A comparative ultrastructural study of cutaneous blue naevi of humans and hamsters.

A comparative study of 12 human blue naevi and 15 carcinogen-induced hamster blue naevi showed no significant ultrastructural difference between these two groups of tumours. Both groups of tumours showed melanotic and hypomelanotic variants, and both were composed almost entirely of cells acceptable as melanocytes and melanophages. However, a few cells in some human and hamster tumours were partially or completely surrounded by a slender or quite thick external lamina. This may indicate a tendency towards a schwannocytic type of differentiation, but no other feature of schwannocytic differentiation such as the formation of mesaxons or pseudomesaxons was detected. Several cutaneous nerves were embedded in some of these tumours but they were clearly uninvolved in the neoplastic process and there was no evidence whatsoever that the tumours had developed from neural elements. We conclude that both human and hamster blue naevi are essentially melanocytomas which develop from dermal melanocytes which failed to reach the epidermis during development. We regard the occasional occurrence of an external lamina as an aberration of the neoplastic state which reflects the close kinship of melanocytes and Schwann cells i.e. their common origin from the neural crest.