Sub-populations of melanocytes in pigmented basal cell carcinoma: a quantitative, ultrastructural investigation

Pigmentation is a characteristic clinical feature of basal cell carcinomas (BCCs) in Japanese patients. The pathogenesis of melanin pigment in pigmented BCCs is poorly understood.

We have combined the techniques of morphometric analysis and electron microscopy to assess accurately the morphologic aspects of melanocytes that occurred in pigmented and non-pigmented areas of pigmented BCCs.

In the pigmented areas melanocytes were not only located along the basal membrane but also interspersed between tumor cells in the central parts of the tumor nest, and had large and numerous dendrites. Those in a supra-basal location displayed some degree of degeneration due to mitochondrion and melanosome swelling. In the non-pigmented areas melanocytes were only basally located, showed fewer dendrites, and frequently showed abortive melanosomes. However, melanocytes in these two different portions were in the active state of melanogenesis and proliferation. Ultrastructural cytomorphometric analysis also showed significant differences in most of the nuclear and cell parameters including nuclear and cell area, the nuclear/cell area ratio, cell perimeter and cell form factor between these two types of melanocytes. Particularly melanocytes in the pigmented areas were twice the cell size of the latter. In addition, the melanosomes remained almost completely in the apoptotic tumor cells, and the phagocytosis of the melanosome-containing apoptotic cells by the neighboring tumor cells appeared to be followed by the formation of the melanosome complexes.

These findings suggest that different populations of melanocytes are probably present in pigmented BCCs, and repeated cycles of phagocytosis of melanosome-containing apoptotic cells may represent the predominant way of forming large melanosome complexes. The present morphological observation and quantitative analysis provide a morphological basis for further studies to interpret other pathologic changes in pigmented BCCs.