Question

1. Describe cleavage and gastrulation in chick embryos. Describe the role of FGFs, Cerberus, Wnt8c, nodal, and Veg1 in mesoderm induction and primitive streak formation. Also describe how BMP and noggin interact in neural induction.

Answer 1

Cleavage begins about 3 hours after fertilization has occurred.Due to the presence of large quantity of yolk, none of the cleavages are able to penetrate the furrow .Thus the divisions are limited to the small blastodisc.

First cleavage division takes place vertically and cleaves the germinal disc superficially. The second division immediately occurs after that. The second division is also vertical but at right angles to the first cleavage division. Subsequent  too is vertical but at right angles to the first. These and several similar subsequent divisions do not extend right across the germinal disc, thus, the early cleavage is confined to a central part and soon the germinal disc comes to have a central mass of cells, and a peripheral undivided cytoplasm, the periblast or marginal zone.

Later, divisions extend into the periblast and cut off larger and less numerous marginal cells, which not only add to the cells but also extend directly over the yolk. Divided blastodisc is hereafter called blastoderm.

While they are still in epiblast,but close to the primitive streak, the mesoderm cells appear to be instructed about their orientation along anterior-posterior axis. At this point, they are induced to express the Hox genes. The migration of the mesodermal cells through the anterior primitive streak and their aggregation to form the chordamesoderm appear to be controlled by the FGF signalling. Fgf8 is expressed in the primitive streak and it repels migrating cells away from the streak. Meanwhile cells continue migrating inward through the primitive streak. As they enter the embryo,the cells separate into 2 layers.THe bottom layer joins the hypoblast (forming the endoderm) and the second layer extends to form a loose layer between the endoderm and the epiblast. The anterior primitive streak thus continues to grow to develop organs while the posterior primitive streak posits itself for further processes. This establishes a clear anterior-to posterio gradient of developmental maturity.

The Posterior Marginal Zone contains cells that are able to induce a primitive streak and Hensen's node. The entire marginal zone produces Wnt8c, which is capable of inducing beta-catenin, and Vg1. Wnt8c and Vg1 act together to induce the expression of Nodal. Nodal' secretion is essential to the formation of primitive streak and Cerberus (an antagonist of Nodal) secreted by primary hypoblast cells prevents primitive streak formation.As the primary hypoblast cells,move away from the PMZ, Cerberus protein is no longer present allowing Nodal activity and therefore formation of the primitive streak in the posterior epiblast. Dorsal mesoderm is able to induce the formation of central nervous system in the ectoderm above it. The cells of Hensen's node secrete BMP antagonist proteins such as the Noggin and Nodal.These proteins repress BMP signalling and dorsalize the ectoderm and mesoderm.