Background Chondroid tissue is an intermediate calcified tissue, mainly involved in desmocranial morphogenesis. Often associated with secondary cartilages, it remained of unprecise embryonic origin. Methods 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. Results 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. Conclusions 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 membranous 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. © Wiley‐Liss, Inc.

The calcified tissues involved in the early morphogenesis of the so-called intramembranous bones of the facial skeleton were studied by microradiographic and histological techniques in 22 chick embryos at the 9th, 12th and 14th days of incubation. On the 9th day, the bones of the upper face and palatal vault are made up of thin sheets of chondroid tissue, deposited in their respective mesenchymal condensations. Woven and lamellar bone formation subsequently takes place in each of them from the 12th day of incubation, mainly on the external side of their chondroid primordia. The same phenomena occur in the lower facial and mandibular bones. These facts indicate that the primitive facial desmocranium of the chick embryo, which is classically considered to be formed by intramembranous ossification, first consists of chondroid tissue. As in the cranial vault, this tissue thus represents the initial modality of the skeletogenic differentiation within the avian facial mesenchyme.

Cryostat sections of E8.5 to E9.5 mouse embryos were hybridized with S-labelled RNA probes to urokinase-type and tissue-type plasminogen activator (uPA and tPA, respectively) mRNA. The sections were screened for expression of either gene. Two main features emerge from the results. (a) The stage of initial detection is different for each transcript. The uPA mRNA is first detected in cephalic mesenchyme at E8.5 when tPA mRNA is already widely distributed in tissues derived from ectoderm and mesoderm; later, the uPA mRNA transcripts were found throughout most mesodermic tissues, (b) Each gene presents a different pattern of expression. The uPA is restricted to single cells or to small groups of cells within tissues; this distribution suggests its involvement in cell migratory mechanisms. On the other hand, tPA was detected in most tissues, with variable intensities. Its expression gains complexity while organogenesis proceeds. This pattern supports the hypothesis that regulatory mechanisms other than a direct gene regulation are involved. In extraembryonic tissues, uPA and tPA genes are constantly expressed at a high level in trophoblastic giant cells and parietal endoderm, respectively. Our results confirm the presence of plasminogen activator during embryonic development and provide detailed picture of the plasminogen activator gene expression in mouse organogenesis.

To reproduce diastematomyelia, a sagittal incision was carried out at the level of the rhomboidal sinus of 36- to 40-h-old chick embryos. A small piece of membrane shell, a small agar screen, or a piece of quail isochronous isotopic notochord was inserted into the gap. The embryos were killed and fixed after 9 days' incubation. Diastematomyelia was obtained in several embryos treated with interposition of a membrane screen or a piece of quail notochord. Microscopic examination revealed two hemicords, each containing its own central canal; in some cases one of the cords showed hydromyelia. Absence of the rump was seen in association with experimental diastematomyelia. The interposition of a resorbable agar screen did not succeed in reproducing diastematomyelia. The results of these surgical manipulations suggest that diastematomyelia cannot be explained by a primary disorder of neurulation. It supports the theory of noninvolution of a firm midline structure (probably the neurenteric canal, rapidly surrounded by mesodermal cells originating from the notochord), which prevents the fusion of the separated parts.

Neural tubes of E8-5 day mouse embryos were dissected and cultured in serum substitute-supplemented medium to allow the emigration of neural crest cells. After 48 h of culture the neural tubes were removed. The neural crest cells were then cultured for 12 h in serum-free medium, and their culture supernatant was studied by electrophoresis and zymography. The cultured cells were shown to secrete both urokinase-type and tissue-type plasminogen activators. When the truncal neural tube was divided in four equal segments, the secretion pattern of the two types of plasminogen activators was similar for the cells from the three most anterior segments; cells having migrated from the most caudal one, i.e. consisting of the neural plate, secreted a higher level of urokinase-type plasminogen activator. The secretion in vitro of plasminogen activators by neural crest cells is in accord with the postulated importance of these proteases in cellular migration.