Female Reproductive Cycle

Female Reproductive Cycle Events

Overview

• The monthly reproductive cycle in females involves two key processes:
  • Development of a group of follicles in the ovary.
  • Ovulation: Release of a mature egg from the ovary surface.
  • Changes in the uterus involving the sloughing and rebuilding of the stratum functionalis.

Follicle Development

• Primordial Follicles:
  • Females are born with a fixed number of primordial follicles.
  • The egg within is diploid (normal chromosome number).
• Follicle Development Initiation:
  • Triggered by FSH (follicle-stimulating hormone) and estrogen.
  • The egg starts meiosis (chromosome number reduction).
  • Follicular cells surrounding the egg mature and divide.
• Primary Follicles:
  • Larger than primordial follicles.
  • Follicular cells become cuboidal.
  • The egg enlarges.
• Secondary Follicles:
  • Multiple layers of follicular cells.
  • Evidence of meiosis is visible (chromosomes sorting).
  • Antrum development: Accumulation of fluid within the follicle.
• Mature Graafian Follicle:
  • A fully mature secondary follicle.
  • Antrum: A fluid-filled cavity containing hormone-rich fluid.
  • Zona Pellucida: A clear glycoprotein layer develops around the egg.
  • Corona Radiata: Special follicular cells that surround the egg and zona pellucida; they accompany the egg upon ovulation.
• Corpus Luteum Formation:
  • After ovulation, the remaining follicular cells and antrum transform into the corpus luteum (yellow body).
  • The follicular cells undergo changes, becoming highly folded.

Uterine Wall Changes

• Stratum Functionalis: This layer of the uterine wall undergoes cyclical sloughing and regrowth.
  • Sloughing: Loss of the stratum functionalis down to the stratum basale (stem cell layer).
  • Regrowth: Rebuilding of the stratum functionalis, requiring the regrowth of spiral arteries.

Phases of the Monthly Reproductive Cycle

• The average cycle is 28 days, with variations among individuals (e.g., 25 to 35 days).
• Ovulation typically occurs around day 14 (mid-cycle).

Phases:

1. Menstrual Phase:
   • Sloughing of the stratum functionalis.
2. Follicular (or Proliferative) Phase:
   • Follicle maturation in the ovary.
   • Regrowth (proliferation) of the stratum functionalis.
3. Ovulation:
   • The release of the egg.
4. Luteal (or Secretory) Phase:
   • Formation of the corpus luteum in the ovary.
   • Continued growth and secretory activity of the stratum functionalis.

Hormone Level Charts

• Charts track:
  • Anterior pituitary hormones: FSH and LH (luteinizing hormone).
  • Ovarian hormones: Estradiol (main estrogen) and progesterone.
  • Basal body temperature (BBT).
• Basal Body Temperature:
  • Body temperature measured upon waking before any activity.

Menstrual Phase (Days 1-5/7)

• Hormone Profile:
  • High FSH levels, low levels of estrogen, progesterone, and LH.
• Ovarian Activity:
  • FSH stimulates the development of a group of primordial follicles (number varies).
  • The oocyte (egg) starts meiosis, and follicular cells change shape and divide.
• Uterine Activity:
  • Sloughing of the stratum functionalis due to the drop in estrogen and progesterone levels from the previous cycle.
  • Menstrual flow consists of blood (from spiral artery constriction) and epithelial tissue from the stratum functionalis.

