Oogenesis

Oogenesis

1. Oogenesis Dr. Ghizal fatima

2. Oogenesis Oogenesis , ovogenesis , or oögenesis is the differentiation of the ovum into a cell competent to further develop when fertilized. It is developed from the primary oocyte by maturation. Oogenesis is initiated in the embryonic stage. Oogenesis , in the human female reproductive system, is a growth process in which the primary egg cell (or ovum) becomes a mature ovum, These cells, known as the primary ova, number around 400,000. The primary ova remain dormant until just prior to ovulation, when an egg is released from the ovary.

3. the first part of oogenesis starts in the  germinal epithelium , which gives rise to the development of  ovarian follicles , the functional unit of the  ovary . Oogenesis consists of several sub-processes: oocytogenesis ,  ootidogenesis , and finally maturation to form an ovum ( oogenesis proper).  Folliculogenesis  is a separate sub-process that accompanies and supports all three oogenetic sub-processes. Oogonium  —( Oocytogenesis )—> Primary Oocyte  —(Meiosis I)—> First  Polar body  (Discarded afterward) + Secondary oocyte  —(Meiosis II)—> Second Polar Body (Discarded afterward) + Ovum Oocyte meiosis, important to all animal life cycles yet unlike all other instances of animal cell division, occurs completely without the aid of  spindle -coordinating  centrosomes .

4. The creation of oogonia The creation of  oogonia  traditionally doesn't belong to oogenesis proper, but, instead, to the  common process  of  gametogenesis , which, in the female human, begins with the processes of  folliculogenesis , oocytogenesis , and  ootidogenesis . Oogonia enter meiosis during embryonic development, becoming oocytes . Meiosis begins with DNA replication and meiotic crossing over. It then stops in early prophase

5. Oogenesis Oogenesis starts with the process of developing primary oocytes , which occurs via the transformation of  oogonia  into primary  oocytes , a process called oocytogenesis . [13]   Oocytogenesis is complete either before or shortly after birth. There are three phases to oogenesis ; namely, multiplication phase, growth phase and maturation phase.

6. Multiplication Phase:  During foetal development, it should be noticed that certain cells present in the germinal epithelium of the female ovary are bigger than others . Hence, these cells split by mitosis, creating a couple of million oogonia or mother egg cells in each ovary present in the foetus . There are no more oogonia which are formed or augmented after birth.

7. Growth Phase:  This particular procedure of the primary oocyte tends to be very long. In this, the oogonium nurtures into bigger primary oocytes . After this, each primary oocyte gets surrounded by a granulosa cells layer to create primary follicle. Later, a large number of follicles get debased during the duration from birth to puberty . Therefore, at puberty around 60,000 to 80,000 primary follicles can be found in each ovary.

8. Maturation Phase:  Similar to a primary spermatocyte , every primary oocyte experiences two maturation divisions. However, the outcomes of maturation divisions under oogenesis are quite different to those which occur in spermatogenesis. Considering the first meiotic division, the primary oocyte segregates into two uneven haploid daughter cells. These are known as the large secondary oocyte and a small polocyte .

9. Later, considering the second maturation separation, the initial polar body might split to create two, second polar bodies. Here, the secondary oocyte once again divides to form unfit daughter cells.

10. Process of Oogenesis The process of oogenesis is completed in the following three stages: Pre-natal Stage The primary oocyte grows while being arrested in meiosis-I. The follicular cells proliferate and form a stratified cuboidal epithelium. Such cells are known as granulosa cells. These cells secrete glycoproteins to form zona pellucida around the primary oocyte .

11. Antral Stage The fluid-filled area, present between granulosa cells, combines to form a central fluid-filled space called the antrum . These are known as secondary follicles. In every month cycle, these secondary follicles develop under the influence of follicle-stimulating hormone and luteinizing hormone.

12. Pre- Ovulatory Stage This stage is induced by LH surge, and meiosis-I completes here. Two haploid cells of unequal sizes are formed within the follicle. One of the daughter cells that receive less cytoplasm forms a polar body. This cell does not participate in ovum formation. The other daughter cell is known as the secondary oocyte . The two daughter cells undergo meiosis-II. The polar body replicates to form two polar bodies, while the secondary oocyte arrests in the metaphase stage of meiosis-II.

13. Ovulation Development of oocyte takes place in ovaries. Every oocyte is neighboured by follicle cells to form a follicle . As the  menstrual cycle   starts, primary oocytes initiate to grow bigger, and follicle cells rise in number, causing the follicle to grow larger too . Normally, some nurturing oocytes degenerate and leave just one follicle to mature. Here, fraternal twins may be born, which are distinct genetically .

14. When a follicle attains maturity, the primary oocyte finishes its primary meiotic division and becomes secondary oocyte . Soon after, the follicle breaks and secondary oocyte is liberated in the fallopian tube even when the second meiotic division has not happened. This release of a secondary oocyte from ovaries is known as ovulation.