Melanocyte

Melanin-producing dendritic cell (of neuroectodermal origin) located between the keratinocytes of the stratum basale of the epidermis and occurring in skin as well as extracutaneously in mucous membranes, inner ear, eye and brain. In part, the extracutaneous melanocytes have completely different functions than UV protection. Remarkably, there are also hidden melanocyte reservoirs in the skin that have no pigment-forming activity.

Melanocytes contact surrounding keratinocytes via a multitude of dendrites. 1 melanocyte contacts approximately 36 keratinocytes.

Melanocytes make up about 1% of epidermal cells. The ratio of keratinocytes to melanocytes varies in different body regions (melanocyte/basal cell ratio varies from 1:4 to 1:9). The face generally has the highest melanocyte density (2000/cmm). On the trunk, it is 1000/cmm. The skin on the palms and soles has only 100 - 200/cmm.

Melanocytes synthesize melanin (melanogenesis) in response to physiological and pathological stimuli and thus cause a tanning of the skin. Skin tanning is caused by increased proliferation of melanocytes and by increased production and transfer of melanin.

Melanogenesis itself is a highly complex process in which several signal transduction molecules, transcription factors and enzymes interact. More than 150 genes are involved in the genetic control of this process.

Melanin is produced in certain organelles of the melanocyte, the melanosomes. From the melanosomes, melanin is transferred from melanocytes to keratinocytes by active phagocytosis. It is deposited (cap-like) around the nucleus and thus protects the DNA of the skin cells from UV damage. Thus, the otherwise pigment-free keratinocytes receive a pigment screen of melanin. Per keratinocyte there are 16 - 132 melanosomes in members of the Caucasian race and 80 - 160 in members of the negroid race.

The UV-irradiated keratinocytes stimulate melanogenesis by releasing paracrine factors such as the peptide hormones ACTH, alpha-MSH as well as GMCSF and c-Kit (see kit below).

Melanin itself is a polymerization product of the amino acid tyrosine and acts as an absorber of light of all wavelengths.

1. Brenner M et al (2010) Basics of skin pigmentation. Dermatologist 61: 554-560
2. Li A (2014) The biology of melanocyte and melanocyte stem cell.
   Acta Biochim Biophys Sin (Shanghai) 46: 255-260.