Question

4) a) What is the cavity inside the blastula called and how does it form? (3%)

b) What happens during gastrulation in the amphibian embryo? (3 %)

c) What are the tissue layers and the cavities present in the amphibian embryo after end of gastrula? (6 %)

d) How does the neural tube form and what does it become in the adult? (4%)

5) a) How are axes determined in the amphibian embryo? (10%)

b) Describe an experiment that can reveal the function of the Nieuwkoop centre. (5%)

c) How and where is the Spemann organiser induced? (3%)

Answer 1

4. a) The cavity inside the Blastula is called Blastocoel. The blastula is the hollow sphere of cells called blastomeres that are produced during the development of an embryo by repeated cleavage of a fertilized egg. The cells of the blastula form an epithelial layer or cover called the blastoderm which enclosing a fluid-filled cavity known as the blastocoel.

b)Gastrulation is the formation of the three primary germ layers that occurs during the first two weeks of development. During gastrulation, the animal cap and non-involuting marginal zone (NIMZ) cells expand by epiboly to cover the entire embryo. The dorsal portion of the NIMZ extends more rapidly toward the blastopore than the ventral portion, thus causing the blastopore lips to move toward the ventral side.

c)Gastrulation is characterized by cell movement and reorganization within the embryo (morphogenetic movements) to the interior of the embryo, forming three primary germ layers: ectoderm, mesoderm, and endoderm. The cells migrate inward at the blastopore, which forms, or is close to, the location of the rectum in the adult
• the ectoderm forms the outer tube of the embryo
• the endoderm is an inner tube that forms the alimentary canal and all its derivative organs
• the mesoderm lies between these two layers.

At the end of gastrulation, the embryo is bilaterally symmetrical, with three discrete cell layers, and rudiments of the notochord and neural tube.

d) Neural tube forms at the end of gastrulation when the embryo is bilaterally symmetrical, with three discrete cell layers, and rudiments of the notochord and neural tube. The neuroectoderm begins to fold inward to form the neural groove. As the two sides of the neural groove converge, they form the neural tube, which lies beneath the ectoderm. The anterior end of the neural tube will develop into the brain, and the posterior portion will become the spinal cord. The neural crest develops into peripheral structures.

5. a) Vertebrate axes do not form from localized determinants in the various blastomeres, as in Drosophila. Rather, they arise progressively through a sequence of interactions between neighboring cells. Amphibian axis formation is an example of regulative development. In Xenopus embryos, the dorso-ventral and antero-posterior axes are established by the Spemann-Mangold organizer. According to the prevalent model of early development, the organizer is induced by the dorsalizing Nieuwkoop signal, which is secreted by the Nieuwkoop center. The formation of the center requires the maternal Wnt pathway, which is active on the dorsal side of embryos.

b) Nieuwkoop performed an experiment that can reveal the function of this particular center. In this experiment, when amphibian animal cells, fated to become neurectoderm, are juxtaposed to endoderm-fated vegetal cells, they switch fate to meso-endoderm. In addition, dorsal vegetal cells induce a “dorsal” type of mesoderm (notochord and muscle), as opposed to the “ventral” type (blood, mesenchyme) induced by ventral vegetal cells. Nieuwkoop’s observations were later incorporated in the three signal model that describes mesoderm formation and patterning in amphibians.

c)The Spemann-Mangold organizer, also known as the Spemann organizer, is a cluster of cells in the developing embryo of an amphibian that induces the development of the central nervous system.

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