Histopathological study of lymphatic invasion in squamous cell carcinoma (O-1N) with high potential of lymph node metastasis.

The process of lymph node metastasis was studied in an animal model (termed O-1N) that was successfully established using a metastatic tumor to the submandibular lymph node from a chemically induced squamous cell carcinoma of the hamster tongue. The model has been maintained by serial transplantation of metastatic tumors into the buccal pouch. Lymphovascular invasion of transplanted O-1N in the tongue was examined in serial histologic sections. Lymphatic vessels were distinguished from blood vessels by Masson's trichrome stain for vascular smooth muscle, BSA-I lectin binding for vascular endothelium, and laminin and type IV collagen immunostaining for the vascular basement membrane. Transplanted tumors enlarged progressively with invasion of surrounding tissues of the tongue and resulted in lymph node metastasis in all animals with successful takes. Local growth of the tumors in the tongue was accompanied by stromal proliferation with abundant dilated lymphatic vessels which contained clusters of tumor cells. On serial sections, the carcinoma cell clusters in lymphatics in the close proximity of tumor nests were in continuity with adjacent tumor nests, whereas such continuity was not recognized in those occurring apart from tumor nests. The formation of isolated carcinoma cell clusters resulting from disintegration of elongated processes of tumor nests with invasion of lymphatics and subsequent transport in lymphatics and deposition in lymph nodes in clusters were well demonstrated in other serial sections. The key step of lymph node metastasis therefore appears to be direct invasion of lymphatic vessels by tumor cells, similar to their invasion of adjacent tissues but different from the way that blood cells escape through vessel walls. Proliferation of lymphatics around tumor nests and transport of tumor cells in clusters would also contribute to the production of metastatic deposits in lymph nodes.