Menstrual Cycle and Fertilization

Menstruation Overview

• Definition: Menstruation is the shedding of the endometrium (uterine lining) when no pregnancy occurs.

• Process: Blood and tissue exit through the vagina, resulting in menstrual bleed.

• Duration: Typically lasts between 3 to 5 days but can vary widely among individuals.

• Hormone Levels:

  • Progesterone levels are low, indicating no pregnancy and allowing for the shedding of the lining.

Follicular Phase

• Definition: This is the first half of the menstrual cycle characterized by the development of the ovarian follicle.

• Hormonal Activity:

  • Follicle Stimulating Hormone (FSH) from the brain stimulates multiple follicles to develop.

  • One follicle becomes dominant and matures, secreting estrogen.

• Estrogen Levels:

  • There is a direct correlation between follicle development and rising estrogen levels, contributing to the repair and growth of the uterine lining.

• Inhibition Mechanism:

  • As estrogen reaches a certain level, it exerts negative feedback that reduces FSH, ensuring typically only one follicle matures.

Ovulation Phase

• Trigger:

  • A spike in Luteinizing Hormone (LH) triggers ovulation, the release of the mature egg from the Graafian follicle.

• Pre-Ovulation Hormones:

  • Estrogen levels rise during follicle maturation before dropping right at ovulation as the follicle is ruptured.

Luteal Phase

• Definition: Post-ovulation phase where the ruptured follicle transforms into the corpus luteum.

• Hormone Production:

  • Corpus luteum secretes progesterone and some estrogen.

• Hormone Levels During Luteal Phase:

  • Progesterone levels rise due to the functioning corpus luteum.

  • If no pregnancy occurs, the corpus luteum degrades, leading to a drop in progesterone and estrogen levels.

• Cycle Reset:

  • Decrease in progesterone triggers the next menstruation and cycle begins anew.

Hormonal Patterns in the Menstrual Cycle

• Estrogen Patterns:

  • Significant rise during the follicular phase due to follicle development, then falls post-ovulation, and may increase again due to corpus luteum activity.

• Progesterone Patterns:

  • Spikes during the luteal phase and drops sharply if there is no pregnancy.

• FSH and LH Patterns:

  • FSH initially rises to stimulate follicles but decreases once estrogen levels rise.

  • LH spikes sharply to induce ovulation and then declines post-ovulation.

Negative Feedback Mechanisms

• High Estrogen Levels:

  • Inhibit FSH secretion, preventing overstimulation of follicles when estrogen is already high.

Fertilization Overview

• Definition: Fertilization is the fusion of sperm and egg to form a zygote.

• Process:

  • Sperm must meet and fuse with the egg, releasing genetic material into the egg cell, resulting in a zygote.

• Role of Cortical Reaction:

  • Prevents polyspermy; upon the entry of a sperm, the egg's outer layer hardens.

• Genetic Combination:

  • 23 chromosomes from the sperm combine with 23 from the egg, forming a total of 46 chromosomes in the zygote.

In Vitro Fertilization (IVF)

• Definition: A reproductive technology that allows fertilization outside the body to help couples conceive.

• Process Overview:

  1. Suppression of menstrual cycle and reproductive hormones to perform a "reset."

  2. Induction of super ovulation to produce multiple eggs instead of one.

  3. Collection of mature eggs via ultrasound.

• Importance of Super Ovulation:

  • Increases the chances of a successful pregnancy by providing multiple viable eggs for fertilization.