Human Anatomy: Uterine Layers and Menstrual Cycle

Uterine Structure

• The uterus has three main layers:
  • Endometrium: Inner mucosal lining.
  • Myometrium: Thick layer of smooth muscle.
  • Perimetrium: Serosa or peritoneal covering of the uterus.

Endometrium Composition

• The endometrium consists of three layers:
  • Stratum compactum: Compact layer.
  • Stratum spongiosum: Spongy layer.
  • Stratum basale: Basal layer.

Menstrual Cycle Phases

• The menstrual cycle includes 3 or 4 overlapping phases:
  • Menstrual phase
  • Proliferative phase
  • Secretory phase
  • Ovulation is also considered a separate phase.

Follicular Phase

• Lasts from the first day of menstruation until ovulation (14 to 21 days).
• Characterized by the influence of estrogen from developing ovarian follicles.
• Involves growth of ovarian follicles, leading to one mature ovum.
• Most fertile period occurs at the end of this phase.

Luteal Phase

• Begins after ovulation and lasts about 14 days, ending with menstrual flow.
• Governed by progesterone secreted by the corpus luteum.
• Progresses through an ischaemic phase if pregnancy does not occur.

Menstrual Phase Dynamics

• Lasts for 3-7 days; marked by shedding of compact and spongy endometrial layers if pregnancy does not occur.
• Degeneration of the corpus luteum leads to a drop in estrogen and progesterone, causing menstrual bleeding.
• Menstrual flow consists of:
  • Blood
  • Degenerated endometrial tissue
  • Secretions from uterine glands (rich in lipids and glycogen).
• Average blood loss during menstruation is about 35-45 mL (range 5-80 mL).

Proliferative Phase

• Lasts for about 6-14 days post-menstruation.
• Coincides with ovarian follicle growth and increased estrogen production.
• Endometrial thickness increases threefold, marking distinct endometrial layers.

Secretory Phase (Luteal Phase)

• Constant duration of 14 days post-ovulation.
• Endometrium reaches maximum thickness under progesterone influence.
• Glands become tortuous, distended with fluid rich in glycogen and lipids.

Hormonal Fluctuations

• Estrogen peaks two days before ovulation, triggering a surge in LH.
• LH surge initiates ovulation; FSH peaks shortly before.
• If fertilization does not occur, granulosa cells secrete inhibin, suppressing gonadotropin release and causing regression of the corpus luteum and shedding of the endometrium.

Signs of Ovulation

• Body Temperature: Biphasic pattern with a drop followed by an increase post-ovulation.
• Cervical Mucus: Becomes thin, clear, and profuse pre-ovulation; thickens post-ovulation.
• Mittelschmerz Pains: Mild lower abdomen pain due to irritation from blood or fluid during ovulation.
• Increased Ferning: Vaginal smears show epithelial cornification at ovulation.

Menstrual Cycle Regulation

• Regulated by the hypothalamus, which secretes GnRH affecting the anterior pituitary to release FSH and LH for ovary cyclic changes.

Anovulatory Menstrual Cycle

• Occurs without ovulation, leading to absence of the luteal phase.
• Minimal changes happen in the endometrium.