Oogenesis |

#oogenesis is the process by which female germ cells, called oocytes or egg cells, develop and mature in the ovaries. It involves multiple stages, starting from the formation of primary oocytes during fetal development and progressing through meiosis to produce secondary oocytes. These secondary oocytes are then released from the ovary during ovulation and can be fertilized by sperm to form a zygote, which eventually develops into an embryo.
1. Formation of Primary Oocytes: Oogenesis begins before a female is even born. While she is still a fetus, her ovaries contain a pool of primary oocytes, which are diploid cells (having the full number of chromosomes, 46 in humans). These oocytes are arrested in the prophase stage of the first meiotic division.
2. Fetal Development: By around the 20th week of gestation, the number of primary oocytes in a female's ovaries reaches its maximum, around 6 to 7 million. However, this number declines significantly by the time of birth, and only about 1 to 2 million remain.
3. Puberty: As a girl reaches puberty, a complex process begins that ultimately leads to the maturation of primary oocytes. Each menstrual cycle, a few primary oocytes are stimulated to resume meiosis. However, only one dominant follicle will fully mature and be released, while the others degenerate.
4. Ovarian Cycle: The maturation process takes place over the ovarian cycle, which is typically around 28 days. It consists of several phases:
(a) Follicular Phase: This phase starts on the first day of menstruation. The anterior pituitary gland releases follicle-stimulating hormone (FSH), which stimulates a group of follicles to grow. Only one follicle, the dominant one, continues developing. The primary oocyte within the dominant follicle continues meiosis and completes the first meiotic division, forming a secondary oocyte and a smaller polar body.
(b) Ovulation: Around day 14 of the cycle, a surge of luteinizing hormone (LH) triggers ovulation. The mature follicle ruptures, releasing the secondary oocyte from the ovary. The released egg is surrounded by the zona pellucida and corona radiata, protective layers.
(c) Luteal Phase: After ovulation, the ruptured follicle transforms into the corpus luteum, which produces progesterone and some estrogen. These hormones prepare the uterine lining for a possible pregnancy.
5. Completion of Meiosis II: Meiosis II is only completed if fertilization occurs. If a sperm successfully penetrates the layers surrounding the secondary oocyte, it triggers the completion of meiosis II, resulting in the formation of a mature ovum (egg) and a second polar body.
6. Fertilization: If fertilization occurs, the male and female pronuclei formed from the sperm and egg nuclei respectively, merge, forming a diploid zygote. This zygote then begins to divide and develop into an embryo.
7. No Fertilization: If fertilization does not occur, the corpus luteum degenerates, causing progesterone levels to drop. This leads to the shedding of the uterine lining, resulting in menstruation, and the cycle starts over.
It's important to note that not all primary oocytes will complete this process. Many will undergo atresia, a process of degeneration, and only a small fraction will actually mature and be released. This process continues until menopause, when the ovaries cease to release eggs, typically occurring in a woman's late 40s or early 50s.
#puberty #ovarian #follicular #ovulation #lutealphase #meiosis #fertilization #chatgpt
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