Female, 19 years of age. Verrucous hyperpigmented papule, skin of back, removed by shave biopsy.Show diagnosis & comments
Papillomatous melanocytic compound naevus
Melanocytic naevus with an intraepidermal (junctional) and an iintradermal component
Junctional component may have a lentiginous, a nested, or a combined lentiginous and nested pattern
Dermal component may have nested pattern superficially, but more disperse architecure usually predominates, especially in deeper parts
Superficial dermal melanocytes are commonly (but not always) slightly larger than junctional ones
Towards the basal part, intradermal melanocytes become progressively smaller, loose whatever pigment they may possess in superficial parts, and mitotic activity is absent. This combinatioon of features is calles 'maturation', which is a misnomer because is bears no relationship to terminal differentiation, which that term suggests. 'Maturation' is an important diagnostic clue in the differential diagnosis between naevi and melanomas, but it is notoriously absent from blue naevus and its variants, whereas melanomas may show smaller cells in deep parts (see 'pseudomaturation'), so that the specific differential diagnosis at hand decides on the applicability of 'maturation' as an indicator of benignancy, and there are pitfalls.
Superficial dermal cells are usually plump and rounded or oval. More deeply in the dermal component, cells are smaller and rounded, whereas in some naevi, the most basal portion consists of more elongated, slightly neuroid cells. These dermal cell types from top to bottom are sometimes referred to as Miescher type A, B and C cells, respectively.
All in all, compound naevi tend to show a regular architecture, with similar cellulkar featiures and organisation from left to right at the same level of depth. This contrasts with many - but not all! - melanomas, which tend to be more variable 'from within'.
Some naevi tend to growt in an exophytic pattern (like this one), with dermal cells accumulating in the expanded upper dermis (Unna naevus), whereas others tend to growth more infiltratively into deeper parts of the dermis (Miescher naevus).