Menstrual Cycle Overview

Introduction

• The menstrual cycle: regular changes in ovaries and endometrium enabling reproduction.
• The endometrium: tissue layer lining inside of uterus.
  • Functional layer: subject to hormonal changes, shed during menstruation.
  • Basal layer: feeds the functional layer.
• Two synchronized processes:
  • Ovarian cycle: development of ovarian follicles and ovulation.
  • Uterine (endometrial) cycle: thickening and shedding of functional endometrium.

Key Events

• Menarche: onset of first menstrual period during adolescence.
• Monthly cycle varies: 20 to 35 days; average 28 days.
• Cycle begins on first day of menstruation, referred to as day one.
• Ovulation: release of oocyte 14 days before next cycle.
  • Pre-ovulatory phase: 14 days leading up to ovulation.
  • Post-ovulatory phase: 14 days following ovulation.

Ovarian Cycle Phases

Follicular Phase (Weeks 1-2)

• Controlled by hypothalamus and pituitary gland.
  • Hypothalamus: secretes gonadotropin releasing hormone (GnRH).
  • Anterior pituitary gland: releases follicle stimulating hormone (FSH) and luteinizing hormone (LH) in response.
• Before puberty: GnRH released steadily.
• After puberty: GnRH pulses.
• Pituitary hormones: control maturation of ovarian follicles.
  • Follicle: primary oocyte + theca and granulosa cells.
• First 10 days:
  • Theca cells bind LH → secrete androstenedione.
  • Granulosa cells bind FSH → produce aromatase → converts androstenedione to 17-β-estradiol.
• Days 10-14:
  • Granulosa cells develop LH receptors.
  • Increased estrogen: negative feedback on pituitary → less FSH.
  • Only dominant follicle continues to grow, secretes estrogen → positive feedback → LH & FSH surge → ovulation.

Luteal Phase (Weeks 3-4)

• Corpus luteum forms from remnant follicle cells.
  • Secretes androstenedione and estrogen (17-β-estradiol).
• Increased progesterone production due to P450scc enzyme.
  • Progesterone: negative feedback on pituitary → decreased FSH and LH.
  • Inhibin: reduces pituitary FSH production.
  • Dominant hormone: progesterone.
• Uterus prepares for possible implantation (secretory phase).
  • Spiral arteries and uterine glands grow and secrete mucus.
  • Cervical mucus thickens post-ovulation.
• Corpus luteum degenerates → corpus albicans → hormone levels drop → menstruation begins.

Uterine Cycle Phases

Menstrual Phase (Days 1-5)

• Shedding of old endometrial lining.

Proliferative Phase (Days 6-14)

• High estrogen: stimulates endometrial thickening, gland growth, and spiral arteries formation.
• Cervical mucus becomes hospitable to sperm.
• Fertilization chance highest between day 11-15.

Secretory Phase (Days 15-28)

• Influenced by progesterone.
  • Spiral arteries coiled, uterine glands secrete more mucus.
  • Cervical mucus less hospitable to sperm.

Summary

• Menstrual cycle:
  • 1st day of menstruation marks beginning.
  • Follicular phase (1st 14 days): ovaries prepare egg, uterus sheds and rebuilds endometrium.
  • Ovulation: day 14 caused by estrogen-induced LH surge.
  • Luteal phase (last 14 days): progesterone dominant, prepares endometrium for implantation.
• Cycle ends with menstruation initiating a new cycle.