# Lecture 22: The Female Reproductive System

## Anatomy and Physiology II

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## Learning Objectives

By the end of this lecture, students will be able to:

1. Identify the major organs of the female reproductive system and their functions
2. Describe the anatomy and histology of the ovary, including follicular stages
3. Outline the process of oogenesis and compare it with spermatogenesis
4. Describe the anatomy of the uterine tubes, uterus, vagina, and external genitalia
5. Explain the ovarian cycle (follicular phase, ovulation, luteal phase)
6. Explain the uterine (menstrual) cycle and its coordination with the ovarian cycle
7. Describe the hormonal regulation of the female reproductive cycle (HPG axis)

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## Lecture Content

### I. Overview of the Female Reproductive System

The female reproductive system performs multiple functions: production of oocytes (oogenesis), production of sex hormones (estrogen and progesterone), reception of sperm, providing the site of fertilization, supporting the developing embryo and fetus, childbirth, and lactation. Its organs include the **ovaries** (gonads) that produce oocytes and sex hormones; the **uterine (fallopian) tubes** that serve as the site of fertilization and transport the oocyte or embryo to the uterus; the **uterus** that provides the site of embryo implantation and fetal development; the **vagina** that serves as the birth canal and receives sperm during intercourse; the **external genitalia (vulva)** including the mons pubis, labia majora, labia minora, clitoris, and vestibular glands; and the **mammary glands** that produce milk (covered in Lecture 23).

### II. The Ovaries

The ovaries are paired, almond-shaped organs approximately 3 cm long and 1.5 cm wide, located in the lateral pelvic wall. They are held in place by the **ovarian ligament** (anchoring the ovary to the uterus), the **suspensory ligament** (attaching the ovary to the pelvic wall and containing the ovarian artery and vein), and the **mesovarium** (a fold of the broad ligament).

Histologically, the ovary is covered by a **germinal epithelium** (simple cuboidal or squamous epithelium, which is a misnomer as it does not produce germ cells). Beneath this lies the **tunica albuginea**, a dense connective tissue capsule. The **ovarian cortex** contains ovarian follicles at various stages of development embedded in connective tissue stroma, while the **ovarian medulla** is the central region containing blood vessels, lymphatics, nerves, and loose connective tissue.

#### Ovarian Follicle Development

At birth, the ovaries contain approximately 1-2 million **primordial follicles**, each containing a primary oocyte arrested in prophase I of meiosis. By puberty, approximately 300,000-400,000 remain (the rest having undergone atresia, or follicular degeneration), and only approximately 400-500 follicles will ovulate during a woman's reproductive years.

Follicular development progresses through several stages. The **primordial follicle** consists of a primary oocyte surrounded by a single layer of flat follicular (granulosa) cells. In the **primary follicle**, the oocyte enlarges, follicular cells become cuboidal and proliferate into multiple layers of **granulosa cells**, and the **zona pellucida** (a glycoprotein layer) forms between the oocyte and granulosa cells. The **secondary (antral) follicle** develops fluid-filled spaces between granulosa cells that coalesce into a central cavity called the **antrum**, and a **theca** layer develops from surrounding stromal cells. The theca interna is vascularized and produces androgens under LH stimulation, while the theca externa provides a fibrous outer layer. The **mature (Graafian) follicle** is a large, fluid-filled structure approximately 2.5 cm in diameter in which the oocyte sits on a stalk of granulosa cells called the **cumulus oophorus**, surrounded by an innermost layer called the **corona radiata**. This follicle is ready for ovulation. After ovulation, the ruptured follicle becomes the **corpus luteum**, which secretes progesterone and estrogen. If no pregnancy occurs, the corpus luteum degenerates into the **corpus albicans**, scar tissue that replaces it.

### III. Oogenesis

Oogenesis is the process of producing mature female gametes (ova). Unlike spermatogenesis, it begins during fetal development, is interrupted, and is completed only if ovulation and fertilization occur.

