Specialized Cells and their Functions
Both gene expression and natural selection are required for “building” traits of an organism, including skin color in humans. The outer cells of the skin provide protection from the sun by synthesizing and distributing pigment. Important in this process are melanocyte and keratinocyte cells.
|Figure (above): Melanocytes (brown) make and transport melanosomes for several “client” keratinocytes. They have long skinny “arms” to accomplish this. The Keratinocytes (pink) have variable numbers and sizes of melanosomes (black) that cluster around the nucleus.|
Keratinocytes and Melanocytes
Human skin pigmentation is the result of gene expression and natural selection. Skin color requires the expression of dozens of genes all interacting to produce the characteristic of “skin color”. Differential reproductive success, the result of natural selection, ensures that an organism’s genes will be carried forward into grandchildren, great-grandchildren, great-great-grandchildren, etc.
|Figure (left): This electron microscope image shows melanosomes clustering near the nucleus of keratinocytes. They protect it from damaging ultraviolet rays by absorbing this relatively high energy light. Dark skin has more and larger melanosomes around the nucleus of keratinocytes that does light skin.|
Skin has four functions: 1) act as a barrier between inside and outside worlds; 2) is a large sensory organ; 3) provides a home to an individual’s microbiome; and 4) interacts with the sun’s UV radiation. The outer layer of skin, the epidermis, is where skin color is expressed. The epidermis has two kinds of cells. Keratinocytes make up the outer layer of the skin and contain packages of pigment that are responsible for skin color. Melanocytes make the packages of pigment, melanosomes, and transfer them to the keratinocytes. There, they cluster around the nucleus. The number of melanosomes is related to the pigmentation or darkness of the skin. Melanosomes are transferred from melanocytes by either phagocytosis or endocytosis. Keratinocytes send paracrine signals to melanocytes to upregulate the formation of melanosomes.