Role of mesenchymal cells in the neovascularization of the rabbit phallus.

The neovascularization of the rabbit phallus at ages between prenatal days 15 and 21 was investigated by light- and electron microscopy, computer-aided light microscopic reconstruction, and immunocytochemistry. The phalli are embedded by an abundance of mesenchymal cells, which are in contact with the neighboring ones or with the endothelial lining of growing capillaries. They often form solid cell cords that eventually make contact with the growing capillaries. The computer-aided reconstruction of the serial light micrographs reveals that these cell cords are involved in connecting the adjacent capillaries. The incorporation of such mesenchymal cell projections into the endothelial lining, occasionally conjugated with simple attachment devices, is frequently observed by transmission and scanning electron microscopy. The contact areas between the mesenchymal and endothelial cells show immunoreactions of fibronectin. These results indicate the successive transformation of mesenchymal cells to endothelial cells of the growing capillaries. As endothelial cells of the growing capillaries show mitotic proliferation, such vasoformative mesenchymal cells seem to be involved in the acceleration of the neocapillarization of the rabbit phallus. Fibronectin actively produced in the mesenchymal cells may participate in their migration and the mechanical linkage with the endothelial cells.