In all of the cases reviewed, there were concerning histological features including the prominence of the junctional lentiginous melanocytic pattern and the presence of occasional benign-looking nevomelanocytes in the middle reaches of the epidermis (acral, and flexural sites), enlargement, crowdedness, dyshesiveness of the junctional and dermal neveomelanocytic nests with variability in their size, shape, and alignments (NOSS at all sites). The nevomelanocytic cells showed mild cytological atypia (nucleomegally, vesicular nuclei, small nucleoli, clear cytoplasm, and fine melanin pigment). Within the dermis, host response in the form of fibroplasia, melanophages and mild lymphocytic infiltrate were seen (genital and acral nevi). Representative cases are shown in Figs. 1–3.

Figure 1.

Immunohistological features of melanocytic nevus of the umbilicus with site-related atypia. A-D: A shave biopsy of the umbilical skin shows a small well-circumscribed, symmetric compound melanocytic nevus. The junctional growth shows the lentiginous and horizontally confluent nested pattern. Within the dermis, there are maturing nevomelanocytic nests, fibroplasia, and some melanophages. E: Melan-A strongly and diffusely stains both the junctional and dermal nevomelanocytes. F: A patchy HMB45 staining is seen in the lentiginous, and the nested nevomelanocytes along the dermoepidermal junction. Rare HMB45 positive cells are seen in the dermal nests. HMB45 also stains occasional solitary nevomelanocytes in the epidermis. (Original magnifications: A: 20x, B: x200, C: x200, D: x 400, E: x100, and F: 200).

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Figure 2.

Immunohistological features of melanocytic nevus of the posterior axillary line with site-related atypia: A-D: There is a symmetric compound melanocytic nevus with a predominantly nested pattern and some solitary nevomelanocytes at the dermoepidermal junction. There are horizontally confluent, junctional nests of nevomelanocytes that mature with their descent into the dermis. The nevomelanocytes are dyshesive, enlarged, and have clear cytoplasm with fine melanin pigment. E: A diffuse HMB45 reactivity is seen in the junctional melanocytic growth as well as in nests of the superficial dermis. F: Ki-67 staining is seen both in the junctional keratinocytesand rare nevomelanocytes. (Original magnifications: A: 20x, B: x100, C: x200, D: x 200, E: x200, and F: 200).

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Figure 3.

Immunohistological features of acral melanocytic nevus of the right big toe with (Melanocytic Acral Nevi with Intraepithelial Ascent of Cells, MANIAC): A-C: Sections show acral skin with a small well-circumscribed, symmetric compound melanocytic nevus. There is no confluence of nests. The nevomelanocytes lack any significant cytological atypia. Within the dermis, there are fibroplasia and few melanophages. D-E: Melan-A decorates the junctional and dermal nevomelanocytes. It shows a lentiginous pattern with the upward pagetoid migration of some nevomelanocytes (lacking cytological atypia) arranged as solitary units in the spinous layer (MANIAC). F: A patchy HMB45 staining is seen in the lentiginous nevomelanocytes along the dermoepidermal junction. HMB45 also stains occasional solitary nevomelanocytes in the lower reaches of the epidermis. Almost all of the dermal nevomelanocytes lack HMB45 protein expression, i.e immunohistochemical maturation. (Original magnifications: A: 20x, B: x200, C: x200, D: x 400, E: x200, and F: 400).