#### Stages

During the **fetal period**, oogonia (diploid stem cells) undergo mitosis to produce primary oocytes (2n). Primary oocytes begin **meiosis I** but arrest in **prophase I** (the dictyate stage), remaining arrested from fetal life until ovulation. **At ovulation**, one primary oocyte completes meiosis I, producing a **secondary oocyte** (n, retaining most of the cytoplasm) and a **first polar body** (n, small, which degenerates) through unequal cytoplasmic division. **After fertilization**, the secondary oocyte completes meiosis II, producing the **ovum** (n, the mature egg that merges with the sperm nucleus) and a **second polar body** (n, small, which degenerates). If fertilization does not occur, the secondary oocyte degenerates without completing meiosis II.

Several key differences distinguish oogenesis from spermatogenesis. Oogenesis produces only one functional gamete per meiotic event (versus four spermatozoa). Oogenesis begins before birth while spermatogenesis begins at puberty. Oogenesis has extended arrest periods while spermatogenesis is continuous. Oogenesis ceases at menopause while spermatogenesis continues throughout life.

<image>A comparative side-by-side diagram of oogenesis and spermatogenesis. Left panel (Oogenesis): Starting with an oogonium (2n) undergoing mitosis during fetal development to produce primary oocytes (2n), which enter meiosis I and arrest in prophase I. At ovulation, meiosis I completes producing a large secondary oocyte (n) and a small first polar body. Upon fertilization, meiosis II completes producing the ovum (n) and a second polar body. The unequal cytoplasmic divisions are clearly shown with size differences. Net result: 1 functional ovum + 2–3 polar bodies. Right panel (Spermatogenesis): Starting with a spermatogonium (2n) undergoing mitosis to produce primary spermatocytes (2n). Meiosis I produces two secondary spermatocytes (n). Meiosis II produces four equal-sized spermatids (n), which differentiate into four spermatozoa. Net result: 4 functional sperm. A timeline along each panel shows the relative duration of each stage.</image>

### IV. Uterine (Fallopian) Tubes

The uterine tubes are paired tubes approximately 10 cm long extending from the uterus toward the ovary. Importantly, they are not directly connected to the ovary; the open end is near but not attached to it. Four regions exist from lateral to medial. The **infundibulum** is the funnel-shaped open end surrounded by **fimbriae**, finger-like projections that sweep over the ovary surface to capture the oocyte at ovulation. The **ampulla** is the widest and longest portion and represents the usual site of fertilization. The **isthmus** is the narrow, medial portion connecting to the uterus. The **intramural (uterine) part** passes through the uterine wall.

The tube wall consists of a mucosa (ciliated columnar epithelium with peg cells that secrete nourishing fluid), muscularis (smooth muscle), and serosa. Transport of the oocyte occurs through a combination of cilia beating toward the uterus and peristaltic contractions of smooth muscle. **Ectopic (tubal) pregnancy**, in which implantation occurs in the uterine tube, is a medical emergency due to the risk of rupture and hemorrhage.

### V. The Uterus

The uterus is a hollow, thick-walled muscular organ located in the pelvic cavity between the bladder and rectum, measuring approximately 7.5 cm long and 5 cm wide in the non-pregnant state. It is supported by several ligaments: the broad ligament, round ligament, uterosacral ligament, and cardinal (lateral cervical) ligament.

The uterus has three regions. The **fundus** is the dome-shaped superior portion above the entrance of the uterine tubes. The **body** is the main portion, narrowing inferiorly to the isthmus. The **cervix** is the narrow, inferior portion projecting into the vagina and containing the cervical canal. The internal os opens into the uterine body, while the external os opens into the vagina. Cervical mucus changes in consistency during the menstrual cycle, becoming thin and watery at ovulation to facilitate sperm passage and thick and sticky at other times to block entry.

#### Uterine Wall Layers

The uterine wall consists of three layers. The **perimetrium** is the serous outer layer (visceral peritoneum). The **myometrium** is the thick smooth muscle layer responsible for labor contractions, consisting of three muscle layers oriented in different directions and responding to oxytocin during labor. The **endometrium** is the mucosal inner lining that undergoes cyclic changes during the menstrual cycle. It consists of two sublayers: the **stratum functionalis** (the superficial layer shed during menstruation and rebuilt each cycle under hormonal influence) and the **stratum basalis** (the deep, permanent layer that gives rise to a new functionalis each cycle and is not shed during menstruation). **Uterine glands** are tubular glands extending through the endometrium that secrete glycogen-rich uterine milk to nourish the early embryo. The blood supply features **spiral (helical) arteries** that supply the stratum functionalis (constricting during menstruation to cause ischemia and shedding) and **straight arteries** that supply the stratum basalis.

