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
Oogenesis can be divided into the following stages:
Oogonial Proliferation (Fetal Development)
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.
Some oogonia enter meiosis and become primary oocytes.
By birth, all oogonia have entered prophase I of meiosis I and are arrested at this stage, remaining as primary oocytes.
Formation of Primordial Follicles
Primary oocytes are surrounded by a single layer of flattened granulosa cells, forming primordial follicles.
These primordial follicles remain dormant in the ovaries from birth until puberty.
Follicular Development (After Puberty)
With the onset of puberty, during each menstrual cycle, a group of primordial follicles is activated to begin growth and development.
Primary oocytes within these follicles complete meiosis I, forming two cells:
A large secondary oocyte (haploid, 1n).
A smaller polar body, which contains excess genetic material and eventually degenerates.
The secondary oocyte enters meiosis II but is arrested in metaphase II and remains there until fertilization.
Ovulation
The mature Graafian follicle, containing the secondary oocyte, ruptures during ovulation, releasing the oocyte into the fallopian tube.
If fertilization does not occur, the secondary oocyte degenerates within approximately 24 hours.
Hormonal Regulation of Oogenesis
Gonadotropin-Releasing Hormone (GnRH): Released by the hypothalamus, it stimulates the anterior pituitary to release FSH and LH.
Follicle-Stimulating Hormone (FSH): Promotes growth and maturation of ovarian follicles and oocytes.
Luteinizing Hormone (LH): Triggers ovulation and the formation of the corpus luteum, which secretes hormones that sustain the uterine lining.
Estrogen and Progesterone: Produced by the follicles and corpus luteum, they regulate the menstrual cycle, promote endometrial growth, and prepare the body for pregnancy.
Significance of Oogenesis
Genetic Diversity: Meiosis during oogenesis contributes to genetic variation through recombination.
Fertility and Reproduction: Oogenesis determines the fertility window and the release of the secondary oocyte for potential fertilization.
Hormonal Balance: Oogenesis regulates the menstrual cycle and hormonal environment crucial for female reproductive health.