Melanin deposition

Melanin

Melanin is an autogenous phenolic pigment. It gives color to the skin, hair, iris and choroid layer of eye. Melanin is formed from the amino acid tyrosine. A copper-containing enzyme tyrosinase is required.

The cells of intermediate lobe of pituitary gland produce α (α-MSH), β (β-MSH) and γ-melanocyte-stimulating hormones (γ-MSH). These MSHs stimulate the production and release of melanin (a process referred to as melanogenesis) by melanocytes.

Occurrence

Normal physiologic deposits occur in-

•      epidermis of skin- melanin exists as minute brown or black granules in the cytoplasm of cells in basal layer of epidermis, the stratum germinativum. Melanocytes transfer melanin to neighboring keratinocytes by way of their dendritic processes,

•      hair- melanin is derived from the epithelium of hair follicle,

•      iris and choroid layer of eye.

Melanin granules are large in dark hairs but smaller in lighter hairs. White rabbits, rats are true albinos (no pigments).

Pathologically, melanin occurs in-

•      melanotic melanoma,

•      melanosis,

•      acanthosis nigricans,

•      abnormalities of human skin, such as, freckles,

•      hyperpigmentation of skin associated with hyperadrenalism,

•      inherited disease in man (Dublin Johnson syndrome).

Melanosis is the deposition of melanin in various organs, especially-

•      lungs and

•      aorta, as brown or black spots of irregular shape.

Albinism is the pathologic absence of melanin due to inability of melanocytes to synthesize sufficient functionally active tyrosinase.

Inability to form melanin occurs in copper deficiency as copper being an essential component of tyrosinase.

Focal depigmentation may occur in scars and radiation burns.

Microscopically, melanin takes the form of minute rounded granules of light or dark brown color located in the cell cytoplasm. Melanin escapes from its intracellular position to extracellular for excretion through urine in case of uncontrolled production (malignant melanoma).

 Melanin is phagocytized by macrophage. When laden with melanin, these macrophages are called melanophores (melanophages).

The DOPA reaction can be used to distinguish melanocytes from melanophores. dopa is an acronym of ‘Dioxyphenylalanine’ an older equivalent of ‘Dihydroxyphenylalanine’.

If a section of fresh tissue is incubated with a solution of DOPA, melanocytes produce a black granular precipitate (identical to melanin) in their cytoplasm. The reaction is caused by the action of an enzyme termed DOPA-oxydase (tyrosinase) which is present in melanocytes.

Thus, the DOPA reaction is not a test for melanin but for the capability of cells to produce melanin. DOPA test must be done with frozen sections of unfixed or slightly fixed tissue.

One may test for the actual presence of melanin granules by using Fontana’s silver solution which turns such granules black.

Significance

•      Melanin itself is not harmful.

•      Melanotic melanomas, in which it may be extremely abundant, are often life-threatening.

•      Melanosis is not harmful.

•      In skin, the amount of pigment increases with increased exposure to sunlight (or artificial ultraviolet rays) and is regarded as a protective mechanism against ultraviolet damage to the skin.