Melanocyte

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A human pigment cell is referred to as a melanocyte or melanocyte , also known as melanophore ( s ) .

Location of epidermal melanocytes.
Bottom left of the block diagram of the structure of the skin from the epidermis ( epidermis ), the dermis ( dermis ) with hair root , and subcutaneous ( subcutaneous tissue ).

Most of these pigment- containing cells are located in the exposed skin and there in the basal cell layer of the epidermis and of hair follicles . Melanocytes are of neuroectodermal origin, synthesize melanin from tyrosine and store the pigment in a membrane- encased melanosome . These oval organelles are transported in the epidermal melanocytes in branching, dendritic processes and at the end of them are released as mature melanosomes to individual surrounding keratinocytes (pigment transfer).

The light-absorbing melanin represents an important protection against the high-energy, ultraviolet radiating (UV) part of light, especially for the genome ; in the keratinocytes of fair-skinned people it is held in melanosome complexes and arranged like a cap over the cell nucleus . With black skin types, the melanosomes are more numerous, larger, individual and distributed over this skin cell. The number of melanocytes is the same, each of which, together with the two to three dozen cells it supplies, forms an epidermal melanocyte unit .

histology

The melanocytes lie in the basal cell layer ( stratum basale ) directly on the basement membrane and are connected to it via hemismosomes . Melanocytes occur in relatively small numbers and are loosely connected with their cytoplasmic branches (dendrites) with about 5–8 keratinocytes. Melanocytes are metabolically active and have numerous mitochondria , a pronounced rough endoplasmic reticulum (rER) and a large Golgi apparatus . Melanocytes are also found in the choroid of the eye, the iris of the eye , the lining of the mouth, and other places. It should be noted here that these special melanocytes do not form melanocyte units with the surrounding cells and usually do not release any melanosomes into their surroundings. Immunohistochemically , melanocytes are S-100 positive. Histochemically, melanocytes stain black-brown (melanin) when incubated in DOPA solution.

embryology

The Melanozytenvorläuferzelle , Melanoblast called, emigrated during the embryonic development of the neural crest forth into the skin. Melanocytes are therefore descendants of the neuroectoderm . In addition to the cutaneous ones in the epidermis and hair follicles, melanocyte populations are later found in the uvea of the eye, in the stria vascularis of the inner ear , in valves and septa of the heart, and in the soft meninges ( leptomeninx ) over the lower brain stem areas.

Melanin synthesis and distribution

The formation of melanin (eu- and pheomelanin), the so-called melanogenesis, begins with the synthesis of the initially inactive enzyme tyrosinase . This is synthesized in the rough endoplasmic reticulum and accumulates in the Golgi apparatus, which then constricts it into vesicles . The activation of the tyrosinase takes place via UV light . After the vesicles have matured further, melanin and crystalline inclusions can be detected under the electron microscope . At this stage the vesicles are called "premelanosomes". The amino acid tyrosine accumulates in the vesicles , which is converted into DOPA by tyrosinase and finally converted into melanin via intermediate steps. The protein Trp-1 also plays a role here. After further maturation, the premelanosome loses its internal structure and is then referred to as a “mature melanosome”. The mature melanosomes migrate into the cytoplasmic extensions of the melanocytes and are released to the surrounding cells. The surrounding cells absorb the melanosomes directly and store them in their own cytoplasm. In lower vertebrates (fish, amphibians), the melanosomes remain intracellular and are not released.

Regulation of melanin synthesis

  • Ultraviolet radiation: Under UV exposure, the melanocytes are activated and more melanosomes are released (skin tanning)
  • MSH (melanocyte-stimulating hormone, melanotropin): MSH also activates the melanocytes. In Addison's disease , among other things, more MSH is produced, which is reflected in increased skin tanning.

Diseases

A decrease in melanocytes or decreased melanin synthesis leads to hypopigmentation . Melanocytes or melanocyte precursors play an important role in the following diseases:

See also

Individual evidence

  1. ↑ K. Steel, C. Barkway: Another role for melanocytes: their importance for normal stria vascularis development in the mammalian inner ear. In: Development. Volume 107, Number 3, November 1989, pp. 453-463, PMID 2612372 .
  2. ^ I. Yajima, L. Larue: The location of heart melanocytes is specified and the level of pigmentation in the heart may correlate with coat color. In: Pigment Cell & Melanoma Research. Volume 21, Number 4, August 2008, pp. 471-476, PMID 18627529 .
  3. ^ M. Goldgeier, L. Klein, S. Klein-Angerer, G. Moellmann, J. Nordlund: The distribution of melanocytes in the leptomeninges of the human brain. In: Journal of Investigative Dermatology. Volume 82, Number 3, March 1984, pp. 235-238, PMID 6699426 .