Cleavage- Salient features and Types of Cleavage

CLEAVAGE- SALIENT FEATURES AND TYPES OF CLEAVAGE

Cleavage:

Cleavage is the series of rapid cell divisions that follow fertilization in the early stages of embryonic development. It involves the division of the zygote into a multicellular structure called a blastula or blastocyst. Cleavage is characterized by rapid mitotic divisions without significant growth or differentiation of the cells. Here are the salient features and types of cleavage:

Salient Features of Cleavage:

1. No Increase in Size:
• Cleavage divisions result in the formation of smaller cells without an increase in the overall size of the embryo.
2. Rapid Successive Divisions:
• Cleavage involves a series of rapid and successive cell divisions.
• The cells divide quickly, and the process is often completed within the first few days after fertilization.
3. No Significant Growth:
• Cleavage divisions are focused on increasing the number of cells rather than the overall size of the embryo.
• Each cell produced is smaller than the parent cell.
4. Absence of G1 and G2 Phases:
• Cleavage divisions lack the typical G1 (gap 1) and G2 (gap 2) phases of the cell cycle.
• The cells rapidly progress through the cell cycle, alternating between S (synthesis) phase and M (mitosis) phase.
5. Mitotic Cell Division:
• Cleavage is primarily a process of mitotic cell division, where the genetic material is replicated, and the cell divides into two identical daughter cells.
6. Formation of Blastomeres:
• The cells produced during cleavage are called blastomeres.
• Initially, blastomeres are totipotent, meaning they have the potential to give rise to all cell types in the organism.
7. Blastula/Blastocyst Formation:
• Cleavage results in the formation of a multicellular structure called a blastula in organisms like frogs and fish or a blastocyst in mammals.

Types of Cleavage:

1. Holoblastic Cleavage:
• Complete cleavage of the egg occurs, involving the entire zygote.
• Common in eggs with little yolk content.
• Found in organisms like amphibians and mammals.
2. Meroblastic Cleavage:
• Incomplete cleavage occurs due to the presence of yolk.
• The cleavage furrow does not penetrate the yolk-rich region.
• Common in eggs with moderate to high yolk content.
• Found in organisms like birds and reptiles.
3. Radial Cleavage:
• Cleavage planes are perpendicular or parallel to the animal-vegetal axis.
• The resulting blastomeres are aligned in a regular pattern.
• Common in deuterostomes, including echinoderms and chordates.
4. Spiral Cleavage:
• Cleavage planes are at oblique angles to the animal-vegetal axis.
• The resulting blastomeres are arranged in a spiral pattern.
• Common in protostomes, including annelids and mollusks.
5. Determinate Cleavage:
• The fate of each cell is predetermined early in development.
• Blastomeres have a fixed developmental potential.
• Common in protostomes.
6. Indeterminate Cleavage:
• The fate of each cell is not predetermined; each blastomere can develop into a complete organism.
• Common in deuterostomes.

Cleavage sets the stage for subsequent stages of embryonic development, such as gastrulation and organogenesis. The type of cleavage is influenced by factors like the amount and distribution of yolk in the egg and the developmental fate of blastomeres.