blastulation
TRANSCRIPT
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BlastulationAsst Prof: Chetana Kanekar
Ashoka College Of Education
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• 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.
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• 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.
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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)
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• 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
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Sea urchin blastula
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Blastula of frog
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A Dendraster blastula (micrograph courtesy of George von Dassow, Center for Cell Dynamics, Univ. of Washington).
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Types of Blastulae
• Coeloblastula
• Stereoblastula
• Periblastula or Superficial Blastula
• Discoblastula
• Blastocyst
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Coeloblastula
• It is hollow sphere and blastocoel is filled with mucopolysaccrides and blastoderm is of single layer of cells.
• E.g. Echinoderms
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Stereoblastula
• It is a Solid blastula because there is no blastocoel cavity.
• This type is formed due to spiral cleavage.
• E.g. Molluscs.
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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
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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.
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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
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Thank you