### VI. Vagina and External Genitalia

#### Vagina

The vagina is a fibromuscular tube approximately 8-10 cm long extending from the cervix to the vestibule. It serves as the passageway for menstrual flow, the birth canal, and the receptacle for the penis during intercourse. Its wall consists of a mucosa (stratified squamous epithelium without glands, lubricated by cervical mucus and transudation), muscularis, and adventitia. The vagina maintains an acidic environment (pH approximately 3.8-4.5) through Lactobacillus bacteria that metabolize glycogen to produce lactic acid, providing protection against pathogens.

#### External Genitalia (Vulva)

The external genitalia include the **mons pubis** (a fatty pad over the pubic symphysis), the **labia majora** (outer folds of skin homologous to the scrotum), the **labia minora** (inner, hairless folds), the **clitoris** (an erectile organ at the anterior junction of the labia minora, richly innervated and homologous to the penis), and the **vestibule** (the area enclosed by the labia minora containing the urethral and vaginal orifices). The **greater vestibular (Bartholin's) glands** secrete mucus for lubrication during arousal.

### VII. The Ovarian Cycle

The ovarian cycle has an average length of **28 days** (range: 21-35 days) and is controlled by the HPG axis: hypothalamus releases GnRH, anterior pituitary releases FSH and LH, and these act on the ovary.

#### A. Follicular Phase (Days 1-14)

During days 1-7, several primordial follicles are recruited and begin to develop under FSH stimulation. Granulosa cells proliferate and follicles grow through primary and secondary stages. Granulosa cells produce estrogen via aromatization of androgens provided by theca cells, following the "two-cell model." During days 7-14, one follicle becomes the **dominant follicle** while others undergo atresia. The dominant follicle produces rising levels of estrogen. At low to moderate levels, estrogen exerts **negative feedback** on GnRH, FSH, and LH, suppressing other follicles. However, when estrogen reaches a high, sustained threshold, it switches to **positive feedback**, triggering the **LH surge**.

#### B. Ovulation (Day 14)

The **LH surge** (along with a smaller FSH surge) triggers several events: completion of meiosis I in the primary oocyte (producing the secondary oocyte and first polar body), enzymatic breakdown of the follicular wall and ovarian surface, and release of the secondary oocyte (with its corona radiata) from the mature Graafian follicle into the peritoneal cavity, where it is captured by the fimbriae of the uterine tube. Signs of ovulation include a slight increase in basal body temperature (approximately 0.5 degrees Celsius due to progesterone), mittelschmerz (mild pelvic pain), and clear, stretchy cervical mucus.

#### C. Luteal Phase (Days 15-28)

After ovulation, the ruptured follicle collapses and transforms into the **corpus luteum** under LH stimulation. The corpus luteum secretes **progesterone** (its primary hormone) and estrogen, which exert **negative feedback** on GnRH, FSH, and LH, preventing new follicle development. If fertilization does not occur, the corpus luteum degenerates after approximately 10-12 days due to declining LH, becoming the corpus albicans. Progesterone and estrogen levels drop sharply, removing negative feedback, and FSH and LH begin to rise, initiating a new cycle. If fertilization does occur, the developing embryo (trophoblast) secretes **human chorionic gonadotropin (hCG)**, which maintains the corpus luteum and its progesterone production, supporting the pregnancy until the placenta takes over at approximately weeks 8-12.

