26.6A: Oogenesis
Oogenesis is the maturation of the female gametes through meiotic division.
- Differentiate among the stages of oogenesis and the actions of hormones during each stage
Key Points
- Oogenesis starts with the process of developing oogonia via the transformation of primordial follicles into primary oocytes, a process called oocytogenesis.
- Oocytogenesis is complete either before or shortly after birth in humans. During the menstrual cycle primary oocytes complete maturation through further meiotic divisions.
- The first meiotic division is coordinated by hormones: follicle stimulating hormone (FSH), estrogen, luteinizing hormone (LH), and progesterone.
- The oocyte is arrested in cell division prior to the second meiotic division, which only occurs after fertilization.
Key Terms
- polar body : A small haploid cell formed concomitantly as an egg cell during oogenesis, but which does not have the ability to be fertilized.
-
ootid
: A nearly mature ovum that results from
meiotic division of a secondary oocyte. - lutenizing hormone : A hormone produced by the anterior pituitary gland. In females, an acute rise of LH triggers ovulation and development of the corpus luteum.
- follicle : A small cavity or sac that is the site of oocyte development in the ovary.
- follicle stimulating hormone : A gonadotropic glycoprotein hormone, secreted in part of the pituitary gland that stimulates the growth of ovarian follicles in female mammals and induces spermatogenesis in male mammals.
The menstrual cycle begins with the maturation of oocytes through the process of oogenesis, as well as concurrent follicle development that stimulates ovulation. Oogenesis starts with the process of developing oogonia via the transformation of primordial follicles into primary oocytes, a process called oocytogenesis. Oocytogenesis is complete either before or shortly after birth in humans. During the menstrual cycle primary oocytes complete maturation through further meiotic divisions.
Follicle development signals the beginning of the menstrual cycle. At the start of the menstrual cycle, some 12-20 primary follicles begin to develop under the influence of elevated levels of follicle-stimulating hormone (FSH) to form secondary follicles. The primary follicles form from primordial follicles, which develop in the ovary as a fetus during conception and are arrested in the prophase state of the cellular cycle.
By around day 9 of the menstrual cycle, only one healthy secondary follicle remain. The rest are reabsorbed into the ovary. The remaining follicle, called the dominant follicle, is responsible for producing large amounts of estrogen during the late follicular phase.
On day 14 of the cycle, a luteinizing hormone surge is triggered by the positive feedback of estrogen. This causes the secondary follicle to develop into a tertiary follicle, which then leaves the ovary 24–36 hours later. An important event in the development of the tertiary follicle occurs when the primary oocyte completes the first meiotic division, resulting in the formation of a polar body and a secondary oocyte. The empty follicle then forms a corpus luteum which later releases progesterone to maintain a potential pregnancy.
Immediately after meiosis I, the haploid secondary oocyte initiates meiosis II. However, this process is also halted at the metaphase II stage until fertilization occurs. When meiosis II has completed, an ootid and another polar body is created.
Both polar bodies disintegrate at the end of meiosis II, leaving only the ootid, which eventually develops into a mature ovum. The formation of polar bodies serves to discard the extra haploid sets of chromosomes that have resulted as a consequence of meiosis.
The Oogenesis Process : Diagram showing maturation of the ovum.