[Cartilage differentiation in the limb bud of the chick embryo. Ultrastructural observations, culture and grafting experiments].

Differentiation of cartilage from mesodermal cells of the chick embryo wing bud is controlled by the ectoderm. Transfilter cultures have shown that this interaction cannot take place at a distance but requires contact conditions as established between mesodermal cells outgrowths and the ectodermal basement membrane in vivo and in vitro. Induction does not pass from one cell to another: when cultured in vitro cartilage differentiating cells do not provoke chondrogenic differentiation of mesodermal cells incapable of autonomous differentiation. During in vitro culture, cartilage differentiation of limb mesodermal cells is obtained in the absence of ectoderm at stage 17 (H. and H.) and rarely at stages 15-16. Is the inductive contact established at these stages for all the cells concerned or is it a continuous phenomenon which lasts as long as condensation formation proceeds? Ultrastructural studies of the space between ectoderm and mesoderm show that at stage 13 (18 to 20 somites), all limb somatopleural cells are capable of establishing a contact with the ectodermal basement membrane. But numerous contacts are also observed up to late stages, when condensations develop. However, at the stages and at the levels of formation of precartilaginous condensations, no movement of cells is observed from the external mesodermal layers towards the condensations, in chimeric quail-chick limbs. It seems therefore that the inductive contact takes place early (24-32 somites, stages 15 to 17 H. and H.), possibly a little later, but long before the formation of precartilaginous condensations (stages 23 to 30 H. and H.). The time between inductive contact and differentiation of the cell differs greatly therefore according to the final localization of the chondrocyte among the proximo-distal axis. This conclusions is discussed in relation to the Progress Zone (P.Z) concept.