Lack of lymphangiogenesis in human primary cutaneous melanoma. Consequences for the mechanism of lymphatic dissemination.

Cutaneous melanoma has an initial preference for lymphatic spread. Remarkably, melanoma progression toward this metastasizing phenotype is accompanied by intense blood vessel angiogenesis (hemangiogenesis), but lymphangiogenesis, the formation of new lymph vessels in the tumor, has never been reported. To investigate how primary melanoma cells interact with the existing lymphatic microvasculature, and whether lymphangiogenesis occurs, an immunostaining was developed that differentially decorates blood and lymph vessels in frozen tissue sections. The density and distribution of both these vessel types in and around thin ( or = 1.5 mm) primary melanoma lesions and in normal and uninvolved skin were determined. Although especially in thick melanoma lesions a significant increase in blood vessel density was observed, lymphatic density remained unaltered, showing that lymphangiogenesis did not occur. Morphological analysis indicated, however, that melanoma progression is accompanied by a sequence of events that involves hemangiogenesis supporting tumor expansion, especially in the vertical growth phase. Often, stromal sepia are formed around the blood capillaries in the tumor neovasculature protecting them from invasion. Lymph vessels inside the tumor were infrequently observed. However, subepidermal lymph vessels often seemed to be entrapped and penetrated by the expanding tumor mass. In this way, hemangiogenesis, as the driving force behind tumor expansion, might indirectly increase the chance of lymphatic invasion in the absence of lymphangiogenesis. This model explains the paradox that, although melanoma metastasis seems to require angiogenesis, a consistent relation of prognosis with blood capillary density in primary cutaneous melanoma is lacking.