Blastulation

1. Blastulation Asst Prof : Chetana Kanekar Ashoka College Of Education

2. The cleavage is the fractionating process which splits zygote or fertilized egg into smaller cells called blastomeres . These cells formed in typical doubling sequence 2,4,8,16,32,64 etc. The Geometrical regularity exist s only in early developmental stages.

3. Later on the process of cleavage becomes entirely irregular. During early cleavage stages the blastomeres form the structure of embryonic stage called Morula . Later on Morula develops into the next embryonic stage called Blastula.

4. Blastula As the cleavage proceeds ,the number of blastomeres of the morula increases and blastomeres undergo rearrangement . The cells adhere to each other and they arrange themselves into true epithelium. This may be single cell thick ( e.g.echinoderms ) or many cells thick ( e.g.vertebrates )

5. This epithelium is called blastoderm . The fluid filled space or cavity called blastocoel appears in the centre of blastoderm . This hollow ,spherical, and unepithelial thick embryonic stage is called blastula and the process of its formation is called blastulation

6. Sea urchin blastula

7. Blastula of frog

8. A  Dendraster  blastula (micrograph courtesy of George von Dassow , Center for Cell Dynamics, Univ. of Washington).

9. Types of Blastulae Coeloblastula Stereoblastula Periblastula or Superficial Blastula Discoblastula Blastocyst

10. Coeloblastula It is hollow sphere and blastocoel is filled with mucopolysaccrides and blastoderm is of single layer of cells. E.g. Echinoderms

11. Stereoblastula It is a Solid blastula because there is no blastocoel cavity. This type is formed due to spiral cleavage. E.g. Molluscs .

12. Superficial Blastula It is also called periblastula . Periblastula is formed in insect eggs due to superficial cleavge and there is no blastocoel in it. The nucei collect in the peripheral layer. Actually cavity is present but filled with yolk from beginning of cleavage

13. Discoblastula This type of blastula is found in yolky eggs of fishes,reptiles and birds. It is called discoblastula because it appears at the animal in the form of small multilayered flat disc separated from yolk by narrow subgerminal cavity.

14. Blastocyst This type of blastula is formed in mammals by regular cleavage The small cavity called blastcoel . The cluster of cells differentiates into two distinct groups called trophoblast cells or nutritive and formative cells

15. Human blastula

16. Thank you