BlastulationAsst Prof: Chetana Kanekar

Ashoka College Of Education

• 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.

• 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.

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)

• 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

Sea urchin blastula

Blastula of frog

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

Types of Blastulae

• Coeloblastula

• Stereoblastula

• Periblastula or Superficial Blastula

• Discoblastula

• Blastocyst

Coeloblastula

• It is hollow sphere and blastocoel is filled with mucopolysaccrides and blastoderm is of single layer of cells.

• E.g. Echinoderms

Stereoblastula

• It is a Solid blastula because there is no blastocoel cavity.

• This type is formed due to spiral cleavage.

• E.g. Molluscs.

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

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.

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

Human blastula

Thank you