Oogenesis is the process by which female germ cells, called oocytes, develop into mature eggs (ova) in the ovaries.
It involves a series of cell divisions and developmental stages that result in the production of a single mature ovum from a primordial germ cell.
Stages of Oogenesis
1. Oogonial proliferation:
During fetal development, primordial germ cells in the ovaries differentiate into oogonia, which are diploid (2n) cells.
Oogonia undergo several mitotic divisions to increase their numbers, and some of them enter meiosis to become primary oocytes.
By birth, all oogonia have entered meiosis and become primary oocytes, which are then arrested in prophase I of meiosis I.
2. Formation of primordial follicles:
Primary oocytes become surrounded by a layer of flattened granulosa cells, forming primordial follicles.
These follicles remain dormant in the ovaries until puberty.
3. Follicular development:
During each menstrual cycle, a cohort of primordial follicles is activated to begin growing and developing.
The primary oocytes within these follicles complete meiosis I, resulting in the formation of two cells: a large secondary oocyte and a smaller polar body, which contains excess genetic material.
The secondary oocyte is now haploid (1n) and enters meiosis II but becomes arrested at metaphase II until fertilization occurs.
4. Ovulation:
The mature Graafian follicle, which contains the secondary oocyte, ruptures and releases the oocyte into the fallopian tube.
If fertilization does not occur, the oocyte degenerates within approximately 24 hours.
5. Completion of meiosis II:
If fertilization occurs, the secondary oocyte resumes meiosis II and divides into a mature ovum and another polar body.
The mature ovum then fuses with the sperm cell to form a zygote, marking the beginning of embryonic development.
Oogenesis is regulated by hormones, including gonadotropin-releasing hormone (GnRH) from the hypothalamus, follicle-stimulating hormone (FSH) and luteinizing hormone (LH) from the anterior pituitary, and estrogen and progesterone produced by the ovaries.
These hormones work together to stimulate and control follicular development and ovulation.
Regulation
The regulation of oogenesis involves hormonal control primarily by the hypothalamus, pituitary gland, and ovarian hormones. Gonadotropin-releasing hormone (GnRH) from the hypothalamus stimulates the pituitary to release follicle-stimulating hormone (FSH) and luteinizing hormone (LH), which in turn regulate the growth and development of ovarian follicles and the progression of meiosis in oocytes.
Significance
Genetic Diversity: Oogenesis, through the process of meiosis, contributes to genetic diversity in the offspring due to the shuffling and recombination of genetic material.
Fertility and Reproduction: The production of viable ova is essential for sexual reproduction, determining the fertility window in females.
Hormonal Balance: The development of ovarian follicles and the corpus luteum during oogenesis plays a critical role in the regulation of hormonal cycles, impacting overall female health.