Chapter 3: Anatomy and Physiology of the Female Reproductive System

Overview

This chapter focuses on the anatomy and physiology of the female reproductive system, including both external and internal structures, as well as the hormonal regulation of the menstrual cycle.

Function of the External Female Reproductive Organs

The external female reproductive organs are collectively called the vulva. Their primary functions include:
- Protecting the urethra and vaginal openings.

Components of the Vulva

Mons pubis: An elevated, rounded, fleshy prominence made up of fatty tissue that overlays the symphysis pubis.
Labia majora: Relatively large and fleshy, containing sweat and sebaceous glands which protect the vaginal opening.
Labia minora: Located just inside the labia majora, surrounding the openings to the vagina and urethra; these structures are vascularized and rich in nerve supply, aiding in lubrication and swelling with stimulation.
Clitoris: A small cylindrical mass of erectile tissue and nerves primarily responsible for sexual stimulation.
Prepuce: A hood-like covering over the clitoris; also the site of female circumcision practiced in some cultures.
Vestibule: An oval area enclosed by the labia minora that includes openings for the urethra, vagina, and two sets of glands (Bartholin and Skene glands) that secrete mucus for lubrication.
Introitus: The vaginal opening, which is surrounded by the hymen and is noted for its entry into the vagina.
Fourchette: A half-moon area located behind the opening of the vagina.
Perineum: The most posterior part of the external female reproductive organs, located between the vulva and the anus, consisting of skin, muscle, and fascia.

Internal Female Reproductive Organs

Layers of the Uterus

Endometrium: The innermost layer that lines the uterine cavity in nonpregnant individuals.
Myometrium: The muscular middle layer that constitutes the majority of the uterus, composed of smooth muscle linked by connective tissue.
Perimetrium: The outer serosal layer covering the body of the uterus.
Cervix: The lower part of the uterus, often referred to as the neck of the uterus; it opens into the vagina and provides a channel for sperm entry and menstrual discharge.

Breasts

The two mammary glands, or breasts, serve as accessory organs specialized for milk secretion following pregnancy.
They overlie the pectoralis major muscles, extending from the second to the sixth ribs, and laterally from the sternum to the axilla.
Each breast contains a nipple surrounded by a pigmented area of skin known as the areola.
Composed of approximately nine lobes (ranging from 4 to 18), which include glandular (alveolar) structures and lactiferous ducts leading to the nipple.
Fibrous connective tissue mixed with adipose tissue surrounds the lobes, with glandular tissue replacing adipose tissue in preparation for milk production.

The Female Reproductive Cycle

The female reproductive cycle encompasses the ovarian cycle, the endometrial (uterine) cycle, and the hormonal changes that regulate these processes.
The endometrium, ovaries, pituitary gland, and hypothalamus are integral to the cyclic changes preparing the body for fertilization.
Menstruation is characterized as the absence of fertilization, marked by the monthly shedding of the uterine lining.
Menopause is defined as the natural cessation of menstrual cycles.
The menstrual cycle is driven by the functional hypothalamic-pituitary-ovarian axis through a sequence of hormones that lead to ovulation.

Phases of the Cycle

Ovulation occurs during the ovarian cycle, while menstruation transpires during the endometrial cycle.
Both cycles are divided at mid-cycle by ovulation, where an ovum is released from its follicle into the fallopian tube, heading toward potential fertilization in the uterus.

Hormonal Regulation of the Menstrual Cycle

Key Hormones

Gonadotropin-releasing hormone (GnRH):
- Secreted from the hypothalamus in a pulsatile manner, increasing during the luteal phase.
- Induces the pituitary gland to release FSH and LH to facilitate ovulation.
Follicle-stimulating hormone (FSH):
- Secreted by the anterior pituitary gland and chiefly responsible for ovarian follicle maturation.
- Its levels peak during the first week of the follicular phase.
Luteinizing hormone (LH):
- Produced and secreted by the anterior pituitary gland, needed for maturation of preovulatory follicles and the luteinization of ruptured follicles.
- Stimulates ovulation and increases progesterone levels afterward.
Estrogen:
   - A steroid hormone promoting female sexual characteristics, secreted by the ovaries, crucial for follicle development.
   - Predominant at the end of the proliferative phase just prior to ovulation.
   - Post-ovulation, estrogen sharply declines as progesterone rises.
Progesterone:
- Secreted by the corpus luteum, vital after ovulation, promoting endometrial changes to prepare for pregnancy.
- Known as the "hormone of pregnancy" due to its calming effects on the uterus.
Prostaglandins:
- Hormone-like mediators of inflammation essential for normal functioning of the female reproductive system.
- Increase during follicular maturation and play pivotal roles in ovulation and menstrual cramps.

