Question

6) a) Why does the chick embryo form as a blastoderm and not as a blastula? (4 %)

b) Where are the 2 chick early organisers and what are they called? (5 %)

c) How are new somites specified in chick? (3%)  

7) a) What is the difference between the mammalian early blastula and other vertebrate embryos and what is the significance of the differences? (5%)

b) How do homeotic genes in mice differ from those in flies? (4%)

c) What is the role of retinoic acid in mouse determination (3%)

Answer 1

6 blastoderm is a single layer of embryonic epithelial tissue that makes up the blastula. It encloses the fluid filled blastocoel. Gastrulation follows blastoderm formation, where the tips of the blastoderm begins the formation of the ectoderm, mesoderm, and endoderm.

The eggs have a large amount of yolk, cell division restricts it to blastoderm (meroblastic cleavage).

b.In the chick embryo, the node is called Hensen’s node because of its discoverer, Viktor Hensen, who first described the node in 1875. The discovery of Hensen’s node has helped to explain axis formation and has allowed experimental embryologists to further investigate vertebrate embryonic development.

The primitive streak is one of the key events in the early development of amniote embryos. The streak is the site where during gastrulation the mesendoderm cells ingress to take up their correct topographical positions in the embryo.

C

Somites are generated in a rhythmic fashion from the presomitic mesoderm and they subsequently differentiate to give rise to the vertebrae and skeletal muscles of the body.

7 a.

the blastulae without accessory or trophoblast tissue, e.g., Amphioxus, frog, etc.

the blastula possessing such auxiliary tissue, e.g., elasmobranch and teleost fishes, reptiles, birds, and mammals.

b.Homeotic genes are master regulator genes that direct the development of particular body segments or structures. ... Hox genes are found in many animals, including fruit flies, mice, and humans. Hox genes difference is that there are more Hox genes on the 5' side of the mouse segment; these correspond to expression in the tail, and flies do not have anything homologous to the chordate tail.

C.During early embryonic development, the major active form of retinoids, all-trans retinoic acid (atRA), regulates germ layer formation, body axis formation, neurogenesis, cardiogenesis, and the development of pancreas, lung, and eye. It is also a critical element for visual function .