Amitosis

True mechanism of zygote cleavage

In educational practice, it is often required to distinguish real developmental mechanisms from erroneous variants, such as amitotic, reductional, or meiotic divisions. Classic cleavage represents a strictly regulated set of mitotic divisions of the zygote.

The main feature of this process is the complete absence of a growth period in the resulting daughter cells, which are called blastomeres. Due to the lack of a growth phase, the total volume of the developing embryo does not increase.

The process has clear dynamics:

• Localization: Begins in the lumen of the uterine tube and ends upon reaching the uterine cavity.
• Early stage (days 2–4): A dense cell cluster forms, the embryo does not yet have a cavity.
• Late stage (days 5–6): Cavity formation occurs, after which the embryo is called a blastocyst.

The factors determining the specific type of cleavage are the amount of yolk and the nature of its distribution in the cytoplasm. Based on this, 4 main types of cleavage and their corresponding blastula types are distinguished.

Proliferation of oogonia: mitosis in the embryonic period

Another process where mitotic divisions play a key role is the proliferation stage in oogenesis. Unlike spermatogenesis, this stage proceeds exclusively prenatally.

• Timeframe: The process begins at months 3–4 of prenatal development and ends by month 7.
• Mechanism: Oogonia actively divide via mitosis within the sex cords.

An important feature is the formation of a germ cell syncytium: after mitosis, daughter cells do not fully separate. They remain connected to each other via cytoplasmic bridges (similarly to spermatogonia). By the end of this stage, the number of oogonia reaches its maximum (5–10 million). This is followed by massive cell death via apoptosis, resulting in the disappearance of up to 95% of the entire population.

Anomalous division variants: polyploidy and multinucleation

Deviations from the normal mitotic scheme are sometimes confused with amitosis, but they have a different nature. Such failures lead to a change in ploidy (rather than just the genome or karyotype) or to an increase in the number of nuclei.

1. Formation of multinucleated cells. Occurs when mitosis proceeds successfully, but cytokinesis (division of the cytoplasm) does not occur. The result is a doubling of the number of nuclei in a single cell (for example, 2 forms 4).
2. Formation of polyploid cells. Occurs via two scenarios:
3. Option 1 (Endoreduplication): After chromosome replication, neither nuclear division nor cytoplasmic division occurs. It is important to distinguish this process from endomitosis, cytokinesis, or amitosis.
4. Option 2 (Alternation): A combination of division without cytokinesis followed by an absolutely normal division.

Class and exam questions

What else to study on this topic

• The influence of the amount and distribution of yolk on the type of cleavage.
• Four main types of cleavage and their corresponding types of blastulae.
• Structure and functions of the germ cell syncytium in the embryonic period.
• Mechanisms and biological significance of massive oogonia apoptosis.
• Differences between prenatal proliferation of oogonia and spermatogenesis.

Other topics in the "Additional Topics" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.