Summary of Hormonal Changes

Before ovulation, estrogen levels rise, leading to an FSH decrease due to negative feedback.
A surge in LH precedes ovulation, leading to corpus luteum establishment, which produces estrogen and progesterone.
After the corpus luteum degenerates, estrogen and progesterone levels decrease, promoting the cycle's restart through LH secretion.

The Ovarian Cycle

This cycle entails predictable changes in oocytes and ovarian follicles within the ovaries.
Females are born with a limited supply (approximately 600,000 to 1,000,000) of oocytes, reduced by half by puberty.
Typically, females ovulate one oocyte per month, spanning nearly 40 reproductive years.
The ovarian cycle contains three phases: follicular phase, ovulation, and luteal phase.

The Follicular Phase

Initiates on day 1 of the menstrual cycle and lasts until ovulation (approximately 10 to 14 days).
Its purpose is to develop a mature oocyte for fertilization.
Estrogen from maturing follicles induces endometrial proliferation, preparing for potential implantation.
FSH stimulates the ovary, leading to the maturation of targeted follicles, culminating in an LH surge for final follicular development and release.

Ovulation

Characterized by the release of an oocyte from an ovary into the fallopian tube where fertilization can occur.
All typically transpire 36 hours post-LH surge, on average around day 14 in a typical 28-day cycle.
Ovulation triggers a drop in estrogen, with the mucosal changes aiding sperm transport.
Symptoms may include cervical mucus changes, spotting, increased discharge, slight temperature rise, and possible mittelschmerz (pain).

The Luteal Phase

Begins immediately after ovulation and lasts until the next menstrual phase (days 15-28 in a 28-day cycle).
The ruptured follicle forms a corpus luteum that secretes rising levels of progesterone, inducing further endometrial changes.
Elevated temperatures indicate progesterone influence and can signal early pregnancy if sustained.
In absence of fertilization, the corpus luteum degenerates which leads to a decline in hormones and subsequent endometrial involution.

The Endometrial (Uterine) Cycle

Reflects the cyclic hormonal changes and contains three phases:

Proliferative Phase:
   - Aligns with the ovarian follicular phase, characterized by enlarged endometrial glands due to elevated estrogen levels.
   - Cervical mucus becomes more favorable for sperm.
   - Begins around day 5 until ovulation.
Secretory Phase:
   - Initiated by progesterone post-ovulation, culminating in a thickened and vascularized endometrium.
   - Lasts from days 15 to 28, fading in absence of fertilization due to corpus luteum degeneration.
Ischemic Phase:
- If fertilization does not occur, hormone levels drop, leading to arterial spasm and eventual shedding of the endometrium, culminating in menstrual flow.

Menstrual Flow and Puberty Considerations

Menstruation marks the end of a cycle and initiates a new one, defined by the shedding of the endometrial lining.
Menarche: The first menstrual period, with an average onset of 12.4 years in the U.S. (range: 10-16 years).
Normal menstruation lasts less than or equal to 8 days and should not disrupt quality of life, with an average volume less than or equal to 80 mL per cycle.

Cultural Considerations

Attitudes toward menstruation vary significantly worldwide; education and access to menstrual care are crucial for managing symptoms and improving perceptions regarding menstruation among young women.

Practice Question

A patient reports their menstrual period occurs every 28 days. When is ovulation expected to occur?
  - Options:
    a. Day 10
    b. Day 14
    c. Day 18
    d. Day 22

Answer

Correct Answer: b. Day 14. Ovulation typically occurs on day 14 in a 28-day cycle, with the follicular phase lasting from day 1 to ovulation and the secretory phase commencing at ovulation through to three days before the next period begins.