Chapter 28G: The Female Reproductive System: The Uterine Cycle (Menses)

The Uterine Cycle (Menses)

Introduction to the Uterine Cycle

• The uterine cycle is also known as menses.

• It involves the degeneration of the functional zone in the uterus.

• This degeneration occurs in patches, and the functional zone is simultaneously repaired as it sloughs off.

Relationship with the Ovarian Cycle

• Recap of Ovarian Cycle: The ovarian cycle consists of two primary phases:

  • Follicular Phase: Pre-ovulatory phase.

  • Luteal Phase: Post-ovulatory phase.

• Uterine Cycle Phases: The uterine cycle has three phases:

  1. Menses

  2. Proliferative Phase

  3. Secretory Phase

• Synchronization:

  • The Menses and Proliferative phases of the uterine cycle occur concurrently with the Follicular Phase of the ovarian cycle.

  • The Secretory Phase of the uterine cycle occurs during the Luteal Phase of the ovarian cycle.

Phases of the Uterine Cycle

1. Menses (Degeneration of the Functional Zone)

• Cause: Constriction of the spiral arteries, leading to reduced blood flow to the functional zone.

• Mechanism: The weakened arterial walls rupture, releasing blood and connective tissue from the functional zone.

• Process: Degenerating tissue detaches in patches and enters the uterine lumen. The entire functional zone is eventually lost.

• Repair: Repair of the functional zone begins almost immediately as patches break away.

• Specificity: Only the functional zone is affected because other parts of the uterus are supplied by straight arteries, which are not impacted in the same way.

• Duration: Lasts from $1$ to $7$ days.

• Blood Loss: Results in the loss of approximately $35$ to $50$ milliliters of blood.

  • For reference: $1$ tablespoon is about $15$ milliliters.

  • Total loss is roughly equivalent to $2$ to $3$ tablespoons of blood.

• Definition: Menstruation specifically refers to the process of the sloughing off (shedding) of the endometrium.

2. Proliferative Phase

• Timing: Occurs during the latter part of the follicular phase of the ovarian cycle.

• Key Activity: Epithelial cells of the uterine glands multiply and spread across the endometrial surface.

• Purpose: This phase is characterized by significant growth and vascularization, aiming to restore the functional zone that was lost during menses.

• Correlation with Ovarian Cycle: This phase occurs at approximately the same time as primary and secondary follicles are developing and enlarging in the ovary.

• Hormonal Stimulation: The proliferative phase is stimulated and sustained by estrogen, which is secreted by the enlarging follicles.

• Characteristic: The functional zone becomes highly vascularized during this phase.

• Affected Arteries: The small arteries are primarily involved in the vascularization and subsequent constriction that leads to menses.

3. Secretory Phase

• Timing:

  • Activities within this phase peak approximately $12$ days after ovulation.

  • The phase itself typically lasts about $14$ days.

  • It occurs during the luteal phase of the ovarian cycle.

• Clinical Relevance (Ovulation Calculation): Since this phase lasts roughly $14$ days, ovulation can be estimated by counting backward about $14$ days from the onset of menses.

  • This calculation is critical for couples attempting to conceive, as the window for successful fertilization (when the oocyte is viable) is relatively short, typically about $12$ to $24$ hours post-ovulation.

• Termination: The secretory phase concludes when the corpus luteum in the ovary stops producing its stimulatory hormones (primarily progesterone and estrogen), leading to the breakdown of the endometrium and the onset of menses for the next cycle.

Key Terms

• Menarche ($\text{men-AR-kee}$):

  • Definition: The first uterine cycle.

  • Timing: Typically begins at puberty, around ages $11$ to $12$, though this can vary among individuals.

• Menopause ($\text{MEN-oh-pawz}$):

  • Definition: The natural termination of uterine cycles.

  • Timing: Generally occurs between the ages of $45$ and $55$, but individual variation is common.

• Amenorrhea ($\text{a-men-oh-REE-ah}$):

  • Primary Amenorrhea: The failure to initiate menses, meaning a failure to start menstrual cycles by the usual age of puberty.

  • Transient or Secondary Amenorrhea:

    • This is a temporary cessation of menstrual cycles after they have already begun.

    • It is not necessarily indicative of a disease or disorder.

    • Often caused by physical or emotional stress.

    • Example: It is not uncommon for endurance athletes, such as marathon runners or triathletes, to experience transient or secondary amenorrhea due to intense training and associated physical stress. The stress on the body can directly affect the regulation of the menstrual cycle.