Dermomyotomal origin of the ribs as revealed by extirpation and transplantation experiments in chick and quail embryos

To elucidate role of the dermomyotome in the formation of the axial skeleton, we performed extirpation and transplantation experiments on the dermomyotomes in chick and quail embryos. When the thoracic dermomyotomes of chick embryos were removed, the intercostal muscles and the distal ribs were deficient, while the proximal ribs were more or less normal. Quail tissues including the dermomyotome, the ectoderm and the medial edge of lateral plate, were transplanted to replace chick dermomyotomes. In these chimeras, the ribs, which would be deficient without the back-transplantation, were recovered. The cells of the recovered part of the ribs as well as the intercostal muscles were derived from the quail transplants. These findings suggest that the distal rib originated from the dermomyotomes and not the sclerotome as previously believed. To localize the origin of the distal rib further, we removed restricted regions of the dermomyotomes along the mediolateral and the rostrocaudal axis. The more lateral the part of the dermomyotomes that we removed, the more distal the part of the ribs affected. On the contrary, when the rostral and caudal edges of the dermomyotomes were removed, only the vertebral ribs showed extensive deficiencies while removal of the middle part between the edges caused less deficiency. The sternal ribs were not deficient in either case, but were extensively affected when the entire lateral edge of dermomyotomes was included in the region removed. We conclude that the lateral edges of the dermomyotomes are the primordia of the sternal ribs, and the rostral and/or caudal edges of the medial part of dermomyotomes are the primordia of the distal part and not of the proximal part of the vertebral ribs.

Plasticity of transposed rhombomeres: Hox gene induction is correlated with phenotypic modifications

In this study we have analysed the expression of Hoxb-4, Hoxb-1, Hoxa-3, Hoxb-3, Hoxa-4 and Hoxd-4 in the neural tube of chick and quail embryos after rhombomere (r) heterotopic transplantations within the rhombencephalic area. Grafting experiments were carried out at the 5-somite stage, i.e. before rhombomere boundaries are visible. They were preceeded by the establishment of the precise fate map of the rhombencephalon in order to determine the presumptive territory corresponding to each rhombomere. When a rhombomere is transplanted from a caudal to a more rostral position it expresses the same set of Hox genes as in situ. By contrast in many cases, if rhombomeres are transplanted from rostral to caudal their Hox gene expression pattern is modified. They express genes normally activated at the new location of the explant, as evidenced by unilateral grafting. This induction occurs whether transplantation is carried out before or after rhombomere boundary formation. Moreover, the fate of the cells of caudally transplanted rhombomeres is modified: the rhombencephalic nuclei in the graft develop according to the new location as shown for an r5/6 to r8 transplantation. Transplantation of 5 consecutive rhombomeres (i.e. r2 to r6), to the r8 level leads to the induction of Hoxb-4 in the two posteriormost rhombomeres but not in r2,3,4. Transplantations to more caudal regions (posterior to somite 3) result in some cases in the induction of Hoxb-4 in the whole transplant. Neither the mesoderm lateral to the graft nor the notochord is responsible for the induction. Thus, the inductive signal emanates from the neural tube itself, suggesting that planar signalling and predominance of posterior properties are involved in the patterning of the neural primordium.

Inhibition of cell spreading on the band of extracellular fibres in early chick and quail embryos

The ventral surface of the upper layer shows a band of extracellular fibrils around the anterior and lateral border of the area pellucida during gastrulation of the chick embryo. Using scanning electron microscopy, we found that this disposition is correlated with the motility of the middle-layer cells of gastrulating chick and quail embryos. Outside the fibrous band, single middle-layer cells and a sheet of mesoblast cells were spread out and possessed lamellae. Single cells on the fibrous band did not form lamellae. The same cell behaviour was obtained with the explants of deep layer on the fibrous band. The fibrous band is assumed to operate as a barrier that inhibits cell motility during gastrulation.

The behaviour of embryonic chick and quail tissues in culture

Pieces of tissue were dissected from early chick and quail embryos (Stages XTII and XIV of Eyal-Giladi & Kochav, 1976; and stages 3–5 of Hamburger & Hamilton, 1951). These tissues were taken from three different regions of the early embryos, and from eight different regions of the older ones, and were derived mainly from the lower layer. Epiblast tissues were also used. The experiments were designed to test the ability of one tissue to penetrate another. A single tissue was grown in culture in a Falcon dish for 18–24 h until it had formed a coherent sheet of cells (Explant I). A second tissue was then combined with it in one of two ways: (a) A small piece of tissue (Explant II) was explanted on top of Explant I. In most cases Explant II penetrated through Explant I and spread on the Falcon dish. (b) Another small piece of tissue (Explant III) was explanted beside (in confrontation with) Explant I. Usually, Explant III penetrated into Explant 1 rather than vice versa. The results were analysed to see if there were any variations in behaviour of the different tissues. The main result was that important differences were found to exist between certain types of chick and quail cells when grown in culture; the implications of this finding for the widely used technique of xenoplastic grafting are mentioned. Another result was that Explant I was more likely to be penetrated when the second tissue was placed on top of it (Explant II) than when it was confronted with it (Explant III). The significance of these results is discussed.

Analysis of endoderm formation in the avian blastoderm by the use of quail-chick chimaeras

The formation of the endoderm has been investigated in chimaeric embryos resulting from the combination of the lower and upper germ layers taken from chick and quail embryos at stages 2–6 of Vakaët (1962). The ability to recognize quail from chick cells made it possible to follow the fate of each germ layer during development. It appeared that the primitive hypoblast participates in the formation of the anterolateral extra-embryonic endoderm while the embryonic endoderm is formed later by migration of cells of the ectomesoblast through Hensen's node and the primitive streak. Further interspecific combinations were carried out between ectoderm and endoderm + mesoderm from quail and chick embryos at stages 5–7 of Hamburger and Hamilton. The explants were grafted into chick embryos for several days and the intestinal structures which developed were observed. No contribution of cells from the neurectoderm to the endoderm was found. In contrast, cells coming from the neural crest colonized the intestinal structures and gave rise to the enteric ganglia. It was concluded from these observations that the enterochromaffin and endocrine cells of the gut epithelium do not originate from the neurectoderm.