Melanin, the most common of the biological pigments, is produced by special cells derived from the neural crest. These cells migrate during development to specific places: one such place is the deep layers of the skin. In the H&E preparation at right, melanin appears as a brown material inside the cells of the skin's deep layers. The cells which produce it (melanocytes) are residents of the deepest layer, and pass the pigment they make on to other cells, that in turn carry it to the surface.
Melanin gives skin part of its coloration. It's produced in the melanocytes via a synthetic pathway that uses the amino acid tyrosine as a raw material. Lack of an enzyme in this pathway can occur due to a spontaneous and fairly common mutation. When this enzyme is missing or inactive, the pathway is blocked, so no melanin can be made. The inability to
synthesize melanin is the condition we call albinism; an true albino
animal has white fur or feathers and no pigmentation in its eyes, either.
The reverse condition (often seen as a color phase variation in rodents
and some carnivores) is melanism, in which more than the usual amount
is made. Such animals are said to be melanistic. The black squirrels
found in the streets and parks of Washington DC are a nice example. They
are melanistic strains of the common grey squirrel, Sciurus carolinensis
and freely interbreed with greys to form a range of color variations from
"normal" to deep blue-black.
Cat footpad, epidermal-dermal boundary; H&E stain, paraffin section,
This image shows melanocytes and keratinocytes in the deepest layer (stratum basale) of the skin. The melanocytes are the cells in this row which are not stained and which do not contain melanin. Since they don't retain the pigment but pass it along to the adjacent keratinocytes, melanocytes are usually unstained in an H&E preparation.
Melanocytes are a stable and long-lived population of cells. The keratinocytes aren't.
The life of a keratinocyte is at best measured in days, but the life
span of melanocytes is years, probably in most cases the entire life of the animal.
As with other normally non-dividing cells, however, sometimes things go haywire.
If melanocytes do start dividing, it's a
very serious matter, as they lack the ability to control their division.
The uncontrolled growth can become highly invasive and move into surrounding areas. The worst case is a malignant melanoma,
a particularly metastatic and often fatal type of cancer. Exactly what
causes it is unknown, but well documented risk factors include overexposure
to the sun and physical damage to a pigmented wart or mole.