Menstrual Cycle Overview

Introduction

• The menstrual cycle is a monthly process involving the ripening and release of a mature egg and the preparation of the uterus for pregnancy.
• It occurs repeatedly between menarche (first menstrual period) and menopause (cessation of cycles).
• The average age of menarche is 12-13 years, with a normal range of 8-16 years.
• Factors like heredity, diet, and overall health can influence the age of menarche.
• The length of the menstrual cycle varies greatly, ranging from 21 to 35 days, with an average of 28 days.
• Menopause typically occurs around 52 years old, with a common range of 45-55 years. Menopause before 45 is considered premature.

Endocrine Control

• The hypothalamus releases gonadotropin-releasing hormone (GnRH) or luteinizing releasing hormone (LHRH).
• The pituitary gland produces follicle-stimulating hormone (FSH) and luteinizing hormone (LH).
• The ovaries produce estrogen and progesterone.

Hypothalamic-Pituitary-Ovarian Axis

• The hypothalamus controls the secretion of pituitary hormones and is influenced by the central nervous system.
• GnRH from the hypothalamus stimulates the anterior pituitary to release gonadotropins (FSH and LH).
• The anterior pituitary controls ovarian function through cyclic secretion of gonadotropins.

Follicle-Stimulating Hormone (FSH)

• FSH levels rise in the late previous cycle and peak around day 5 of the new cycle, before rising again around day 12.
• FSH is responsible for:
  • Ripening and maturation of ovarian Graafian follicles.
  • Stimulating estrogen secretion by growing follicles.
  • Contributing to ovulation alongside LH.

Luteinizing Hormone (LH)

• LH levels remain relatively constant except for a sudden peak around day 12 or 13 of the cycle.
• LH is responsible for:
  • Promoting follicular ripening alongside FSH.
  • Triggering ovulation (rupture of the mature Graafian follicle).
  • Transformation of the ruptured follicle into the corpus luteum.
  • Stimulating the corpus luteum to produce estrogen and progesterone.

Prolactin

• Prolactin release is inhibited by FSH and LH.
• Its role in ovulation control is not fully understood, but it may prevent further ovulation after fertilization.

Physiology of the Menstrual Cycle

• The cycle is divided into the ovarian cycle and the uterine cycle.

The Ovarian Cycle

• Consists of three phases:
  • Follicular phase (day 1-13)
  • Ovulatory phase (day 14)
  • Luteal phase (day 15-28)

Follicular Phase (Day 1-13)

• FSH stimulates the development of a primordial follicle into a mature Graafian follicle.
• Multiple follicles may start maturing, but usually only one reaches full maturity and ovulates.
• The empty follicle (corpus luteum) eventually atrophies, becoming the corpus albicans.
• As the Graafian follicle matures, it increasingly produces estrogen, which inhibits FSH secretion (negative feedback).

Ovulatory Phase (Day 14)

• Ovulation is the release of a mature ovum from the Graafian follicle into the fallopian tube.
• The LH surge, starting around day 12 and lasting 48 hours, triggers ovulation.
• The mature ovum is about 20 mm in diameter.
• Once released, the ovum is swept into the fallopian tube by the fimbria.
• An unfertilized ovum disintegrates in the fallopian tube after about a day.
• Fertilization usually occurs in the ampulla of the fallopian tube.
• A fertilized ovum undergoes embryogenesis.

Luteal Phase (Day 15-28)

• The empty follicle (corpus luteum) produces estrogen and progesterone.
• If the ovum is not fertilized, the corpus luteum degenerates, estrogen and progesterone levels fall, and menstruation begins.
• If the ovum is fertilized, the developing embryo secretes human chorionic gonadotropin (HCG), which prevents the degeneration of the corpus luteum.
• The corpus luteum continues to produce estrogen and progesterone, preventing menstruation and indicating pregnancy.

Menstrual Cycle Feedback Mechanisms

• Decreased estrogen levels stimulate the hypothalamus to release GnRH at a higher rate (positive feedback).
• Increased GnRH stimulates the anterior pituitary to produce FSH (positive feedback).
• Increased estrogen levels inhibit GnRH release (negative feedback).

The Uterine Cycle

• Consists of three phases:
  • Menstruation
  • Proliferative phase
  • Secretory phase

Menstruation

• The first phase of the uterine cycle, marking non-pregnancy.
• Involves periodic shedding of the endometrium with blood loss.
• Occurs due to the degeneration of the corpus luteum.
• Normal menstruation lasts 3-5 days, but 2-7 days is considered normal.
• Average blood loss is 50-80 ml.
• Menstrual flow is typically dark red and consists of red blood cells, cervical mucus, epithelial cells, and bacteria.

Proliferative Phase

• The second phase, where estrogen stimulates endometrial growth (proliferation).
• Maturing ovarian follicles secrete increasing amounts of estradiol.
• Estrogen initiates the formation of a new endometrial layer.
• It also stimulates cervical crypts to produce fertile cervical mucus.

Secretory Phase

• The final phase of the uterine cycle, where progesterone, produced by the corpus luteum, prepares the endometrium for implantation.
• Progesterone increases blood flow and uterine secretions, while reducing uterine muscle contractility.

Explore the phases of the menstrual cycle, from menarche to menopause, and understand the endocrine control involved. This quiz covers key hormonal interactions and factors affecting menstrual health, providing essential insights into a vital physiological process.