EMBRYONIC ORIGIN AND FATE OF CHONDROID TISSUE AND SECONDARY CARTILAGES IN THE AVIAN SKULL

Background: Chondroid tissue is an intermediate calcified tissue, main ly involved in desmocranial morphogenesis. Often associated with secon dary cartilages, it remained of unprecise embryonic origin. Methods: T he latter was studied by performing isotopic isochronic grafts of quai l encephalon onto 30 chick embryos. The so-obtained chimeras were sacr ificed at the 9th, 12th, and 14th day of incubation. The contribution of graft- and host-derived cells to the histogenesis of chondroid tiss ue, bone, and secondary cartilages was analyzed on both microradiograp hs of thick undecalcified sections and on classical histological secti ons after several DNA or ECM specific staining procedures. Results: Ch ondroid tissue is deposited in the primitive anlage of all membranous bones of the avian skull. Also present on their sutural edges, it unif ormly arises from the neural crest, In the face, bone and secondary ca rtilages share this mesectodermal origin. However, secondary cartilage s located along the basal chondrocranium and bone formed on the chondr oid primordium of the cranial vault, originate from the cephalic mesod erm. Conclusions: These facts provide evidence that chondroid tissue a rises from a specific differentiation of neural crest derived cells an d that this original skeletogenic program differs from that of seconda ry chondrogenesis. Moreover, they obviously indicate that in membranou s bone ontogenesis, chondroid tissue replaces functions devoted to mes odermal primary cartilages of the cranial base, and so corroborates at the tissue level, the dual embryonic and phyletic origin of the skull . (C) 1996 Wiley-Liss, Inc.