<image>A comprehensive multi-panel figure correlating the ovarian cycle, hormonal levels, and uterine cycle over 28 days. Panel A (top): Anterior pituitary hormone levels — FSH and LH plotted over 28 days, showing the LH surge at day 14 and smaller FSH peak. Panel B (middle-upper): Ovarian hormone levels — estrogen rising during the follicular phase, dipping briefly at ovulation, then rising again during the luteal phase along with progesterone (which peaks at ~day 21), both declining sharply in the final days if no pregnancy. Panel C (middle-lower): Ovarian cycle — a series of illustrations showing the stages of follicular development from primordial follicle through primary, secondary, and mature Graafian follicle, ovulation at day 14 with release of the oocyte, formation and growth of the corpus luteum during the luteal phase, and its degeneration to corpus albicans. Panel D (bottom): Uterine (endometrial) cycle — a cross-section of the endometrium showing the menstrual phase (days 1–5, shedding of the functionalis with blood), proliferative phase (days 6–14, regrowth under estrogen), and secretory phase (days 15–28, thickened glandular endometrium under progesterone with coiled spiral arteries). Vertical dashed lines align the phases across all four panels.</image>

### VIII. The Uterine (Menstrual) Cycle

The uterine cycle reflects cyclic changes in the endometrium coordinated with the ovarian cycle.

#### A. Menstrual Phase (Days 1-5)

Declining progesterone and estrogen from the degenerated corpus luteum cause spiral arteries to constrict, producing ischemia of the stratum functionalis and tissue necrosis. The stratum functionalis is shed along with blood and uterine fluid as menstrual flow (menses), with approximately 30-80 mL of blood lost. The blood does not clot because of fibrinolysins in the menstrual fluid. The stratum basalis remains intact throughout.

#### B. Proliferative Phase (Days 6-14)

This phase corresponds to the follicular phase of the ovarian cycle. Rising estrogen from the developing follicle stimulates regeneration of the stratum functionalis from the stratum basalis, proliferation of endometrial cells, uterine glands, and spiral arteries. The endometrium thickens from approximately 0.5 mm to 2-3 mm. Cervical mucus becomes thin, clear, and stretchy (spinnbarkeit), facilitating sperm entry.

#### C. Secretory Phase (Days 15-28)

This phase corresponds to the luteal phase of the ovarian cycle. Progesterone (plus estrogen) from the corpus luteum stimulates further thickening of the endometrium (up to approximately 5-6 mm), causes uterine glands to become coiled and secrete glycogen-rich "uterine milk" to nourish a potential embryo, and makes spiral arteries more coiled and elaborate. Cervical mucus becomes thick and sticky, blocking sperm and pathogens. The endometrium is now prepared for implantation, with days 20-23 constituting the "implantation window." If no implantation occurs, the corpus luteum degenerates, hormone levels fall, and the cycle restarts with menstruation.

### IX. Hormonal Regulation Summary

**GnRH** from the hypothalamus is released in a pulsatile pattern to stimulate the anterior pituitary. **FSH** stimulates follicle growth and estrogen production by granulosa cells. **LH** stimulates theca cells to produce androgens, triggers ovulation (via the LH surge), and stimulates corpus luteum formation and progesterone production. **Estrogen** from the follicle and corpus luteum promotes follicular growth, endometrial proliferation, and development of female secondary sex characteristics. At low to moderate levels it exerts negative feedback on FSH and LH, while at high sustained levels it exerts positive feedback to trigger the LH surge. **Progesterone** from the corpus luteum maintains the endometrium in the secretory phase, promotes glandular secretion, inhibits uterine contractions, raises basal body temperature, and exerts negative feedback on GnRH and LH. **Inhibin** from granulosa cells selectively inhibits FSH.

### X. Clinical Correlations

**Polycystic ovary syndrome (PCOS)** is a hormonal imbalance with elevated androgens, irregular cycles, multiple ovarian cysts, and infertility. **Endometriosis** occurs when endometrial tissue grows outside the uterus, causing pain and infertility. **Cervical cancer** is often caused by HPV (human papillomavirus) and is screened by Pap smear. **Amenorrhea** is the absence of menstruation, classified as primary (never having had a period) or secondary (cessation for more than 3 months). **Premenstrual syndrome (PMS)** involves physical and emotional symptoms in the luteal phase linked to progesterone effects. **Menopause** is the cessation of reproductive cycles occurring around age 45-55, resulting from declining ovarian function and reduced estrogen, which produces hot flashes, bone loss, and vaginal atrophy.

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