The Integumentary System
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The Integumentary System

It’s Professor Dave, let’s look at the
integumentary system. With some basics covered regarding two of
the types of tissue in the human body, we are ready to start looking at some of the
different systems found in the body, and we are going to start with the outermost system,
which is called the integumentary system. This is made up of the skin that covers and
protects all your other body parts, as well as its derivatives, like certain glands, as
well as hair and nails. Let’s look at the structure and function
of these components now. The skin is a remarkable organ, acting as
the first line of defense from the elements that surround us. It keeps bacteria out, and water and heat
in, so let’s check out the structure of skin. Our skin is made of two regions, a thin outer
layer, which is the epidermis, and a thicker inner layer, called the dermis. The dermis, which is a tough layer of fibrous
connective tissue, is vascularized, getting its nutrients from the bloodstream. The epidermis, which is made of epithelial
tissue, is avascular, so it gets nutrients by diffusion through the tissue fluid from
the dermis. In addition, below the dermis is the subcutaneous
layer, also called the hypodermis. This is made predominately of adipose tissue,
and it anchors the skin to other structures below. Now let’s zoom in on the epidermis, the
thin outer layer of the skin. This is made of keratinized stratified squamous
epithelium, made of four different cell types in five layers. The deepest layer, the one attached to the
dermis, is called the stratum basale, meaning basal layer. This is made of a single row of cells, most
of which are called keratinocytes. These are rapidly dividing all the time, pushing
new cells up into the layers above, to help regenerate dead skin, and producing lots of
keratin as they do so. This is necessary because millions of dead
keratinocytes rub off your skin every day, due to friction, and even more from your hands
and feet, so these constantly dividing cells in the stratum basale ensure that a new epidermis
forms every few weeks, so that we always have our skin intact. This layer also contains melanocytes, which
produce melanin, and tactile cells, which act as the sensory receptor for touch. Above this is the stratum spinosum, meaning
prickly layer. This section is several layers of cells thick,
and is full of cells with a weblike system of intermediate filaments attached to desmosomes. They look kind of spiky, so they are sometimes
called prickly cells. Here we can also find dendritic cells, which
ingest foreign substances and activate the immune system, which we will talk about later. Next is the stratum granulosum, or granular layer. This is four to six cell layers thick, and
in this section, keratinization begins as cells continue to move their way upwards from
the basal layer. This is when the cells get far enough from
the dermal capillaries below to receive sufficient nutrients, so the cells fill up with keratin
as they die, and they flatten while the organelles disintegrate. This makes the cell tougher and scalier, which
allows for the outer layers to better protect the body. Above this is the stratum lucidum, or the clear layer. This is two or three cell layers thick, made
of dead keratinocytes that have become flat and clear. This is where they begin to aggregate into
arrays called tonofilaments. And lastly, there is the stratum corneum,
or horny layer. This outermost section is twenty to thirty
cell layers thick, and all of these cells are anucleated, meaning the nucleus has disintegrated. So it’s incredible to think that all the
cells covering the outside of your body are actually dead, but they are dead in a specialized
way, with thick plasma membranes surrounding lots of keratin. These dead cells protect all the living ones
inside from all the outside dangers. With the epidermis covered, let’s go a little
deeper and talk about the dermis. This is made of strong and flexible connective
tissue, and is full of nerves and blood vessels. This is also where hair follicles begin. The dermis has two sections, the papillary
layer, and the reticular layer. Descending from the epidermis we enter the
papillary layer, which is very thin, made of areolar connective tissue and a loose network
of collagen and elastic fibers. This leaves room for defensive cells to patrol
the area for bacteria that may have made it through the skin. The word papillary refers to the projections
from the surface of this layer, which are called dermal papillae, that stick out into
the epidermis above. These contain lots of tactile cells as described previously. In areas where there is lots of friction,
like the hands, these papillae sit on mounds called dermal ridges, which cause ridges in
the epidermis as well, which are meant to enhance the gripping ability of the fingers,
and they are visible as the lines on our fingertips that make our unique fingerprints. Below this papillary layer sits the reticular
layer, which is most of the dermis, and it is made of dense fibrous connective tissue
that is arranged irregularly. A network of blood vessels sits below this,
just before the hypodermis. What else can we say about the skin? We mentioned that the stratum basale also
contains melanocytes, which produce melanin. This is a pigment molecule, and this is one
of the components of the skin that determines its color, and protects the skin from ultraviolet
radiation. The other pigments responsible for skin color
