Embryonic origin and fate of chondroid tissue and secondary cartilages in the avian skull.

Abstract

Chondroid tissue is an intermediate calcified tissue, mainly involved in desmocranial morphogenesis. Often associated with secondary cartilages, it remained of unprecise embryonic origin. The latter was studied by performing isotopic isochronic grafts of quail encephalon onto 30 chick embryos. The so-obtained chimeras were sacrificed at the 9th, 12th, and 14th day of incubation. The contribution of graft- and host-derived cells to the histogenesis of chondroid tissue, bone, and secondary cartilages was analyzed on both microradiographs of thick undecalcified sections and on classical histological sections after several DNA or ECM specific staining procedures. Chondroid tissue is deposited in the primitive anlage of all membranous bones of the avian skull. Also present on their sutural edges, it uniformly arises from the neural crest. In the face, bone and secondary cartilages share this mesectodermal origin. However, secondary cartilages located along the basal chondrocranium and bone formed on the chondroid primordium of the cranial vault, originate from the cephalic mesoderm. These facts provide evidence that chondroid tissue arises from a specific differentiation of neural crest derived cells and that this original skeletogenic program differs from that of secondary chondrogenesis. Moreover, they obviously indicate that in membraneous bone ontogenesis, chondroid tissue replaces functions devoted to mesodermal primary cartilages of the cranial base, and so corroborates at the tissue level, the dual embryonic and phyletic origin of the skull.