Body processes

Furrows – Function, Task & Diseases


Cleavage of the zygote is a cell division in early embryogenesis . It follows fertilization and is part of preembyonic development. Flaws in cleavage are associated with gene mutations such as trisomy or parental disomy.

What is the furrow?

At the beginning of embryogenesis, cell division occurs in the fertilized egg cell by constriction. This process is also known as cleavage or cleavage of the zygote and occurs in all multicellular organisms.

The embryo does not enlarge during this cell division, but is segmented. Cleavage division is extremely rapid, since it does not require new production of biological components. In theory, a nuclear division can take place every seven to eight minutes.

In contrast to conventional cell division, the divisions are synchronous and the nucleus-plasma ratio changes permanently in a shortened cell cycle consisting of an S and M phase. The end product of the cleavage divisions is the so-called morula. This is a cell-filled sphere containing the blastomeres. These blastomeres are the cells formed during division .

A total of three different types of cleavage are distinguished: the meroblastic cleavage of fish, reptiles, insects and birds, the holoblastic cleavage of mammals and amphibians and the helical cleavage of annelids and molluscs.

function & task

Cleavage divisions belong to the pre-embyonic phase of embryonic development and follow the fertilization of the egg cell. Nuclear fusion in the egg cell precedes fissuring. A few hours later, the first cell division begins, which gives rise to the morula from a two-cell stage through a four-cell and eight-cell stage. 

The morula is a rounded cell cluster that forms on the third or fourth day after fertilization. Cell divisions take place in the form of mitosis . The subsequent cells are genetically identical to the zygote and are therefore also referred to as clones.

Within the next five to six days, the morula becomes a germinal vesicle or blastula in the fallopian tube and in this form nests in the mucous membrane of the uterus . The cleavage of the zygote is therefore an important preparatory step in embryonic development and is of correspondingly high value for reproduction .

At the beginning of each cleavage, the cells, but not the plasma , become more. The cells therefore grow later and currently only divide into blastomeres of decreasing size.

In the plasma of the zygote there is yolk, with different distribution. Usually a relatively yolk-poor side faces a comparatively yolk-rich side. The transition between these two sides is called the gray crescent. The yolk distribution decides how the mitotic division of the zygote proceeds in detail. Areas rich in yolk are referred to as the vegetative pole and are subject to slow furrowing. At the so-called animal pole of the yolk-poor side, cleavage takes place at a higher speed. The different types of cleavage are therefore related to the distribution of the yolk.

Totally equal cleavage occurs, for example, in isolecithalem yolk. The yolk is relatively evenly distributed in these zygotes. The totally equal cleavage gives rise to evenly distributed blastomeres of approximately the same size and occurs primarily in holoblastians.

The opposite is the totally inequal or discoidal cleavage. It assumes a telolecithal distribution of the yolk, in which the yolk is mainly located at the vegetative pole of the zygote. Eggs with a particularly large amount of yolk at the vegetative pole undergo discoidal cleavage. For example, the meroblastians are creatures with this type of cleavage.

The third type of cleavage is superficial cleavage. It takes place on centrolecithal eggs, the yolk of which lies inside. The new cells form on the surface of the egg and the yolk remains inside the cell.

Diseases & Ailments

Chromosomal losses or duplications can already occur at the first cleavage division. As a rule, division disorders of the egg or sperm cell before fertilization are responsible for such phenomena. For example, during mitotic division, embryos with a trisome or disome cell line develop where it is not required. 

A uniparental disomy can develop into an isodisomy, in which a parental chromosome is completely or partially duplicated. The genetic researcher knows such chromosomal anomalies as mosaics. For example, Patau syndrome is a related disease associated with a trisomy of chromosome 13. The syndrome is associated with high mortality and is associated with stillbirth . Malformations of the limbs are also part of the clinical picture, as are heart defects or developmental disorders of the brain and malformations of the central nervous system .

Edwards syndrome is known as trisomy 18 . Heart defects, developmental disorders in the brain and malformations in the central nervous system are also present in this disease. Malformations of the extremities and the abdominal area are also common symptoms.

Disorders such as Prader-Willi syndrome or Angelmann syndrome are also associated with uniparental disomy . In the case of Prader-Willi syndrome, in addition to a predisposition to obesity , there is usually short stature and a mental disability . The Angelmann syndrome is characterized by epilepsies , deformities and ataxia as well as convulsions , perceptual disorders and severe psychomotor retardation.

The extent to which trisomies or disomies are pronounced in individual cases and how much they affect the lives of those affected varies considerably from person to person.

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Hello! I am Lisa Newlon, and I am a medical writer and researcher with over 10 years of experience in the healthcare industry. I have a Master’s degree in Medicine, and my deep understanding of medical terminology, practices, and procedures has made me a trusted source of information in the medical world.