Oogenesis

Oogenesis (from gr oon, egg and gr génesis, emergence, becoming), also Ovogenese, from Latin ovum, the egg, is the development of an egg capable of fertilization ( ovum ) from a cell of the germline in multicellular animals.

  • 3.1 growth phases
  • 3.2 maturation phase

Stages of development

After fertilization, the zygote divides many times. The diploid cells of the cell line of germline cells are referred to as primordial germ cells, one of which can be found in women about 500,000.

Multiplication phase

The propagation phases take place mitotically. Already at an early embryonic stage migrate the primordial germ cells (with full set of chromosomes ) in the ovary ( the ovary ). You will then be referred to as oogonia ( Ureizellen ). In the ovary, the oogonia proliferate by mitotic cell divisions. At the end of the growth phase are in humans 700.000 to 2 million oogonia. It begins in the fourth week of gestation and ends with the first year of life.

Meiotic division

The set of chromosomes is halved. It forms a single layer of cells around the oocyte. It's called the " primary follicles ". This phase lasts until 2 years of age.

Resting phase

Growth phases

The single cover is multi-layered. Are the mitotic divisions completed, store the oogonia a yolk substances and increase in size. They are surrounded by follicular cells.

The growth phase is in Drosophila few days, a few months in humans ( approximately 2 to 7 months of female embryonic development ).

At the end of the growth phase, the oogonia are called oocytes ( Eimutterzellen ) first order. You start with the first meiotic division of meiosis, the reduction division, which leads to a haploid cell, the second-order oocyte ( Promordialfollikel ).

During the first meiotic division the oocyte usually lays a resting phase which is terminated only by an activation from the outside.

When people are in the 10 to 11 months after fertilization, all oocytes in diplotene of the first order, a late stage of the prophase of the first meiotic division. This stage may persist as so-called dictyotene decades until it comes to ovulation of the corresponding oocyte. In the period until the onset of puberty, most oocytes die off, leaving only about 40,000 oocytes are present, about 500 reach ovulation.

Maturation phase

After completion of the first meiotic division of meiosis there is a haploid cell, the second-order oocyte. This now leads through the second meiotic division, the Äquationsteilung so that a haploid egg cell ( ovum ) is present with a chromosome from one chromatid.

When people start in each menstrual cycle only a few oocytes 2nd order hormone controlled the second meiotic division. This is usually only creates an oocyte to mature oocyte, the remaining degenerate. In metaphase II of the second meiosis, the egg leaves the ovary and goes after fertilization by the sperm into the subsequent stages of meiosis a.

In meiosis find unequal ( Stomata ) cell divisions take place: It is abgegliedert of the large oocyte only a small cell, either immediately dissolved ( absorbed ) is or the second meiotic division performs. This small cell is inoperative and is referred to as a pole or polar bodies.

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