Wiktionary
n. A cell in the skin that produces the pigment melanin.
WordNet
n. a cell in the basal layer of the epidermis that produces melanin under the control of the melanocyte-stimulating hormone
Wikipedia
Melanocytes ( or ) are melanin-producing cells located in the bottom layer (the stratum basale) of the skin's epidermis, the middle layer of the eye (the uvea), the inner ear, meninges, bones, and heart. Melanin is the pigment primarily responsible for skin color. Once synthesised, melanin is contained in a special organelle called a melanosome and moved along arm-like structures called dendrites, so as to reach the keratinocytes.
Usage examples of "melanocyte".
Anyway, basically, what a lab is going to be looking for is whether that hyperpigmentation is intracellular, as in melanocytes, which are the dark, pigmented cells that are much more concentrated in non-Caucasians, or intercellular.
For some years it was assumed that nothing like intermedin existed in mammals, but in 1955 biochemists at the University of Oregon were able to isolate from the mammalian pituitary a hormone that stimulated the activity of the melanocytes (mel'uh-noh-sites), which are the cells that produce the skin pigment melanin.