The Menstrual Cycle

The Menstrual Cycle

Overview

The menstrual cycle includes two interconnected cycles: the ovarian cycle and the uterine cycle, which are represented on the back of pages 8 and 10.

Estrogen and progesterone levels overlap in both diagrams.

We will begin by examining the role of the pituitary gland in relation to the developing follicle and corpus luteum, focusing on the ovarian cycle (page 7).

Ovarian Cycle

Follicle Stimulating Hormone (FSH)
  • Label A on the diagram represents FSH levels.
  • FSH is regulated by negative feedback, meaning that high levels of estrogen suppress its release.
Luteinizing Hormone (LH)
  • Label B represents LH.
  • LH is also regulated by negative feedback, with high levels of progesterone inhibiting its production.
  • Both LH and FSH are produced by the anterior pituitary gland.
Phases of the Cycle
  • The shaded area indicates the flow phase, during which menstruation occurs.
  • During the flow phase, a new follicle begins to develop.
  • The follicle matures until ovulation, which typically occurs around day 14 of a 28-day cycle.
  • After ovulation, the remaining follicle transforms into the corpus luteum.
  • The corpus luteum can be either fresh or degenerating.
  • The length of the menstrual cycle can vary, typically ranging from 4 to 7 days.
Step-by-Step Hormone Regulation

  1. Low Hormone Levels:

    • Low levels of LH, FSH, estrogen, and progesterone stimulate the release of gonadotropin-releasing hormone (GnRH) from the hypothalamus.
    • Low hormone levels can lead to mood changes, such as crankiness or emotional sensitivity, due to hormone withdrawal.

  2. GnRH Release:

    • GnRH stimulates the anterior pituitary gland to release FSH and LH.
    • Hormone levels are at their lowest around day zero, but GnRH causes them to rise quickly.

  3. Follicle Development and Estrogen Release:

    • FSH stimulates the development of a new follicle in the ovary.
    • As the follicle develops, it releases estrogen.

  4. Ovulation Trigger:

    • High levels of estrogen trigger a surge of GnRH, leading to a massive release of LH and FSH (positive feedback).
    • This hormone surge causes ovulation.
    • The surge can also increase body temperature by up to half a degree, indicating ovulation.

  5. Corpus Luteum Development:

    • LH supports the development and maintenance of the corpus luteum.
    • The corpus luteum releases estrogen and progesterone.
    • High levels of progesterone exert negative feedback on LH production, causing LH levels to decrease.

  6. Corpus Luteum Degeneration:

    • With lower levels of LH, the corpus luteum is no longer supported and degenerates.
    • Hormone levels return to low levels, restarting the cycle (approximately every 28 days).
Phases of the Ovarian Cycle (Detailed)
  • Follicular Phase (Days 0-14):
    • The follicle is developing.
  • Ovulation (Day 14):
    • Ovulation typically occurs.
  • Luteal Phase (Days 15-28):
    • The corpus luteum is present.
  • As the oocyte develops, follicular cells surround it.
  • The follicle produces increasing amounts of estrogen.
  • Estrogen triggers the release of GnRH, LH, and FSH, leading to ovulation.
  • After ovulation, the remaining follicle becomes the corpus luteum.
  • Granulosa cells convert to corpus luteum cells, which release progesterone and estrogen.

Uterine Cycle

Overview

The uterine cycle occurs simultaneously with the ovarian cycle, described on page 8.

Estrogen and Progesterone Levels
  • Estrogen levels peak first, causing a surge of GnRH.
  • Progesterone levels become very low as the corpus luteum degenerates.
  • The uterine lining (endometrium) changes in thickness in response to these hormones, as shown in the bottom graph.
Stages of the Uterine Cycle

  1. Menstruation:

    • Low levels of all hormones induce menstruation, during which the uterine lining is shed.

  2. Proliferative Phase (Days 4-14):

    • As estrogen levels increase from the developing follicle, the uterine lining thickens and becomes more vascular and glandular.

  3. Secretory Phase (Post-Ovulation):

    • The corpus luteum secretes estrogen and progesterone, which continue to thicken the uterine lining.
    • Progesterone causes the glands to secrete mucus to help the egg stick to the uterine lining.

  4. Menstruation (If No Pregnancy):

    • If pregnancy does not occur, the corpus luteum degenerates, and estrogen and progesterone levels drop.
    • The uterine lining is no longer supported by hormones, leading to menstruation.
Development from Ovulation to Implantation (Assuming Fertilization)
  1. Zygote Formation
  2. Early Cleavage (Four-cell stage)
  3. Morula Stage
  4. Early Blastocyst
  5. Late Blastocyst (Ready for implantation)
  • Implantation should occur during the late blastocyst stage.
If Pregnancy Occurs
  • The developing embryo secretes high levels of human chorionic gonadotropin (hCG).
  • hCG prevents the corpus luteum from degenerating.
  • The corpus luteum continues to secrete progesterone and estrogen, maintaining the uterine lining.

Hormone Control of Ovarian and Uterine Cycles

The uterine cycle is controlled by hormones produced during the ovarian cycle.

  • LH and FSH, which are from the pituitary gland, influence the developing follicle and corpus luteum, which in turn release estrogen and progesterone.
    *The Ovarian can be divided into the Follicular phase and luteal phase.
    *The Uterine cycle can be divided into the menstrual phase or menses (Days 0-4), the proliferative phase (Days 4-14), and the secretory phase (Days 14-28).
  • During the end of the cycle we see that the lining starts to shed at the end when the hormones are very low.

Birth

Initiation of Labor
  • Birth is initiated by changing hormone levels in the fetus, signaling the placenta to initiate labor.
  • The placenta produces prostaglandins, causing the uterus to contract and push the fetus downwards.
  • Uterine contractions cause the release of oxytocin, creating a positive feedback loop.
  • Pressure from the baby's head against the cervix signals the mother's hypothalamus.
  • The hypothalamus signals the posterior pituitary to release more oxytocin.
  • Oxytocin and prostaglandins cause continuous uterine contractions until the baby is born.
Stages of Labor
  1. Dilation: The body starts to dilate and position the baby's head.
  2. Delivery: The baby is delivered, ideally head first.
  3. Placental Delivery: After the baby is born, the body releases a surge of hormones to deliver the placenta (afterbirth).

Development After Implantation

  • The umbilical cord starts to form from the body stalk.
  • The amnion, amniotic fluid, forming mesoderm, endoderm, and ectoderm are present.
  • The embryo develops and implants.
  • The surrounding tissues develop into the placenta.