Furrowing of the zygote is a cell division in early embryogenesis. It follows fertilization and is part of preembyonic development. Errors in furrow division are associated with gene mutations such as trisomies or parental disomies.
What is furrowing?
Furchung of the zygote is a cell division in early embryogenesis. It follows fertilization and is part of pre-embryonic development. At the beginning of embryogenesis, cell division by constriction occurs on the fertilized egg. This process is also called furrowing or furrow division of the zygote and occurs in all multicellular organisms. The embryo does not increase in size during this cell division, but is segmented. Furrow division exhibits a rapid rate because it does not require new production of biological components. A nuclear division can theoretically occur every seven to eight minutes in this way. The divisions proceed synchronously, unlike in conventional cell division, and the nuclear-plasma relation changes permanently with a shortened cell cycle consisting of an S and M phase. The end product of the furrow divisions is the so-called morula. This is a cell-filled sphere containing the blastomeres. These blastomeres are the cells formed during the division. Overall, three different types of furrows are distinguished: meroblastic furrowing of fish, reptiles, insects, and birds; holoblastic furrowing of mammals and amphibians; and helical furrowing of annelids and mollusks.
Function and task
Furrow divisions are part of the preembyonic phase of embryonic development and follow fertilization of the egg. Furrowing is preceded by nuclear fusion in the oocyte. 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 roundish cluster of cells that develops on the third or fourth day after fertilization. Cell divisions take place in the form of mitosis. The subsequent cells are therefore genetically identical to the zygote and are therefore also called clones. The morula becomes a germinal vesicle or blastula in the fallopian tube within the next five to six days and nests in this form in the mucosa of the uterus. Thus, the furrowing of the zygote is an important preparatory step in embryonic development and has a correspondingly high value for reproduction. At the beginning of each furrowing division, the cells, but not the plasma, become more. Thus, the cells grow later and currently divide only 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 determines how the mitotic division of the zygote proceeds in detail. Yolk-rich sites are called the vegetative pole and undergo slow furrowing. At the so-called animal pole of the yolk-poor side, furrowing occurs at a higher rate. Thus, the different types of furrowing are related to the distribution of the yolk. For example, total equilibrium furrowing occurs in isolecithal yolks. The yolk is relatively evenly distributed in these zygotes. Total equilibrium furrowing gives rise to evenly distributed blastomeres of approximately equal size and occurs primarily in holoblastians. The opposite is totally inequivalent or discoidal furrowing. It assumes a telolecithal yolk distribution in which the yolk is located primarily at the vegetative pole of the zygote. Eggs with particularly abundant yolk at the vegetative pole undergo discoidal furrowing. For example, the meroblastians are creatures with this type of furrowing. The third type of furrow division is superficial furrowing. It takes place on centrolecithal eggs with the yolk inside. The new cells are formed on the egg surface and the yolk remains inside the cell.
Diseases and ailments
During the first furrow division, chromosomal losses or duplications may already occur. Usually, division disorders of the egg or sperm cell before fertilization are responsible for such phenomena. For example, during mitotic division embryos with a trisomal or disomal cell lineage are produced where it is not required.A uniparental disomy may develop into an isodisomy, in which one parental chromosome is completely or partially duplicated. Such chromosomal anomalies are known to geneticists as mosaics. For example, Pätau syndrome is a disease associated with trisomy of chromosome 13. The syndrome is associated with high mortality and is associated with stillbirths. Malformations of the limbs are also included in the clinical picture, as are heart defects or developmental disorders of the brain and malformations of the central nervous system. Edwards syndrome, on the other hand, is known as trisomy 18. In this disease, heart defects, developmental disorders of the brain and malformations of the central nervous system are also present. Malformations of the extremities and abdomen are also common symptoms. Uniparental disomy is now also associated with conditions such as Prader-Willi syndrome and Angelmann syndrome. In Prader-Willi syndrome, small stature and mental retardation are usually present in addition to a predisposition to obesity. Angelmann syndrome is characterized by epilepsies, deformities and ataxias, as well as convulsions, perceptual disturbances and severe psychomotor retardation. The degree to which trisomies or disomies are pronounced in individual cases and the extent to which they affect the lives of affected individuals vary considerably from person to person.