Q.16 The formation of hypoblast in birds is a result of
(1) Epiboly
(2) Delamination
(3) Invagination
(4) Involution
Understanding Hypoblast Formation in Birds
In avian embryology, the hypoblast plays a crucial role in early chick embryo development. It forms part of the extraembryonic membrane and contributes to the yolk sac. For students preparing for exams like NEET or biology research, knowing the exact mechanism is key—especially in MCQs like: The formation of hypoblast in birds is a result of (1) Epiboly (2) Delamination (3) Invagination (4) Involution.
Correct Answer: (2) Delamination
This process ensures the blastoderm splits into distinct layers, setting the stage for gastrulation.
Why Delamination? Step-by-Step Explanation
Delamination involves cells peeling off or separating from the upper layer of the blastoderm in a sheet-like manner. Here’s how it works in birds:
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After fertilization, the chick egg forms a blastoderm—a flattened disc of cells atop the yolk.
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Around 12-18 hours of incubation, cells from the posterior margin of the epiblast (upper layer) delaminate.
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These cells migrate downward and spread anteriorly, forming a thin, continuous hypoblast layer beneath the epiblast.
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The hypoblast doesn’t contribute to the embryo proper but secretes enzymes for yolk digestion and induces the primitive streak.
This differs from mammals, where hypoblast forms via different mechanisms. Evidence comes from classic studies by Pasteur and modern live imaging, confirming delamination as the primary mode in chickens and quail.
Breakdown of All Options: Why Not the Others?
To master hypoblast formation in birds, compare all choices:
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(1) Epiboly: Incorrect. Epiboly is a spreading process where cells thin out and expand over a large surface, like in amphibian blastulae or fish embryos. In birds, it occurs later during blastoderm expansion but doesn’t form the hypoblast.
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(2) Delamination: Correct, as explained above. It’s a hallmark of avian hypoblast genesis, creating a lower layer via cell separation.
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(3) Invagination: Incorrect. This is folding inward of cells, typical in amphibian gastrulation (e.g., bottle cells in frogs forming archenteron). Birds lack this; their flat disc prevents invagination.
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(4) Involution: Incorrect. Involution involves cells rolling inward at a specific site, seen in amphibian gastrulation or bird primitive streak formation (mesoderm migration). It happens after hypoblast formation, not for it.
| Option | Mechanism | Example Organism | Role in Birds? |
|---|---|---|---|
| Epiboly | Cell spreading/thinning | Fish, amphibians | Blastoderm expansion, not hypoblast |
| Delamination | Layer peeling/separation | Birds | Forms hypoblast ✓ |
| Invagination | Inward folding | Amphibians | Not in avian flat disc |
| Involution | Cell rolling inward | Amphibians, birds (later) | Primitive streak, post-hypoblast |
Key Takeaways for Avian Gastrulation
The formation of hypoblast in birds via delamination highlights adaptations to yolky eggs. It precedes primitive streak formation, enabling efficient nutrient uptake. For deeper study, refer to Gilbert’s Developmental Biology or videos of chick embryo time-lapses.
This MCQ tests precise knowledge of comparative embryology—delamination is unique to sauropsids like birds and reptiles.
