Menstrual Cycle Overview

Overview

This lecture explains the regulation and phases of the menstrual cycle, highlighting the hormonal control and interplay between the ovarian and uterine cycles.

Menstrual Cycle Basics

• The menstrual cycle involves changes in female reproductive hormones and structures, averaging 28 days but varying between individuals.
• The cycle prepares the female reproductive system for potential pregnancy.
• The menstrual cycle consists of two concurrent cycles: the ovarian cycle (ovary changes) and the uterine cycle (uterus changes).

Structures Involved

• Ovaries release an egg, fallopian tubes transport it, and the uterus can host implantation.
• Typically, one ovary releases one egg per cycle; if fertilized, the egg implants in the uterus.

Phases of the Ovarian and Uterine Cycles

Ovarian Cycle

• Follicular phase: Follicles in the ovary grow, each containing an egg; only one will ovulate.
• Ovulation (around day 14): Dominant follicle releases an egg.
• Luteal phase: Ruptured follicle becomes the corpus luteum, which secretes hormones.

Uterine Cycle

• Menstrual flow phase: Shedding of uterine lining (menses/period, days 1–5).
• Proliferative phase: Uterine lining (endometrium) regrows, preparing to support a fertilized egg.
• Secretory phase: Endometrium continues building, ready for blastocyst implantation.

Hormonal Regulation of the Menstrual Cycle

• Hypothalamus releases GnRH (gonadotropin-releasing hormone), stimulating the anterior pituitary.
• Anterior pituitary releases FSH (stimulates follicle growth) and LH (triggers ovulation, supports hormone production).
• Ovaries secrete estradiol (a form of estrogen) and progesterone, which regulate the uterine lining.

Hormonal Feedback Mechanisms

• Low estradiol levels cause negative feedback, inhibiting GnRH, FSH, and LH secretion, keeping their levels low.
• High estradiol levels in late follicular phase cause positive feedback, leading to an LH surge and ovulation.
• After ovulation, corpus luteum produces estradiol and progesterone, inhibiting GnRH, FSH, and LH by negative feedback.
• If no implantation occurs, corpus luteum breaks down; estradiol and progesterone drop, uterine lining sheds, and a new cycle begins.

Pregnancy and Hormonal Changes

• If implantation occurs, blastocyst releases hCG, maintaining the corpus luteum and high progesterone to support the uterine lining.
• The placenta eventually assumes hormonal roles during pregnancy.

Key Terms & Definitions

• Menstrual cycle — recurring changes in reproductive hormones and structures preparing for pregnancy.
• Ovarian cycle — events and changes in the ovary.
• Uterine cycle — changes in the uterus; includes menstrual flow, proliferative, and secretory phases.
• Follicle — a structure containing an egg in the ovary.
• Corpus luteum — hormone-secreting structure from the ruptured follicle after ovulation.
• GnRH — hormone from hypothalamus prompting pituitary hormone release.
• FSH — pituitary hormone stimulating follicle growth.
• LH — pituitary hormone triggering ovulation.
• Estradiol — an estrogen important for growth of the uterine lining.
• Progesterone — hormone maintaining the uterine lining.
• hCG — hormone from a blastocyst that signals pregnancy and maintains the corpus luteum.

Action Items / Next Steps

• Review provided diagrams of hormone levels throughout the cycle.
• Read further on feedback mechanisms and selection of the dominant follicle if desired.