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How does the amount of yolk in an egg affect early cell division?
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Animal development
The embryos of many animals appear similar to one another in the earliest stages of development and progress into their specialized forms in later stages.
In animal development, the amount and distribution of yolk significantly influence the pattern of cleavage, as yolk masses can hinder the cytoplasmic movements required for mitosis.2
The impact of yolk on early cell division is generally categorized as follows:
- Holoblastic Cleavage: In eggs with little yolk, division is complete, meaning the entire egg is subdivided into blastomeres. While these divisions are often uniform, they can be uneven, such as in amphibians where cells are smaller at the animal pole and larger and more yolk-rich at the vegetal pole.12
- Meroblastic Cleavage: When eggs contain large amounts of yolk, it can retard or completely suppress cleavage in yolk-rich regions. In these cases, division is partial.12
- Discoidal: Common in birds, fish, and reptiles, cleavage is restricted to a small disk of cytoplasm at the animal pole, leaving the bulk of the yolk uncleaved.2
- Superficial: Common in many arthropods, the surface layer of cytoplasm divides while the central yolk mass remains uncleaved.2
Essentially, as yolk concentration increases, cleavage is increasingly restricted to yolk-poor regions of the egg.2