Question

Compare and contrast the processes of gastrulation (early, mid, late stages) in Xenopus Laevis (frog) and Danio Rerio (zebrafish)? All 3 stages?

Answer 1

Process of Gastratulation in Xenopus Laevis or Frog The embryo consists of 16 to 64 numbers of cells which is called morula. The cell divides and form 128-cells making the blastocoel clear forming blastula. The blastocoels help in migration of the cells during gastrulation. The process of gastrulation is initiated on the dorsal side that is going to be formed in the future. It is followed by involulation of marginal zone cells and converge at the blastopore. It mostly undergoes a midblastula transition slowing down the cell cycle, synchronising cell cycle, allowing cells to move to their original positions and there is transcription of new mRNA. The process of gastrulation starts with invagination initiated at future dorsal side. The endodermal cells sink into the embryo forming blastopore and cells become flask shaped called bottle cells. They stay in contact with the outer surface with association of cytoplasmic strands and the main body is displaced inside. The gastrulation starts near marginal zone near the blastula equator. The next stage is involution which involves involution of the marginal zone cells. These cells turn inward on reaching the blastopre tip and travel along inner surface of outer cell sheets. The endodermal cells forms the dorsal lip which forms archenteron by invagination and later become pharyngeal cells of foregut. The chorda mesoderm cells are next set of cells that forms over the dorsal lip of blastopore and forms the notochord. The last or late gastrulation is epiboly, where once the new cells enter the embryo, it displaces off the blastocoels to opposite side of the dorsal blastoporal lip. It widens the blastopore that develops lateral lips and then ventral lips which passes over the additional mesoderm and endodermal precursor cells. Gastrulation process in Zebrafish In zebrafish, after fertilisation the yolk moves to the vegetal pole with help of microtubules that leaves non-yolky animal pole where the process of gastrulation is described by large quantity of yolk. In case of zebrafish, the complete embryo forms only one side of the mass and the early cells are loosely associated. The first phase of gastrulation is epiboly of the ectoderm throughout the cell mass. The cells undergoes involution as the ectoderm reaches halfway and becomes precursor cells for mesoderm. The notochord is formed next by combination of mesendodermal and ectodermal cells along the future dorsal side. The epiblast is the ectodermal precursor and hypoblast is the mesendodermal cell precursor. The gastrulation process reaches its end after the formation of the spinal cord.