Individual differences in hair color are correlated chiefly to the amount, character and distribution of pigment in the hair shaft. As in the epidermis, hair pigment is formed in melanocytes, whose most important function is to synthesize and transport melanins to the keratinocytes of the hair cortex and medulla. The difference between follicular and epidermal melanocytes is that the former produce the yellow pigment pheomelanin as well as black eumelanin and that they do not produce pigment almost continuously, like the melanocytes in the epidermis, but only in a particular phase of hair growth.
Melanocytes develop from melanoblasts, which originate from primordial cells of the ectodermal neural plate. The melanoblasts migrate to the relevant mesodermal region during early embryonic development, their infiltration being dependent on interaction of the surrounding cells. The presence of tendritic melanocytes has already been demonstrated in human embryos in the tenth week of intra-uterine development. In he embryonic follicle the melanocytes are distributed not only in the bulb, but also in other segments in which they are absent in postnatal life.
In the anagen follicle the melanocytes are localized between the keratinocytes in the bulb above the papilla and their dendritic processes project into the intercellular spaces. Apart from their shape, they differ from the surrounding cells by having no desmosomes and by not containing any submicrotopic fibrils.The melanocyte nucleus is surrounded by mitochondria and large numbers of dense particles 10—20 nm in diameter are seated on the surface of the membranes of the endoplasmic reticulum. Melanosomes — ovoid or rounded granules about 1 mm in diameter — are present near the Golgi complex. Immature melanosomes (premelanosomes) are characterized by a longitudinally fibrous surface with periodic indentations. Eumelanosomes, which produce black pigment, are to be distinguished from pheomelanosomes, which produce yellow pigment. Pheomelanosomes are spherical and in the immature state their membranes contain small vesicles.
Melanosome formation is determined by the interaction of four basic components — proteins, tyrosinase, membranes and evidently a number of basic enzymes. The structural and enzymatic proteins are synthesized according to a genetic program. They are wrapped in membranes which may come either from vesicles of the Golgi complex or from the endoplasmic reticulum. The premelanosome, covered with a membrane, contains protein complexes forming a matrix composed of large numbers of concentrically organized protein lamellae.
The formation of pigment granules thus first of all requires polypeptide synthesis on the ribosomes and formation of the specific enzymatic protein, i. e. tyrosinase. In the second phase the tyrosinase molecules are organized in the form of the premelanosome and in the third phase the melanin itself is synthesized and accumulated; the premelanosome in this way becomes the melanosome.
Melanosomes are transferred from the dendritic processes to the surrounding cells by a "cytocrine" process. The keratinocytes evidently play an active part in this type of phagocytosis, since parts of the melanocyte dendrites, together with melanosomes, are taken up by their cytoplasm. These transferred melanosomes differ as regards their numbers and their organization and this influences the colour of the hair shaft. The melanosomes can be differently distributed in the hair medulla and cortex, they can be unequally distributed in these layers of the hair and they can also have different diameters (from 0.6 to 1.6 mm).
One important factor influencing hair colour is the way the melanosomes are organized in the cytoplasm of the keratinocytes. They may be non-aggregated, i. e. scattered singly in the cytoplasm, or they may form aggregates of two or more melanosomes localized in small membrane-bound cavities. Later the melanosomes disintegrate, leaving pigment granules scattered throughout the hair cells.
The most important factor deciding the final colour of the hair, however, is whether the melanocytes produce eumelanin or pheomelanin. Eumelanin — black pigment — has a high molecular weight and is insoluble in most solvents. Like eumelanin, pheomelanin - yellow pigment — contains nitrogen, but it also contains sulphur and its biosynthesis is a derivation of the process leading to eumelanin synthesis (for details see the chapter on pigmentation defects).
Hair pigmentation is primarily genetically controlled. Genetic studies in mice demonstrated that gene mutations determined the different colour variations of their coat and that every stage of melanogenesis was genetically controlled, from formation of the melanocytes, via formation of the melanosomes, to the transfer and distribution of melanins in the target cells.
In the anagen follicle the melanocytes are localized between the keratinocytes in the bulb above the papilla and their dendritic processes project into the intercellular spaces. Apart from their shape, they differ from the surrounding cells by having no desmosomes and by not containing any submicrotopic fibrils.The melanocyte nucleus is surrounded by mitochondria and large numbers of dense particles 10—20 nm in diameter are seated on the surface of the membranes of the endoplasmic reticulum. Melanosomes — ovoid or rounded granules about 1 mm in diameter — are present near the Golgi complex. Immature melanosomes (premelanosomes) are characterized by a longitudinally fibrous surface with periodic indentations. Eumelanosomes, which produce black pigment, are to be distinguished from pheomelanosomes, which produce yellow pigment. Pheomelanosomes are spherical and in the immature state their membranes contain small vesicles.
Melanosome formation is determined by the interaction of four basic components — proteins, tyrosinase, membranes and evidently a number of basic enzymes. The structural and enzymatic proteins are synthesized according to a genetic program. They are wrapped in membranes which may come either from vesicles of the Golgi complex or from the endoplasmic reticulum. The premelanosome, covered with a membrane, contains protein complexes forming a matrix composed of large numbers of concentrically organized protein lamellae.
The formation of pigment granules thus first of all requires polypeptide synthesis on the ribosomes and formation of the specific enzymatic protein, i. e. tyrosinase. In the second phase the tyrosinase molecules are organized in the form of the premelanosome and in the third phase the melanin itself is synthesized and accumulated; the premelanosome in this way becomes the melanosome.
Melanosomes are transferred from the dendritic processes to the surrounding cells by a "cytocrine" process. The keratinocytes evidently play an active part in this type of phagocytosis, since parts of the melanocyte dendrites, together with melanosomes, are taken up by their cytoplasm. These transferred melanosomes differ as regards their numbers and their organization and this influences the colour of the hair shaft. The melanosomes can be differently distributed in the hair medulla and cortex, they can be unequally distributed in these layers of the hair and they can also have different diameters (from 0.6 to 1.6 mm).
One important factor influencing hair colour is the way the melanosomes are organized in the cytoplasm of the keratinocytes. They may be non-aggregated, i. e. scattered singly in the cytoplasm, or they may form aggregates of two or more melanosomes localized in small membrane-bound cavities. Later the melanosomes disintegrate, leaving pigment granules scattered throughout the hair cells.
The most important factor deciding the final colour of the hair, however, is whether the melanocytes produce eumelanin or pheomelanin. Eumelanin — black pigment — has a high molecular weight and is insoluble in most solvents. Like eumelanin, pheomelanin - yellow pigment — contains nitrogen, but it also contains sulphur and its biosynthesis is a derivation of the process leading to eumelanin synthesis (for details see the chapter on pigmentation defects).
Hair pigmentation is primarily genetically controlled. Genetic studies in mice demonstrated that gene mutations determined the different colour variations of their coat and that every stage of melanogenesis was genetically controlled, from formation of the melanocytes, via formation of the melanosomes, to the transfer and distribution of melanins in the target cells.
No comments:
Post a Comment