Lecture 2B

Hormonal Control of Oocyte Maturation

• Hormonal control of oocyte maturation sets in with the onset of puberty, when the primary oocyte transforms into the secondary follicle.
• Folliculogenesis, the formation of granulosa cells around the oocyte, is coupled with oogenesis, the growth of the oocyte.

Role of Gonadotropic Hormones

• Gonadotropic hormones, specifically FSH (follicle-stimulating hormone) and LH (luteinizing hormone), are produced with the onset of puberty.
• FSH targets granulosa cells, stimulating them to increase in number, which in turn leads to an increase in estrogen levels.
• The higher the FSH stimulation, the greater the increase in granulosa cells and estrogen levels.
• Estrogen levels then activate a feedback regulation on the pituitary gland, stimulating the release of LH.

Role of LH

• LH is released at lower levels than FSH and is activated by the levels of estrogen released by the granulosa cells.
• LH acts on theca cells, stimulating the synthesis and secretion of testosterone.
• Testosterone is converted into estrogen by an aromatase enzyme and a cyclic AMP generating system.
• The increased estrogen levels further stimulate the release of LH, creating a cyclical hormonal activity.

Maturation and Growth of the Oocyte

• The maturation and growth of the oocyte, along with the formation of follicle cells, are governed by the interplay of hormones: FSH, LH, and estrogen.
• The FSH acts on the granulosa cells, which then secrete and synthesize estrogen.
• The estrogen, in turn, stimulates the release of LH, which acts on the theca cells to synthesize and secrete testosterone.

Sequence of Follicle Maturation

• At birth, all female mammals have primary oocytes in their ovaries, which go through various stages of maturation.
• Only one primordial follicle completes maturation up to the Graafian follicle per month, becoming the secondary oocyte released at ovulation.
• The process of releasing the secondary oocyte from the ovary is ovulation, which occurs at the middle of the month.

Post-Ovulation

• After ovulation, the antrum (cavity filled with liquid) releases its liquid, crumpling into a shrunken structure called the corpus luteum.
• The corpus luteum undergoes further crenation, eventually degenerating into a corpus albicans, which is incorporated into the connective tissue of the ovary.

Difference between Atretic Follicle and Corpus Luteum

• Corpus Luteum: bigger size, distinct membrana granulosa, and thicker theca externa and interna.
• Atretic Follicle: undifferentiated wall.