Melanin-producing cells (melaninocytes) are of neurogeneous origin. They may become the origin of tumor-like formations (nevi) and melanomas. Nevi are benign tumors of skin consisting of melanocytes of epidermis and derma. Neurogeneous origin of melanocytes is generally recognized. Nevi are defects of development of neuroectodermal pigment elements. They look like brown spots of different size, and may

be either flat or elevated over the surface or be wartlike. Sometimes their size is enormous (giant pigmented nevus).

According to the WHO classification (1974), there are the following types of nevi: 1) junction nevus, 2) compound nevus, 3) intradermal, 4) epithelioid nevus (intracellular), 5) balloon-cell nevus, 6) halo-nevus, 7) giant pigmented nevus, 8) involution nevus (fibrous papule of the nose), 9) blue nevus, 10) cellular blue nevus.

Junction nevus. Nests of nevus cells are found on the border of epidermis and dermis. The nests are round or oval. Their cytoplasm is homogeneous, slightly granular. The nevus cells are localized in the area of reticular layer apices.

Compound nevus. Together with the nevus cells located on the border of dermis and epidermis, there are nests of nevus cells in derma itself.

Intradermal nevus. Nevus cells are located only in derma. Some of them can be found on the border between derma and epidermis. They resemble nests. The nevus cells look like compact mass. The cells in mature nevi may be polynuclear. Macroscopically they have papillomatous appearance and may contain hairs.

Epithelioid nevus can often appear on the face, especially in children. It looks like flat or ball-like node. The surface of the skin is smooth, sometimes papillomatous changes are observed. Microscopically it looks like compound nevus with borderline changes.

Sometimes marked acanthosis is present. The amount of melanin is small, it may also be absent. The cells have light basophilic cytoplasm and hyperchromic nuclei. Epithelioid cells with large foamy light cytoplasm may be present. Mitoses are not numerous. Uni- or polynuclear cells resemble Touton's cells. There are a lot of vessels.

Blue nevus. Macroscopically this looks like bluish or bluish-brown or bluish-gray sport, its shape is round or oval, it does not elevate over the surface of the skin. Microscopic examination reveals stretched melanocytes.

Melanoma. In the case of malignant melanoma, at the age of 20, only one person per 300 000 (0.3 per 100 000) has the cancer, and at the age of 80 about 30 per 300 000 (10 per 100 000) have it. The numbers of skin cancers rise with age because the main cause of all types of skin cancers is sunlight exposure. Sunlight contains ultraviolet light (UV), and this is what does the harm, particularly to the skin of babies and young children. The numbers of skin cancers vary from country to country. In tropical countries with large white populations, the numbers are proportional to the amount of sunlight. Australia, South Africa and the Southern American states all have a very high incidence of skin cancer in their white populations. Black people are better protected by their skin colouring.

Melanoma is one of the most malignant tumors, it spreads through the lymphatic and hematogenic routs. 70% of melanomas develop on the skin of the face, body and extremities.

Two kinds of melanoma are known.

1. Melanoma against a background of pigmented Hutchinson's sport (freckles) or malignant lentigo.

2. Superficially disseminated melanoma (invasive melanoma, nodular melanoma). Melanomas may not contain pigments. In the tumor, there are a lot of mitoses, hemorrhages and necroses. At the tumor decomposition, a great amount of melanin and chromelanin enter the bloodstream, which is accompanied by melaninemia and melaninuria. The tumors are localized on the skin, pigment membrane of the eye, meninges, medullar layer of adrenal glands, in rare cases mucous membranes.

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