Q.26 Embryonic cleavage in mammals is
(1) Rotational cleavage
(2) Radial cleavage
(3) Bilateral cleavage
(4) Discoidal cleavage
What is Embryonic Cleavage?
Embryonic cleavage refers to the rapid series of mitotic cell divisions that occur in the zygote immediately after fertilization, transforming it into a multicellular blastula without significant growth. In mammals, this process is unique due to the large amount of yolk and the structure of the egg. Understanding cleavage types is key for biology students preparing for exams like NEET.
Correct Answer: Embryonic Cleavage in Mammals is Rotational Cleavage (Option 1)
The correct answer is (1) Rotational cleavage.
In mammals (e.g., humans, mice), the zygote undergoes rotational cleavage, a holoblastic (complete) division where the first cleavage is meridional (vertical, passing through the animal-vegetal axis). The blastomeres rotate slightly around each other, leading to unequal cell sizes by the 4-cell stage—one pair larger, one smaller. This pattern suits the moderate yolk distribution in mammalian eggs and results in a morula, then blastocyst.
Why rotational? Mammalian eggs have little yolk, allowing full cleavage, but the rotational asymmetry prepares for compaction and implantation.
Explanation of All Cleavage Options
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(1) Rotational cleavage: Characteristic of mammals. First division is meridional; second blastomeres rotate. Leads to a loose morula. Example: Human embryo at 2-cell stage shows rotation.
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(2) Radial cleavage: Typical in echinoderms (sea urchins) and some chordates (amphioxus). Blastomeres divide parallel or perpendicular to the axis, stacking radially like slices of a pie. Cells remain aligned; common in isolecithal (equal yolk) eggs.
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(3) Bilateral cleavage: Seen in cephalochordates and some tunicates. Divisions create mirror-image halves along a bilateral plane. Blastomeres align symmetrically but not radially.
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(4) Discoidal cleavage: Found in birds, reptiles, and some fish with megalecithal (yolk-rich) eggs. Cleavage is superficial, limited to a disc atop the yolk (blastodisc), not penetrating deeply.
| Cleavage Type | Common Examples | Egg Type | Key Feature |
|---|---|---|---|
| Rotational | Mammals (humans) | Mesolecithal | Rotating blastomeres, holoblastic |
| Radial | Echinoderms | Iolecithal | Stacked alignment |
| Bilateral | Cephalochordates | Iolecithal | Mirror symmetry |
| Discoidal | Birds | Megalecithal | Yolk-limited disc |
Why Does This Matter for Mammalian Development?
Rotational cleavage in mammals ensures proper blastocyst formation, with the inner cell mass developing into the embryo and trophectoderm aiding implantation. This differs from radial cleavage’s even divisions or discoidal’s yolk restriction, highlighting evolutionary adaptations.