Follicular (Proliferative) Phase

• Hormone Profile:
  • Increasing estrogen (estradiol) levels, no significant LH or progesterone production at the start
• Ovarian Activity:
  • Estrogen produced by developing follicles (follicular cells) causes:
    • Primary follicles develop into secondary follicles.
    • Enlargement of the egg and increase in follicular cell number and function.
    • Early antrum formation (fluid pockets).
    • Development of the zona pellucida.
  • Typically, only one secondary follicle matures into a Graafian follicle.
• Uterine Activity:
  • Estrogen stimulates the regrowth of the stratum functionalis, which begins to enfold.
  • Enfolding increases surface area, leading to glandular tissue formation (secretory epithelial cells).
  • These cells secrete lubricating and nutrient fluid for a potential fertilized egg.
• Late Follicular Phase & Cervical Mucus:
  • High estrogen levels trigger thinning of cervical mucus.
  • Cervix: The narrowed distal region of the uterus connecting to the vaginal tract.
  • Cervical Canal: Lined with cells that secrete mucus.
  • Infertile Mucus: Thick, sticky mucus that blocks sperm entry into the uterus.
  • Ferning: Under microscope, infertile mucus shows strands that prevent sperm passage. Sperm can't get through due to the random structure of the mucus threads.
  • Fertile Mucus: High estrogen levels cause the mucus to become thinner, slippery, and with avenues for sperm passage.
  • Women can track fertility by monitoring cervical mucus consistency, although this is not a method of birth control.

Ovulation (Event)

• Hormone Profile:
  • Luteinizing hormone (LH) surge: A rapid increase in LH secretion.
• Mechanism:
  • High estrogen levels from a mature Graafian follicle trigger the LH surge.
  • The LH surge causes the mature egg to rupture from the ovary surface.
• Ovulation Kits: Detect the LH surge in urine to predict ovulation.
• Visual Observation:
  • The Graafian follicle bulges on the ovary surface before rupture.
  • The egg, with its corona radiata, is released.
• Basal Body Temperature:
  • BBT drops slightly just before ovulation and then rises after ovulation due to progesterone secretion.
  • Elevated BBT for a few days indicates ovulation has occurred.
• Fertility Monitoring:
  • BBT tracking provides clues about fertility but is not a reliable method of birth control.
  • The egg's lifespan after ovulation varies.

Luteal (Secretory) Phase

• Hormone Profile:
  • Predominant hormones: Estradiol and progesterone from the corpus luteum.
• Ovarian Activity:
  • The corpus luteum forms from the remaining follicular cells and antrum after ovulation.
  • The corpus luteum secretes both estrogens and progesterone.
  • Progesterone causes the increase in basal body temperature.
  • The corpus luteum is yellow due to the lipids contained within the lutein cells.
• Fate of Corpus Luteum:
  • If no fertilization occurs: The corpus luteum persists for about 14 days, then degenerates and ceases hormone production.
  • If fertilization occurs: Human chorionic gonadotropin (HCG) from the fertilized egg/embryo maintains the corpus luteum.
• Pregnancy Tests: Detect HCG in urine, indicating pregnancy.
• Uterine Activity:
  • Uterine glands (enfolded epithelium of the stratum functionalis) continue to grow and become more secretory.
  • Glands become increasingly twisted to fit more cells and secrete nutrient-rich fluid (glycogen and mucus).
  • This provides a suitable environment for a fertilized egg to implant and survive.

Hormonal Changes Summary

• Estrogen: Rises as follicles mature, peaks before ovulation, and is then secreted by the corpus luteum.
• Progesterone: Not significant until after ovulation, when the corpus luteum produces it.
• No Fertilization: The corpus luteum degenerates, causing a sharp drop in estrogen and progesterone levels.
  • This drop triggers spiral artery constriction and the sloughing of the stratum functionalis (menstruation).
• Fertilization Occurs: HCG maintains the corpus luteum; progesterone and estrogen levels remain high.
  • Initially maintained by the corpus luteum, hormone production is taken over by the placenta around 3-4 months of pregnancy.

Postpartum Hormonal Changes

• After childbirth and placenta delivery, the source of progesterone and estrogen is lost.
• Hormone levels drop, leading to the sloughing of the stratum functionalis and the resumption of the menstrual cycle.
• During pregnancy, sustained hormone levels maintain the uterine lining, preventing menstruation.