Cutaneous melanocytes and their tumours
Melanocytic naevi of the skin are benign neoplasms of cutaneous melanocytes. Normal melanocytes are present at the dermoepidermal junction and the hair follicle, and small numbers are found in the dermis. The latter play an important role in skin colour of poikilotherms, but have become sparse and genetrally nonpigmented in homoiotherms such as mammals, including man. The idea that dermal melanocytes are normally absent, and that their presence indicates some pathological 'migratory arrest' of precursor melanoblasts normally destined to reach the epidermis, is obsolete.
It is tempting to speculate that cutaneous melanoses and blue naevi arise from dermal melanocytes, but this is, of course, unproven. The two main populations of cutaneous melanocytes - of the epidermis and hair follicle - probably give rise to most common naevus subtypes. Naevi of non-hair-bearing skin (such as volar skin) probably arise from epidermal melanocytes. With regard to naevi of hair bearing skin, it is unclear, where they originate. Many very small naevi presenting as flat hyperpigmented macules, are entirely located at the dermoepidermal junction, so that the junctional melanocytes normally situated in that location are the most likely cells of origin. However, small, usually nonpigmented dermal naevi are exceedingly common and are generally unnoticed, even in histoloigcal slides from excisions performed for some other lesion). The idea that all of such small dermal naevi have previously gone through a junctional and a compound phase is entirely conjectural. Finally, the contour of many compound and dermal naevi, with presence of naevus cells along much of the depth of hair follicles resulting in a shaply pointed wedge shape, might point to a closer relation between hair follicles and common naevi than is often assumed. 'Abtropfung' of naevus cells from the dermoepidermal junction into the dermis probably does play some role in the morphogenesis of a compound or intradermal naevus, but is unlikely to be the whole story. In fact, it is fair to say that cutaneous melanocytic naevi and melanomas orginiate form skin melanocytes, but that the morphogenesis and the sites of the melanocyte of origin have largely remained unclear.