Melanocyte

Melanin-producing dendritic cell (of neuroectodermal origin) that lies between the keratinocytes of the stratum basale of the epidermis and is found in the skin and extracutaneously in the mucous membranes, inner ear, eye and brain. Some of 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 come into contact with the surrounding keratinocytes via a large number of dendrites. 1 melanocyte contacts about 36 keratinocytes(epidermis-melanin unit).

Melanocytes make up about 1% of the epidermal cells. The ratio of keratinocytes to melanocytes varies in different regions of the body (melanocyte/basal cell ratio varies between 1:4 and 1:9). The face generally has the highest melanocyte density (2000/cmm). On the trunk it is 1000/cmm. The skin on the palms of the hands and soles of the feet 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. Dermatology 61: 554-560
2. Casalou C et al. (2022) Loss of 'Epidermal Melanin Unit' Integrity in Human Skin During Melanoma Genesis. Front Oncol 12:878336.
3. Li A (2014) The biology of melanocyte and melanocyte stem cell.
   Acta Biochim Biophys Sin (Shanghai) 46: 255-260.