The ultrastructure of congenital naevocytic naevi. III. Morphological variability of melanosomes.

In congenital naevocytic naevi (CNN), the ultrastructure of melanosomes has been demonstrated only in a few case reports. In the present study, five defined skin levels were systematically compared by electron microscopy in five giant and seven medium-sized CNN. The ages of the patients ranged from 5 days to 29 years. Six common acquired naevocytic naevi (ANN) served as controls. Melanosomes were classified morphologically into several groups. Epidermal melanosomes were normal in all cases. In 10 of the 12 patients, the morphology and size of dermal melanosomes corresponded well with those in ANN, although in four young children (up to 4-years-old) a few enlarged or atypical forms were found. On the other hand, two new-born babies showed rather different, grossly enlarged dermal melanosomes of various shapes and some unusual internal features. They were seen focally even in lower dermal layers and in naevus cells in subepidermal sweat ducts. We conclude that dermal naevus cells in CNN of new-borns, at least in some cases, may contain melanosomes with unusual morphological variations. This does not necessarily indicate malignant transformation.