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Inverted Type A Nevus - Submitted on: Tuesday December, 04, 2001
Contributed by:Sam Soheil Dadras, M.D., Ph.D., Artur Zembowicz, M.D., Ph.D.
Classification of melanocytic nevi is important in their differentiation from melanoma. Inverted type A nevus is an important variant, as it can, and hsa been in the past, confused with malignancy.

Definition:
Inverted type A nevus is a distinctive type of melanocytic nevus. The dermal component of which is formed, at least in part, by type A melanocytes. Some deep penetrating (1) and plexiform spindle cell nevi (2) may be considered variants of inverted type A nevi. Inverted type A nevus can also be a component of a combined nevus (3).

Clinical Features:
Inverted type A nevi are found on the face, upper trunk, proximal extremities, and vulva of patients between the
ages of 11 and 81 years. They are typicall darkly pigmented macules ranging from 2 to 7 mm. or may be tan with a focal blue area.

Histology:
Figures
(Click on an image for a larger view)
Figure 1. Nodular pigmented lesion with intraepidermal nests and dermal component composed of type A melanocytes infiltrating all the way into the deep reticular dermis toward subcutaneous fat along a hair follicle in the left labia of a 34 year-old woma
Figure 2. Wedge-shaped pigmented lesion with dermal component infiltrating all the way into deep reticular dermis and dissecting through the skeletal muscle in the right lower eye lid of a 30 year-old pregnant woman.
Figure 3. . The epitheloid melanocyte or type A nevus cell contains a round-to-oval nucleus of a size slightly smaller than the nuclei of adjacent keratinocytes. The nucleus contains finely dispersed chromatin and occasionally a single faintly eosinophil
At low magnification, inverted type A nevi are nodular or wedge-shaped. They sometimes have extensions pointing toward the subcutaneous fat (Fig 1) or, in some anatomical sites such as the face, skeletal muscle (Fig. 2). At least a portion of the dermal component of inverted type A nevus is formed by epithelioid meelanocytes or type A nevus cells. In contrast in common nevi, type A mmelanocytes are confined to the junctional or superficial dermal component. Type A melanocytes contain round-to-oval nuclei of a size slightly smaller than the nuclei of adjacent keratinocytes (Fig. 3). The nuclei contain finely dispersed chromatin similar to neuroendocrine cells and a single faintly eosinophilic macro-nucleolus. They have ample gray-blue cytoplasm and coarse or fine melanin granules (4). In inverted type A nevus, type A cells are seen in the deep dermis arranged as single cells or in nests of large pigmented epithleioid cells interspersed with melanophages (Fig 1). They often surround skin appendages and extend along neurovascular bundles infiltrating deep dermis (Fig.2). Although slight nuclear atypia is very characteristic, mitoses are absent or very rare. Some lesions have mild to moderate lymphocytic infiltration.


Treatment:
Complete excision is the treatment of choice for inverted type nevus.

Bibliography:
  • Seab JA, Graham JH, and Helwig EB (1989) Deep Penetrating nevus. The American Journal of Surgical Pathology 13(1):39-44.
  • Barnhill RL, Mihm MC, Magro CM (1991) Plexiform spindle cell nevus. Histopathology 18, 243-247.
  • Pulitzer DR, Martin PC, Cohen AP, and Reed RJ (1991). Histologic classification of the combined nevus. The American Journal of Surgical Pathology 15(12):1111-1122.
  • Imber MJ and Mihm MC. (1999) Melanocytic lesions, chapter 3 in Diagnostic Surgical Pathology, eds Sternberg. Lippincott Williams & Wilkins.
     

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