The control of muscle and cartilage development in the chick limb: the role of differential vascularization.

The results of previous studies on the effects of nicotinamide levels on chick limb mesodermal cell expression have indicated that there may exist a metabolic gradient situated radially across the limb which has a controlling influence on whether a limb cell will differentiate into either a myogenic or chondrogenic phenotype. This study investigates the possible role of the vascular pattern in establishing such a metabolic gradient. Observations are reported which show that prospective myogenic and chondrogenic areas become differentially vascularized before the onset of molecular differentiation, thus indicating that the vascular pattern is capable of establishing metabolic gradients across the limb. Furthermore, predictions based on these results have been tested and verified in vitro. The hypothesis is presented that the spatial differentiation of cartilage and muscle in the developing chick limb is under the controlling influence of gradients of metabolic potential which can be established by the differential vascularization of the limb.