Oogenesis
Oogenesis ( ancient Greek ᾠόν ō (i) ón "egg", γένεσις génesis "arising, becoming"), also ovogenesis , from Latin ovum , the egg, is the development of a fertilizable egg cell ( ovum ) from a cell in the germ line in multicellular animals .
Development phases
After fertilization , the zygote divides many times. The diploid cells of the germ line cell line are called primordial germ cells that are already present in the female embryo , of which about 500,000 are found in the ovaries of sexually mature women.
Propagation phase
During the multiplication phases, the cells divide through mitosis . The primordial germ cells (with a complete set of chromosomes ) migrate into the ovary (the ovary) at an early embryonic stage . They are then referred to as oogonia (urine cells). In the ovary, the oogonia multiply through mitotic cell division. At the end of the reproduction phase, there are 700,000 to 2 million oogonia in humans. The reproductive phase begins in the unborn girl in the mother's womb in the 4th week of pregnancy and ends with the girl's 1st year of life .
Meiotic division
Now comes a phase in which the set of chromosomes is halved. A single layer of cells is formed around each egg cell. This is how the "primary follicles" arise. This phase lasts until the age of 2.
Resting phase
Growth phases
The single-layer shell becomes multilayer. When the mitotic divisions are complete, the oogonia store yolk substances and enlarge. They are surrounded by follicular cells , creating the primordial follicle .
The growth phase is a few days in Drosophila and a few months in humans (approx. 2 to 7 months of female embryonic development). At the end of the growth phase, the oogonia are referred to as first-order oocytes (egg mother cells) . They begin with the 1st meiosis , the reduction division, which leads to a haploid cell, the second order oocyte.
During the first meiosis, the oocyte in the primordial follicle usually has a resting phase (dictyotene), which is only ended by external activation.
In humans, in the 10th to 11th month after fertilization, all first-order oocytes are in the diplotene , a late phase of the prophase of the 1st meiosis . This stage, called a dictyotan, can persist for decades until the corresponding oocyte ovulates. In the period up to the beginning of puberty , most of the oocytes die, so that only about 40,000 oocytes remain, about 500 reach ovulation .
Maturation phase
After completion of the first meiosis , a haploid cell, the second order oocyte ("Secondary Oocyte" in the picture) is present. This now carries out the second meiosis, the equation division, so that a haploid egg cell ( ovum ) with a chromosome from a chromatid is present.
In humans, only a few second-order oocytes begin the second meiotic division in each menstrual cycle , under hormone control. In most cases, only one oocyte makes it to the mature egg cell, the rest degenerate. In metaphase II of the second meiotic division, the egg leaves the ovary ( ovulation ) and only enters the further stages of meiotic division after being fertilized by a sperm.
In meiosis, unequal (inequitable) cell divisions take place: only a small cell is separated from the large oocyte, which is either immediately dissolved (resorbed) or carries out the second meiosis. This small cell has no function and is known as a polar or directional body.
See also
Web links
Individual evidence
- ↑ In addition to many compound terms such as oocyte, oogonium or oolemma, there are also Latin-Greek mixed forms such as ovocyte, ovogonium and ovolemma in the literature.
- ↑ NCBI: Oogenesis
- ↑ Oogenesis. Retrieved May 16, 2019 .
- ↑ Debora Dankert: Effects of postovulatory aging on molecular parameters and the developmental competence of murine oocytes, pages 8-10