Skin color is a function of the size, number and the distribution of melanin cells ( not their density )
In fact, the number of melanin cells is the same is the same in all races, however melanin cells of darkly pigmented skin
have thicker, longer and branched dendrites. The differences in racial skin pigmentation depend on the quantity of melanin
pigments produced and on the distribution and the deposition of these pigments throughout the epidermis.
Tyrosinase is rate-limiting enzyme for melanin synthesis, and defects in the enzymes activity lead to the albinism in
human. It also seems likely that racial differences in human color may primarily be due to differences in the tyrosinase
activity in the melanin cells from varying skin types. Melanin cells derived from black skin have up to ten times more
tyrosinase activity and produce up to ten times more melanin than do melanin cells in white skin.
Melanin The Skin Pigment
Visible pigmentation in human results from the synthesis and distribution of melanin in the skin, hair bulbs and eyes. Melanin plays a crucial role in the absorption free radicals generated within the cytoplasm and in shielding the host from various types of ionizing radiation, including the UV light. Thus, melanin protect the skin against sun burn .
The essential enzyme in melanin biosynthetic pathway is tyrosinase, the only enzyme absolutely required for melanin production.
Tyrosinase, The Enzyme Behind The Dark Skin
Tyrosinase is multifunctional, glycosylated, copper-containing oxidase with a molecular weight of approximately 60 to 70 kDa. In mammals, it is exclusively found in melanin cells. It is therefore a good specific marker for these cells. Tyrosinase is synthesized in melanosomal ribosome's found on the rough endoplasmic reticulum. After synthesis, tyrosinase is glycosylated within Golgi, then delivered to melanosomes via coated vesicles .
Tyrosinase is rate-limiting, essential enzyme in the biosynthesis of the skin pigment melanin. As such it catalyzes three different reactions in the biosynthetic pathway of melanin.
The rate limiting steps in melanogenesis are the oxidation of tyrosine and DOPA. The quantity of melanin synthesized is thus proportional to the amount of tyrosinase activity present in the cell.
Regulation of Melanogensis
The regulation of pigmentation in mammals is controlled at many different levels and is quite complex at each level. Melanin cells are initially derived from the neural crest and migrate throughout the embryo during development. These migration patterns are under strict genetic control and can lead to some interesting patterns when final melanin distribution in the skin is not uniform, as can be seen in zebras and giraffes.
Pigmentation is also regulated at the cellular level by melanin cells synthesizing melanin within melanosomes, which can be produced in varying sizes, numbers and densities. Lastly, melanogenesis is regulated at the sub cellular level where synthesis and expression of various melanogenic enzymes and inhibitors play a critical role.
Skin pigmentation depends upon the organization and function of epidermal melanin unit and several separate but related events:
Melanoblast migration from the neural crest.
Melanoblast differentiation into melanin cells.
The rate of synthesis and melanization of melanosomes.
The size of melanosomes.
Synthesis of melanin.
The efficacy of melanosome transfer into keratin layer.
The rate of melanosome degradation within the keratin layer.
The rate of synthesis and inhibition of the tyrosinase enzyme.
Activity of tyrosinase in melanosomes.
Melanin cells work in close harmony with their neighboring cells in the epidermis. They are influenced by a variety of biological factors and environmental factors with the most important factor is UV- exposure ( sun exposure) which increase the melanin cells activity up to 100-fold.
