7.  Pigment Transfer in Skin Cells
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7.  Pigment Transfer in Skin Cells

The preservation of the Developmental Biology Film Series was made possible by generous contributions from Publisher of “The Biggest Picture” Producer of the documentary, “Symbiotic Earth” Mammalian skin consists of a superficial epidermis and an underlying dermis. In the epidermis dead keratinocytes make up the outer layer. Immediately below them are differentiating keratinocytes which arise from actively dividing stem cells that lie closest to the dermis. Other cells, the melanocytes, are found among the dividing keratinocytes. These cells manufacture granules of the pigment melanin and transfer them to the keratinocytes. This diagram shows the structure of this epidermal system. The melanocyte extends long processes called dendrites between the closely packed keratinocytes in the dividing layer. Melanin granules are formed in the melanocyte and then transferred through the dendrites to the keratinocytes. When keratinocytes mature and slowly reach the surface of the skin where they die, they carry the melanin with them. This is a microscopic preparation made from a piece of guinea pig skin. The dermis has been removed with trypsin and we are looking at the basal layer of the epidermis from underneath. The specially stained dendritic melanocytes appear dark against the lightly stained keratinocytes. As the plane of focus has changed it can be seen that melanocytes lie in different levels in the skin. To study the dynamic interactions between these two types of cells, they are isolated with trypsin and cultured in a cell suspension. Here the two cell types are easily distinguished by their shapes. But very often it is much more difficult to recognize the melanocytes. In this scene a melanocyte develops two long dendrites which introduced packages of pigment granules, or melanosomes, contained within vacuoles, into a flat keratinocyte. While the individual melanosomes are too small to be seen in the light microscope the transferred vacuoles, containing many melanosomes are visible. The melanosomes are left behind as the dendritic processes start to withdraw. The vacuoles open up inside the keratinocyte dispersing pigment into the cytoplasm. Let’s watch the pigment donation again. [Music]


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