are carotene, which is yellow-orange, and hemoglobin, which is red when oxygenated. We will discuss this molecule in more depth
later in the series. That wraps things up for the skin, so let’s
just briefly touch on the other components of the integumentary system, which are called
skin appendages. First, there is hair. Of course most of us have lots of hair on
our heads, but there is also hair all over the body, including eyelashes and nose hairs,
and these all have specific protective functions. So what is a hair made of exactly? As it turns out, a hair is a flexible strand
made largely of dead, keratinized cells. This is hard keratin, which is a bit different
from the soft keratin found in cells of the epidermis, which makes them more durable,
and not as flaky. Hairs are produced by hair follicles. These possess a root, which is the part deep
inside the follicle where keratinization is happening, and a shaft, the part closer to
the surface of the skin and then extending outside the body, where keratinization is complete. The hair itself consists of three layers of cells. The innermost is the medulla, containing large
cells and soft keratin. Next is the cortex, which is several layers
of flattened cells. And lastly there is the cuticle, which is
a single layer of overlapping cells, the most keratinized cells in the hair. As for the follicle the hair sits in, this
also has some structure to it. In general, a hair follicle is a pocket that
folds down from the surface of the epidermis down into the dermis, about four millimeters
below the surface. The deep end then expands slightly to form
a hair bulb. A bundle of nerve endings attach to the bulb
and act as a receptor, responding to any bending of the hair and alerting the brain in case
an insect is there, or something of the like. A little bit of dermal tissue called a hair
papilla protrudes into the bulb, which supplies signals and nutrients to the hair so it can grow. The wall of each follicle has a few layers. The outermost is the peripheral connective
tissue sheath, derived from the dermis. Next is the glassy membrane, derived from
the basal lamina. And then there is the innermost epithelial
root sheath, derived from the epidermis. This last section has an external part and
an internal part, which thins as it approaches the bulb. The cells that actively divide are found in
the hair matrix, which push existing cells upwards as they divide, causing the hair to grow. In addition, each follicle has an arrector pili. This is a small bundle of muscle cells that
can contract and pull the follicle in such a way that the surface of the skin dimples
out, producing what we refer to as goose bumps, when cold or afraid. Our body hair is pretty sparse so this no
longer serves much purpose, but for much furrier mammals it is an important defense mechanism
for trapping heat and intimidating enemies. Hair can either be vellus hair, which is pale
and fine, or terminal hair, which is darker and more coarse, like hair of the eyebrows
and scalp. The nails found on our fingers and toes are
also part of the integumentary system. These are products of a modification of the epidermis. Just like with hair, in contrast to the soft
keratin of the skin, nails contain hard keratin, making them great tools for scratching or
picking up objects. Each nail has a free edge, which is the very
tip, then a body, which is most of what we see, and then a proximal root, which is embedded
in the skin. The part of the epidermis the nail sits on
is called the nail bed, and the nail grows out of the nail matrix, which pushes the nail
outwards across the nail bed as these cells divide. There are also skin folds overlapping the
borders of the nail which are called nail folds, sitting on the lateral and proximal
borders, and the latter extends onto the nail as the eponychium. At the edge of the finger is the hyponychium,
where dirt tends to collect. The last part of the integumentary system
we must mention is the vast collection of glands that can be found. First up are the sweat glands, also known
as sudoriferous glands. These are found almost everywhere on the surface
of the skin, totalling up to around three million. There are two types of sweat glands. The first is called eccrine, or merocrine
sweat glands. Most of them are of this type, and it consists
of a coiled tube. Secretion occurs in the dermis, and the resulting
fluid, or sweat, travels through the tube towards a funnel-shaped opening called a pore. Sweat is ninety-nine percent water, but it
contains some salts and metabolic wastes, among other things. Sweat is also secreted by apocrine sweat glands,
which are far fewer, found only in certain areas, and secrete fat and protein components
along with the normal mixture, which is the cause of body odor. Ceruminous glands and mammary glands are also
types of apocrine glands, which produce earwax and breast milk. Beyond sweat glands there are sebaceous glands,
also known as oil glands. These are branched alveolar glands that secrete
sebum, which is made of oily lipids. This will soften and lubricate hair and skin,
slowing water loss and killing certain bacteria. So that covers the basics of the integumentary
system, which consists of skin, hair, nails, and glands. This system acts a barrier, separating what’s
outside of us from what’s inside of us. It has the ability to repair quickly, regulate
body temperature, and respond to stimuli outside of the body, among other things. Now that we have this system covered, let’s
head inside the body and check out the rest.


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