kind of spiraling, we’re just cutting through
part of the spiral. It spirals up and out through the surface
and that still the space there, they’re even going through the corneal layer that allows
the sweat to come out to the surface. So when you get sweaty feet or sweaty palms,
this is how it’s happening. And so this structures called the acrosyringium. The acrosyringium is this little twirly outflow
tract of eccrine sweat ducts that you see most prominently on acral skin. The eccrine ducts exit out the surface, also
in the skin too, but you really notice the slight kind of swirled, a spiral like pattern
in the palms and the soles. There’s another one, so you can see the eccrine
duct. You can see it’s kind of cut. There’s another section of it, there comes
up to the top. So again, this all connects together, we’re
only just seeing one plane of section. So these are 3 dimensional structures we’re
just cutting one slice through. So that’s a nice example of an acrosyringium. And I think that about covers everything except
for one other abnormal thing, but it’s so common that it’s worth bringing up because
we see it all the time. So I showed you before… Sorry. This guy showed you before, we said that there’s
epidermis and then there’s the dermis which is composed of pink collagen bundles, but
looking at this you can tell, even from here you can tell, this is not pink. This is actually blue or gray. This is made of tangled little wavy fibers
that are made, that are blue or gray in color. And you can see a little tiny bit of pink
collagen that is left over in here. Let’s move over a little bit. You can see there’s more collagen over there,
that’s what the dermis should look like, right. But here, most of the dermis is being replaced
by all of this pink. Sorry, all of this bluish gray stuff. And those are elastic fibers, and so this
is called solar elastosis. And when you spend years and years out in
the sun, getting sun damaged, this is what happens. This is not from one sunburn, this is from
decades and decades of chronic daily sun exposure. And you can see that when people that are
older and had a lot of sun exposure, what happens is, it basically pushes down and replaces
the whole dermis. And so now the whole dermis is now composed
of a thick layer of elastic fiber. So we call this solar elastosis, and when
you see that, it’s a good sign that you’re dealing with someone who’s had a lot of sun
exposure. And so you know you can keep an eye open for
things that tend to occur in the setting of sun exposure, like in this case. This is actually an invasive squamous cell
carcinoma. All this pink stuff growing down here, this
is squamous cell carcinoma. A type of skin cancer that occurs secondary
to long term sun exposure. Alright, so that’s solar elastosis and in
the part of the country where I practice, we see this so commonly, most cases actually
probably end up having this when people are of a certain age because people just are out
in the sun a lot, there’s a lot of high intensity sunlight here. So it’s so common that even though it’s not
normal, it’s worth bringing up. And this is actually what causes wrinkles. So when you get, when you see people that
are very wrinkly skin from long term sun exposure, this is why. It’s the elastosis makes the skin kind of
move into different shapes and doesn’t let it go back to its normal shape over time. So not only does sun damage cause certain
types of cancer, but also causes the skin to wrinkle and age more rapidly. So I think that wraps up normal skin histology
and even a few abnormal things as well.


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