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In all cases reviewed, there was a strong and diffuse expression of S100 and Melan-A proteins throughout the entire lesion (percentage of positive cells ranged from 91% to 89.5%). There was a strongly expressed HMB45 protein in the junctional component (lentiginous and nested nevomelanocytes). Within the dermis, HMB45 protein expression decreased, following a gradient from junctional to deeper dermal localization being decreased (occasional positive nevomelanocytes) in the mid dermal portion and nearly completely lost in the deep dermal portions of the nevi, indicating immunohistochemical maturation (a feature favoring of benignancy) of these lesions. The mean value of the percentage of HMB45 positive cells was 39.6 ± 4.5. HMB45 protein expression values were high in the acral and head and neck nevi. The mean values of the percentage of S100 or Melan-A positive cells were significantly higher as compared to those of HMB45 protein (Mann-Whitney U test, p < 0.0001). There was no correlation between the HMB45 protein expression values and the clinical features of the lesions (age and gender of the patient, or site, size of the lesions). Ki-67 labeling index was extremely low in NOSS (Ki-67 mean labeling index was 1.1 ± 0.1, a feature favoring of benignancy). A summary of these findings is shown in Table 2 and Figs. 1–3. A summary of these findings is shown in Table 2 and Figs. 1–3.

The histological characteristics of NOSS in this series concur with previous studies.1–5 Despite these worrisome morphological changes, histological features favoring benignancy included the presence of symmetry, circumscription, the predominance of the nested over the lentiginous component, dermal maturation, and lack of notable upward scatter of the nevomelanocytes or significant mitotic activity.4,6 In NOSS, the possible pathogenetic mechanisms underlying the development of alarming histological features (large, crowded, dyshesive nests, and low-level intraepidermal pagetoid spread) include the underlying site-specific hormonal effects10 (NOSS of the milk line or in the genital sites), site-related repetitive traumas (NOSS of the acral and head and neck regions), site related-UV irradiation (NOSS of the head and neck regions),11 alteration of the melanocytic–keratinocytic interactions,10 and some mutational changes.

The dyshesive nested pattern in NOSS, may be due to downregulation or loss of cell adhesion molecules. The alarming histological futures of NOSS in the head and neck regions may be due to the effects of UV irradiation. In support, UV irradiation can induce morphological changes reminiscent of melanoma by enhancing the proliferative and metabolic activities of the nevomelanocytes.11

The presence of low-level pagetoid spread (in acral and genital nevi) may represent “a passive ‘passenger’ mechanism via the maturing keratinocytic flow”12,13 or reflect alteration of the melanocytic–keratinocytic interactions.10 The pagetoid single cell spread of the nevomelanocytes into the spinous layer (pagetoid melanocytosis) can be marked in some acral nevi, also known as MANIAC (Melanocytic Acral Nevi with Intraepithelial Ascent of Cells). These nevi are separated from acral melanomas by several clues including the fact that these pagetoid nevomelanocytes lack cytological atypia and they do not spread beyond the center of the lesion. MANIAC generally shows symmetry, circumscription, and maturation of the lesions with descending into the dermis.14,15

This study reports strong HMB45 protein expressionin the junctional component of NOSS, indicating that these nevi focally have an activated phenotype with active melanosome formation. The mechanisms underlying this activated phenotype include the release of melanocytic growth factors.16 This study also revealed a gradual decrease in the HMB45 protein expression, following a gradient from junctional to deeper dermal localization in NOSS, i.e. immunohistochemical maturation. The HMB-45 reactivitycan attest to the presence or absence of ‘‘immunohistochemical maturation’’ of a given melanocytic lesion. The activated junctional or superficial, type A melanocytes (epithelioid cells) express HMB45, while the deeply located type C melanocytes (spindle cells) do not express this antibody.17–19 In this author's view, the extremely low Ki-67 labeling index together with immunohistochemical maturation are important features that favor the benignancy of a given melanocytic lesion (at the special sites) with alarming histological features. The decreasing staining HMB45 pattern, as well as the lack of Ki-67 in the deep parts of the lesional cells of NOSS, has previously been successfully studied as a diagnostic clue of nevus versus melanoma.20–23

To conclude, the diagnosis of NOSS should be considered in any nevomelanocytic proliferation occurring at particular sites (head and neck region, milk line, genitalia, perineum, conjunctiva and acral sites. The expression of HMB45 protein in NOSS indicates that their nevomelanocytic cells have an activated phenotype. The decreased HMB45 protein expression following a gradient from junctional to deeper dermal localization in NOSS is indicative of their immunohistochemical maturation.