[Perivascular and osteogenic cells in periosteal ontogenesis].

The structure of the perivascular cells, their interrelations with the vascular wall and differentiating osteoblasts has been studied in the white rat and rabbit fetuses and newborns during periosteal osteogenesis by means of electron microscopic radioautography using 3H-thymidine. The blood bed in the perichondrium in the zones, where osteogenesis is developing, has a capillary-sinusoid type of structure. Among the perivascular cells, poorly differentiated cells, adjoining the endothelium and intensively incorporating 3H-thymidine, are revealed, as well as the cells that are territorially isolating from the endothelium and demonstrating certain signs of their differentiation into the osteogenic ones. This supports the previously obtained data on historadioautography with 3H-thymidine that during the periosteal osteogenesis a successive differentiation of the perivascular cells (via the periosteoblast stage) into osteoblasts takes place. The process is accompanied by a decreased proliferative cell activity, at the expense of a prolonged time of generation and a progressive yield of cells out of the mitotic cycle. The poorly differentiated perivascular cells are considered as induceable ones, and the preosteoblasts--as the determined osteogenic precursor-cells. The cells growing into the perichondrium with capillaries, when the periosteal ontogenesis develops, represent a peculiar population of cells with polypotent properties, which participate in genesis of osteoblasts and periocytes.