- The part of the embryo from which the ectoderm, mesoderm and endoderm are formed in chick is known as
(1) primitive streak (2) hypoblast
(3) epiblast (4) cytotrophoblast
Introduction
In avian development, particularly in the chick embryo, the germ layers ectoderm, mesoderm, and endoderm emerge from a specific embryonic region called the epiblast during the process of gastrulation. The primitive streak forms in the epiblast and serves as the conduit through which cells migrate to establish these three primary germ layers, a critical early event in the formation of the body plan.
The Epiblast as the Source of Germ Layers
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The epiblast is the upper layer of the blastoderm in the chick embryo, comprising pluripotent epithelial cells.
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These epiblast cells give rise to all three germ layers: ectoderm (surface layer), mesoderm (middle layer), and endoderm (inner layer).
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Unlike the hypoblast, which contributes to extra-embryonic tissues, the epiblast is the true embryonic germinal layer for the embryo proper.
Formation and Role of the Primitive Streak
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The primitive streak appears as a thickened band of epiblast cells along the midline at the posterior region of the blastoderm.
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It marks the site where epiblast cells ingress during gastrulation to internalize and form mesoderm and endoderm.
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The cells that remain in the epiblast become ectoderm.
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This streak also establishes the anterior-posterior body axis and bilateral symmetry.
Other Options Explained
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The hypoblast (Option 2) primarily contributes to extra-embryonic membranes and does not form the three germ layers.
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The cytotrophoblast (Option 4) is a layer of trophoblast cells important in placental formation in mammals and is not involved in avian germ layer formation.
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The primitive streak (Option 1) is not itself the source of the germ layers but the site where germ layer formation begins via cell ingress from the epiblast.
Summary Table: Germ Layer Origin in the Chick Embryo
| Structure | Contribution to Germ Layers | Explanation |
|---|---|---|
| Epiblast | Ectoderm, Mesoderm, Endoderm | Embryonic germinal layer from which all three layers form |
| Primitive Streak | Site of cell migration and germ layer formation | Not a tissue source but a structure guiding gastrulation |
| Hypoblast | Extra-embryonic membranes | Does not contribute to embryonic germ layers |
| Cytotrophoblast | Mammalian placental layer (not in chick) | Not involved in chick germ layer formation |
Conclusion
In chick embryogenesis, the epiblast serves as the cellular origin for the ectoderm, mesoderm, and endoderm. The primitive streak, formed within the epiblast, functions as a morphogenetic center where cells ingress and migrate to differentiate into the mesoderm and endoderm, while those remaining in the epiblast form the ectoderm. Therefore, the correct answer is that the epiblast produces all three germ layers.
FAQ
Q: What is the epiblast’s role in chick embryonic development?
It gives rise to all three primary germ layers: ectoderm, mesoderm, and endoderm during gastrulation.
Q: What function does the primitive streak serve?
The primitive streak is the site where epiblast cells ingress to form mesoderm and endoderm, marking the start of gastrulation and body axis formation.
Q: Does the hypoblast form embryonic germ layers?
No, the hypoblast mostly contributes to extra-embryonic tissues such as the yolk sac membranes.
This detailed overview provides a foundational understanding of germ layer formation in chick embryos vital for developmental biology and related academic preparation.
