Development of avascularity during cartilage differentiation in the embryonic limb. An exclusion model.

The differentiation of cartilage and muscle in limb-bud mesenchyme has been interpreted by some investigators in terms of a vascular pre-pattern model. It has been argued that a pre-pattern of the early limb vasculature compartmentalises the mesenchyme into specific microenvironmental areas in which, depending on the oxygen tension and nutrient supply, cartilage or muscle will differentiate. However, recent analyses of the development and differentiation of blood vessels in limbs have shown that regional variations in vascularization develop co-incidentally with the earliest indication of cartilage formation or mesenchymal condensation. The simple model described in the present study suggests that the mechanical compression/tension forces generated by the condensing mesenchyme are sufficient to constrict and eventually close off the thin-walled undifferentiated vessels caught in the condensation foci, thus leading to the avascularity of cartilage rudiments. This view suggests that the vasculature has no major function in governing the pattern of cartilage differentiation.