Selective proliferation and accumulation of chondroprogenitor cells as the mode of action of biomechanical factors during secondary chondrogenesis.

Secondary cartilage fails to differentiate on membrane bones of embryonic chicks which have been paralyzed by the in ovo injection of D-tubocurarine chloride at ten days of incubation. A planimetric analysis of serial sections of a membrane bone (the quadratojugal) from control (mobile), and from paralyzed embryos, indicated that osteogenesis was not slowed in paralyzed embryos. However the rate of accumulation of periosteal progenitor cells was significantly lower in paralyzed than in mobile embryos. Quantitative analysis of 3H-thymidine-labelled progenitor cells indicated that the slowed accumulation of progenitor cells was the result of fewer progenitor cells initiating DNA synthesis and mitosis. Between 10 and 11 days of incubation, 60 to 75 more 3H-thymidine-labelled progenitor cells accumulated in mobile embryos than accumulated on each quadratojugal in paralyzed embryos. This subpopulation of cells could represent the chondroprogenitor cells which produce secondary cartilage in mobile embryos. If this is so, then biomechanical factors control the ability of the embryo to produce secondary cartilage by allowing the selective accumulation of chondrogenic progenitor cells.