Question

1. Detail the events associated with fertilization in sea urchins. Include the acrosomal/cortical reactions and how polyspermy is prevented.

2. Describe the processes of cleavage, gastrulation, and neurulation in frogs.

Answer 1

(1) fertilization in sea urchins includes basic 5 events which are as follows:-

1. Sperm attraction:- it is maily through chemotaxis

2. Acrosomal reaction:- ie, the interaction of sperm and egg jelly. In this process, the acrosomal enzymes lysis the protein of envelope in the region of sperm head. This is followed by fusion of egg and sperm.

3.fusion of egg and sperm cell membrane

4. Block to polyspermy: prevention of polyspermy is by 2 mechanisms in sea urchin egg which are:

(a) Fast reaction: This is done by electric change in egg plasma membrane

(b) slow reaction: it is accomplished by exocytosis of cortical granules.

5. Activation of egg metabolism.

(2) process of cleavage, gastrulation and neurulation in frogs:

1. Cleavage: cleavage is holoblastic and unequal in frogs. The process includes

(1) first cleavage plane is meridional. It cuts the egg from its median into 2equal sized blastomeres.

(2) second cleavage : meridional. It cuts first cleavage furrow at right angles and forms 4 blastomeres.

(3) third cleavage is horizontal and forms 8 blastomeres.

(4) fourth cleavage is meridional. It is unequal and forms 16 blastomeres.

(5) fifth cleavage is horizontal. And it forms 4 tiers of blastomeres.

(6) further cleaveges forms a ball of several blastomeres.

2. Gastrulation: it is the formation of hollow gastrula from blastula. It includes dynamic movement of blastomeres which is termed as morphogenic movements and rearrangement of blastomeres.

The movements are of 3 types:

A. Invagination: active infolding of blastomeres.

B. Involution: rolling movement of blastomeres

C. Epiboly: growth of one layer of cell over the other.

3. NEURULATION is the process which creates neural tube,neural crest and bona fide epidermis.

By creating nerural tube, it give rise to central nervous system. By creating neural crest, which migrates away from dorsal surface of neural tube and gives rise to diverse set of cell types. By creating bonafied epidermis, it covers over neural tube once its created.

Hope this helps you..Thank you!!