22. Movement of epithelial sheet spreading as a unit to enclose deeper layers of the embryo is termed as
    (1) Epiboly (2) Emboly
    (3) Involution (4) Ingression

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Introduction

Embryonic development involves a series of intricate cell movements that sculpt the early embryo into its foundational form. Among these, epiboly is a distinctive process characterized by the spreading and thinning of a sheet or multiple layers of epithelial cells to envelop underlying layers. It is essential in the gastrulation stages of many animals, contributing significantly to shape the embryo and organize its germ layers.

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What is Epiboly?

• Epiboly involves the expansion and thinning of epithelial sheets, typically ectodermal cells, as they move to cover the yolk or inner layers.

• This movement results in large-scale surface coverage by coordinated cell shape changes, intercalation, and rearrangements.

• Epiboly effectively encloses deeper tissues like mesoderm and endoderm, ensuring proper body plan formation.

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Examples and Mechanism

• In species like fish, amphibians, and sea urchins, epiboly is observed during gastrulation where the blastoderm spreads over the yolk.

• Mechanisms include radial intercalation, cell flattening, and coordinated cytoskeletal dynamics.

• Epiboly is driven by actomyosin contraction and membrane remodeling facilitating epithelial expansion.

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Biological Significance

• Epiboly ensures the embryo’s surface layer forms correctly, protecting and covering internal structures.

• It contributes to the spatial organization of germ layers for further differentiation.

• Defects in epiboly can disrupt embryogenesis, leading to developmental abnormalities.

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Comparison with Other Movements

• Epiboly is distinct from involution (rolling inward of cells), invagination (buckling inward), and ingression (individual cell migration).

• Unlike the inward cell movements, epiboly is an outward, spreading movement of cell sheets.

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Conclusion

Epiboly represents a fundamental embryonic movement where a sheet of cells spreads as a unit, key to enveloping and organizing deeper embryonic layers. Its study illuminates critical aspects of developmental biology and morphogenesis.

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Final Answer:
(1) Epiboly