Q. 95 During the gastrulation stage of amphibian development, ectoderm formation takes place by the
expansion of epithelial cell sheet over mesodermal cells. This type of cell movement is termed as
(A) ingression.
(B) epiboly.
(C) involution.
(D) delamination
This article breaks down a key multiple-choice question on amphibian embryonic development, focusing on ectoderm formation during gastrulation. It highlights the correct answer—epiboly—and explains all options with biological context, ideal for biology students, researchers in developmental biology, and exam prep in genetics or embryology.
What is the Correct Answer?
The correct answer is (B) epiboly.
During amphibian gastrulation, ectoderm forms as animal pole cells expand and thin out, spreading as an epithelial sheet over the mesodermal cells. This vegetative spreading, driven by cell shape changes and increased surface area, defines epiboly—a hallmark movement in frogs like Xenopus laevis.
Why Epiboly?
Epiboly involves the blastula’s animal cap cells enlarging and flattening, covering the vegetal region. It starts post-fertilization around the 32- to 512-cell stage, reducing cell layers from multilayered to a single epithelial sheet. This positions presumptive ectoderm externally, setting up the three germ layers.
Detailed Explanation of All Options
Understanding these cell movements helps grasp gastrulation dynamics in amphibians, where the blastula reorganizes into a gastrula with ectoderm (skin, nervous system), mesoderm (muscles, bones), and endoderm (gut).
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(A) Ingression: Cells detach from the epithelial layer and migrate inward individually or in groups, forming mesoderm or endoderm. In amphibians, this occurs at the blastopore, where bottle cells ingress to initiate invagination—not ectoderm expansion over mesoderm.
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(B) Epiboly (Correct): Thin, sheet-like spreading of ectodermal cells from the animal pole, covering mesoderm. It converges with involution at the blastopore lip, driven by microtubule-dependent cell wedging and yolk dynamics.
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(C) Involution: Bulk inward rolling of cells at the blastopore’s dorsal lip, displacing superficial cells mesenchymally. Head mesoderm involutes here, heading toward the interior—not ectoderm sheet expansion.
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(D) Delamination: Splitting of a single epithelial layer into two parallel sheets, often seen in chick blastoderm or vertebrate neurulation. In amphibians, it doesn’t describe ectoderm overgrowth; that’s epiboly’s role.
| Option | Cell Movement Type | Role in Amphibian Gastrulation | Applies to Ectoderm Expansion? |
|---|---|---|---|
| (A) Ingression | Individual inward migration | Forms bottle cells at blastopore | No |
| (B) Epiboly | Sheet thinning and spreading | Ectoderm covers mesoderm/endoderm | Yes |
| (C) Involution | Bulk inward folding | Mesoderm rolls into embryo | No |
| (D) Delamination | Layer splitting | Minor in some vertebrates | No |
Why This Matters in Developmental Biology
Gastrulation morphogenetic movements like epiboly rely on cytoskeletal changes (actin, microtubules) and signaling (Wnt, BMP pathways). Disruptions lead to defects, studied in model organisms. For exams like NEET or CSIR-NET, memorize: epiboly = ectoderm spread; involution = mesoderm ingress.
This MCQ tests precise terminology from texts like Gilbert’s Developmental Biology.
