Cleavage in biology refers to the series of rapid mitotic cell divisions that occur after fertilization of an egg, dividing the large volume of egg cytoplasm into numerous smaller cells called blastomeres. This process is a crucial stage in early embryonic development.
Understanding Cleavage
Here's a more detailed look at cleavage:
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Purpose: The primary purpose of cleavage is to subdivide the enormous volume of egg cytoplasm into smaller, more manageable cells. It doesn't initially increase the overall size of the embryo. Instead, it increases the number of cells.
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Process: Cleavage involves repeated mitotic divisions. These divisions are characterized by a lack of significant cell growth (G1 and G2 phases are shortened or absent). This allows for rapid cell proliferation.
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Blastomeres: The cells produced during cleavage are called blastomeres. As cleavage progresses, the blastomeres become progressively smaller.
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Morula: As cleavage continues, the embryo becomes a solid ball of cells called a morula.
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Blastula: Further cleavage leads to the formation of a blastula, a hollow ball of cells with a fluid-filled cavity called the blastocoel.
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Types of Cleavage: Different types of cleavage patterns exist, depending on the amount and distribution of yolk in the egg. These patterns include:
- Holoblastic Cleavage: Complete cleavage, where the cleavage furrow extends through the entire egg. This occurs in eggs with little yolk (isolecithal) or a moderate amount of yolk (mesolecithal).
- Meroblastic Cleavage: Incomplete cleavage, where the cleavage furrow does not completely divide the egg due to a large amount of yolk (telolecithal).
Significance of Cleavage
Cleavage is essential for:
- Cellularization: Dividing the cytoplasm into discrete cellular units.
- Preparation for Gastrulation: Creating a foundation for the next major developmental event, gastrulation, where cells migrate and reorganize to form the three primary germ layers (ectoderm, mesoderm, and endoderm).
- Establishing Cell Fate: The early stages of cleavage can influence the developmental fate of the blastomeres, although this varies depending on the organism.
In summary, cleavage is the rapid series of cell divisions that occur after fertilization, partitioning the egg's cytoplasm into smaller cells, and setting the stage for further development.
