Morphological Characterization of Hemangiomatous Tumors Derived from a Novel Murine Vascular Endothelial Cell Line (F‐2)

Hemangiomatous tumors were induced in Balb/c nude mice by inoculating F‐2 cells (5 times 106) from a novel tumorigenic murine endothelial cell line which had been established and maintained in our laboratories. These tumors were morphologically investigated in the course of development. Subcutaneous hemorrhage was observed at the inoculation site within 12 hours after the injection of F‐2 cells, followed by development of skin tumors of various sizes at the same sites. They were dome‐shaped, glossy surfaced, black, soft tumors. Mice finally died of massive blood loss due to internal and/or external hemorrhage (on day 10–77). Light microscopically, F‐2 cells formed aggregates immediately after inoculation and then branched into a network of channels and cysts containing erythrocytes. Thereafter, spongiform structures composed of various sizes of cysts appeared with subsequent formation of a single large blood‐filled cyst lined by one or two layers of thin cells. Under an electron microscope, F‐2 cells, possessing large amounts of cytoplasm, formed narrow spaces which were occasionally incomplete with ambiguous basal lamina at the early stages. However, later, in large cysts, they became attenuated, tightly connected, and produced complete lumen surrounded by a basal lamina. Immuno‐histological demonstration of H‐2 K, D antigen showed that tumors induced by F‐2 cells mainly consisted of the inoculated F‐2 cells. These results indicate that F‐2 presents a good experimental system for investigation of vascular endothelial cell tumorigenesis and differentiation.