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#blastodisc

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chick development-l

The blastodisc in chick eggs is located at the:

During avian oogenesis yolk accumulates vegetally while active cytoplasm enriched with maternal mRNAs, mitochondria, ribosomes and nucleus migrates animally. After oviposition and fertilization in infundibulum, rapid cleavages occur only in this cytoplasmic disc, forming blastodisc sitting atop yolk at animal pole. Vegetal pole packed with inert yolk incapable of division, so embryonic axis must originate animally. Animal positioning facilitates contact with albumen, gas exchange, and delamination generating hypoblast beneath epiblast, while area pellucida overlies subgerminal cavity, setting stage for posterior marginal zone signaling.

Ref: Alberts, Molecular Biology of the Cell, 6th ed., Chapter 21: Animal-vegetal polarity and blastodisc formation in telolecithal eggs.

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Chick development-ll

Chick blastodisc is situated at the:

Chick oocyte telolecithal with massive yolk accumulates at vegetal pole, cytoplasm including nucleus, mitochondria and maternal RNAs concentrates at animal pole forming blastodisc. Cleavage restricted there produces area pellucida and opaca atop yolk. This polarized arrangement persists through laying, embryo developing from animal-most disc while yolk provides nutrition vegetally. Position contrasts vegetal pole which is yolk, equator lateral, posterior pole defined later after axis formation. Therefore blastodisc situated at animal pole analogous to zebrafish blastoderm at animal pole of heavily yolky egg. This illustrates conserved developmental logic of morphogen gradients patterning embryonic axes through Wnt and BMP antagonism.

Ref: Gilbert, Developmental Biology 12th ed., Chapter 11: Avian blastodisc at animal pole.

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