#cleavage patterns

Developmental biology Basics

Cleavage pattern wherein only part of the egg forms the embryo is called:

Cleavage patterns classify based on yolk content and distribution affecting division planes. Meroblastic cleavage restricts division to small cap of cytoplasm atop large yolk mass, so only part of egg contributes to embryo while remainder serves nutritive function. Characterized as discoidal meroblastic in birds, reptiles, bony fish, and superficial meroblastic in insects centrolecithal eggs. Holoblastic cleavage completely bisects egg forming blastomeres comprising entire embryo. Meroblastic pattern allows evolution of large yolky eggs supporting external development without placenta, demonstrating adaptation linking egg architecture to embryonic cleavage strategy.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 7: Meroblastic discoidal cleavage.

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Developmental biology Basics

Which of the following organisms exhibits meroblastic cleavage?

Meroblastic cleavage occurs in eggs with dense yolk concentrated at vegetal pole that prevents complete division. Only cytoplasmic disc at animal pole undergoes cleavage, forming blastoderm sitting on yolk mass. Birds such as chickens and reptiles possess telolecithal eggs with massive yolk for external development, necessitating meroblastic discoidal pattern. Mammals and frogs have holoblastic cleavage because isolecithal or mesolecithal eggs contain less yolk, allowing cleavage furrows to bisect entire egg. Holoblastic cleavage creates smaller equal or unequal blastomeres, while meroblastic supports large nutrient stores required for oviparous development.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 7: Cleavage patterns and yolk influence.

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Fertilization in mammals-lll

Rotational cleavage occurs at:

Mammalian embryonic cleavage differs from canonical radial cleavage by incorporation of rotational division at second cell cycle. First division meridional produces two blastomeres; second division blastomeres divide at right angles to each other: one meridionally, other equatorially. This orthogonal orientation yields four-cell embryo with varied cell-cell contacts and slight asynchrony facilitating compaction. Rotational cleavage is hallmark of eutherian regulative development, enabling flexibility in fate allocation and inside-outside positioning before blastocyst formation. First, third and fourth cleavages are not defined by this rotational switch; second cleavage uniquely establishes mammalian cleavage geometry influencing early embryonic polarity and cell fate plasticity.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 10: Mammalian cleavage patterns - rotational cleavage at second division.

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Animal Classification

Spiral cleavage during embryogenesis is seen in

Enterocoelous formation of mesoderm and true coelom characterizes deuterostome development seen in Echinodermata Hemichordata and Chordata contrasting with schizocoelous mode in protostomes. During gastrulation archenteron wall evaginates as paired mesodermal pouches pinching off from endoderm expanding into blastocoel lumina coalescing to become eucoelom lined by mesoderm. Process termed enterocoely indicates gut origin of coelom. In echinoderms three pairs form axocoel hydrocoel somatocoel. Contrasts with schizocoelous where coelom forms by splitting mesodermal mass. Enterocoely produces trimeric organization emphasizing archenteric mesoderm source and deuterostome monophyly defined in embryology.

Ref: Campbell Biology, 12th ed., Chapter 32: Spiral cleavage protostomes; NCERT Animal Kingdom

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