Melanocytic neoplasms

According to the WHO classification (1995), the following types of melanocytic nevi are distinguished: borderline; complex (mixed); intradermal; epithelioid and/or spindle cell; balloon cell nevus; halo nevus; giant pigmented nevus; fibrous papule of the nose (involutional nevus); blue nevus; cellular blue nevus.

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Common acquired melanocytic nevi

Common acquired melanocytic nevi (syn.: pigment nevi, moles) are benign melanocytic neoplasms. Each nevus goes through several stages in its development. At first, it is a borderline, intraepidermal nevus, which looks like a uniformly pigmented spot of brown or black color with a smooth surface, is not palpable. It usually appears in children in the first years of life, but is especially active in adolescence. Most often it is located on open areas of the body - the face and trunk. It gradually increases in size, but, as a rule, the diameter at this stage does not exceed 5 mm. Over time, the nevus becomes epidermo-dermal, or mixed, while it is much more convex, on average does not exceed 7 mm, is a papule with clear contours or a papillomatous formation on a leg, but is less intensely pigmented - it can be light brown and even flesh-colored. The surface of the nevus is smooth or warty. Being an exophytic formation, the mixed nevus is susceptible to injury, and with its active growth, itching may be felt. Then the mixed nevus turns into an intradermal nevus, which also rises above the skin surface, although it can flatten and sometimes completely loses pigment, acquiring a flesh-colored or pinkish color, especially in the case of a pronounced vascular component. The subsequent development of the dermal nevus takes from several years to several decades, until its involution.

In rare cases, spontaneous immunologically mediated regression of melanocytic nevi with the development of perinevus vitiligo (see Halonevus) is possible.

Pathomorphology

In borderline nevus, nests of nevomelanocytes are found in the epidermis at the dermal-epidermal junction, especially at the ends of epidermal outgrowths. Lentiginous proliferation of melanocytes is possible. Nevus cells are usually larger than normal melanocytes, with round or, much less frequently, elongated nuclei, small basophilic nucleoli, with abundant light cytoplasm, which may contain a large amount of pigment. A refractive artifact is observed, due to which the melanocytes of the nevus do not adhere to the surrounding keratinocytes. In cases where the nevus is heavily pigmented, elimination of melanin through the overlying layers of the epidermis is determined.

At the stage of mixed nevus, nevomelanocytes migrated into the dermis more or less uniformly over the entire area of the nevus, are determined as nests mainly in the papillary layer of the dermis. Mixed nevus is a symmetrical formation, clearly delimited from the surrounding tissues, the epidermal component of the nevus does not extend beyond the dermal.

A sign of benignity is the so-called maturation of the nevus from the outside inward. Some authors distinguish three types of cells in the histological architectonics of a mixed nevus: type A cells, relatively large, located in the upper parts of the dermis, have a cuboid shape, abundant cytoplasm containing varying amounts of pigment. Type B cells, located in the middle parts of the dermis, are significantly smaller in size compared to type A cells, do not contain melanin, and are located in the form of aggregates. Type C cells, located in the lower parts of the dermis, have a spindle-shaped shape, are located in the form of bundles and strands, separated by layers of connective tissue (neurotized nevus).

At the stage of dermal nevus, nevomelanocytes are found only in the dermis, mainly type B and C cells. In long-existing dermal nevi and during their involution, the following changes are found: fibromatous, angiomatous, neuromatous, replacement of nevus cells by mature adipose tissue. Often in papillomatous nevi, so-called pseudovascular cavernous spaces are found, sometimes with the presence of giant multinucleated cells in them.

At all stages of the evolution of common acquired melanocytic nevi, cellular atypia, mitosis, necrosis, and lymphocytic reaction are not characteristic.

Immunomorphological examination reveals a positive reaction to the S-100 antigen in most nevi cells.

Histogenesis

According to modern concepts, the fact of preferential, albeit temporary, growth of nevus cells in relation to surrounding tissues testifies in favor of their assessment as a neoplasm, and not a developmental defect. Nevus cells differ from normal melanocytes by the absence of processes, the ability to group into "nests" (groups of 3-5 cells or more) and accumulate pigment in the cytoplasm, as well as a tendency to migrate from the basal layer of the epidermis to the dermis.

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Fibrous nasal papule

Fibrous papule of the nose (syn.: involutional nevus) is a solitary formation of flesh color, protruding above the surface of the skin, hemispherical in shape, usually not exceeding 5 mm in diameter. It can be located on any area of the skin of the face, but mainly on the skin of the nose, sometimes bleeding after a minor injury.

Pathomorphology

The histological picture is very similar to angiofibroma. The dermis is fibrotic and hyalinized, a large number of vessels with dilated lumens are revealed. The stroma is characterized by the presence of many fibroblast-like and stellate cells, sometimes multinucleated cells are encountered. In the epidermis, there is often lentiginous proliferation of melanocytes.

Histogenesis

Previously, fibrous papule of the nose was considered a fibrous nevus and is still mentioned in the WHO classification in the group of melanocytic nevi. However, the data of electron microscopic and immunohistochemical studies refute this opinion. Currently, the formation is considered to be a consequence of reactive proliferation of dermal dendrocytes based on the detection of spindle-shaped and stellate cells in the dermis, in which factor XIIIa and vimentin are detected.

Halonevus

Haloneus (syn. nevus of Setton) is clinically a pigmented nevus surrounded by a zone of depigmentation in the form of a halo, most often developing on the skin of the back in children and young people. It can spontaneously regress, leaving behind a depigmented area.

Pathomorphology

In halo nevi, depending on the stage of the process of nevus destruction by lymphoid elements (mainly CD8+ T-lymphocytes), either a continuous lymphocytic infiltrate in the dermis without signs of melanocytic proliferation is detected, or single nevomelanocytes or their clusters in the epidermis and dermis are determined, sometimes with the presence of pronounced cellular atypism. In the surrounding epidermis, corresponding to the depigmented crown around the nevus, destruction of melanocytes of the basal layer is detected.

Histogenesis

In patients with regressing halo nevus, the circulation of antibodies to malignant melanoma cells has been noted. Direct immunofluorescence of halo nevus cells with the serum of patients with halo nevus or malignant melanoma has also been noted. Electron microscopy has shown that all nevus cells within the infiltrate are destroyed. Some lymphocytes are transformed into plasma cells. The data of O. Stegmaier et al. (1969) indicate that nevus cells produce antigen and that infiltrate lymphocytes secrete antibodies against these cells, destroying them.

Balloon cell nevus

Balloon cell nevus is extremely rare, usually occurs in young people. Clinically, it is not different from a normal melanocytic nevus.

Pathomorphology

Nevus cells have a light cytoplasm, and their size is usually increased by 10 times compared to normal cells, the nuclei do not differ from the nuclei of nevus cells and are located either in the center or displaced; multinuclear balloon cells are found. The number of balloon-shaped cells varies, they can be only a component of intradermal, as well as mixed nevi, or there are very many of them. In the first case, balloon-shaped cells are located in the form of a node or scattered among normal nevus cells. In the second case, they are located alveolarly or in the form of foci in which they do not have cellular boundaries.

Histogenesis

According to some data, the formation of balloon-shaped cells is a consequence of dystrophic processes in the nevus cells.