Lecture: on Female Reproductive System – Anatomy, Physiology, and Glandular Functions

🌸 Lecture: The Female Reproductive System – Anatomy, Physiology, and Glandular Functions

Introduction

The female reproductive system is a complex and finely tuned biological system responsible for producing female gametes (ova), housing and nourishing a developing fetus, and producing female sex hormones. Its functions are cyclical, culminating in either menstruation or pregnancy.

Part 1: Anatomy of the Female Reproductive System

The female reproductive system can be divided into external and internal organs.

A. External Genitalia (Vulva)

These structures protect the internal organs and play a role in sexual arousal.

• Mons Pubis: A fatty, rounded area overlying the pubic symphysis, covered with hair after puberty.

• Labia Majora: Two prominent folds of skin extending from the mons pubis, enclosing the other external structures. They are homologous to the male scrotum.

• Labia Minora: Two smaller, hairless folds of skin located medial to the labia majora. They protect the vaginal and urethral openings.

• Clitoris: A small, highly sensitive erectile organ homologous to the male penis. It is a key organ in female sexual arousal, containing abundant nerve endings.

• Vestibule: The space between the labia minora, containing the openings of the urethra, vagina, and the ducts of the greater vestibular glands.

• Perineum: The diamond-shaped region between the thighs, containing the external genitalia and anus.

B. Internal Genitalia

These organs are located within the pelvic cavity and are crucial for reproduction.

• Ovaries (Paired Gonads): Almond-shaped organs located on either side of the uterus. They are the primary female reproductive organs with dual functions:

  1. Oogenesis: Production of female gametes (ova).

  2. Endocrine: Production of female sex hormones, primarily estrogen and progesterone.

• Uterine Tubes (Fallopian Tubes or Oviducts): Tubes extending from the uterus towards the ovaries, but not directly connected to them. They are the usual site of fertilization. Each tube has distinct regions:

  • Infundibulum: The funnel-shaped distal end, fringed with finger-like projections called fimbriae that sweep the ovum into the tube.

  • Ampulla: The widest and longest part, where fertilization most commonly occurs.

  • Isthmus: The narrower, medial portion joining the uterus.

• Uterus (Womb): A hollow, thick-walled, muscular organ located between the bladder and the rectum. Its primary functions are to receive, retain, and nourish a fertilized ovum. It has several regions:

  • Fundus: The rounded superior portion above the entrance of the uterine tubes.

  • Body (Corpus): The major portion of the uterus.

  • Cervix: The narrow, inferior neck that projects into the vagina.

    The uterine wall has three layers:

  1. Perimetrium: Outermost serous layer.

  2. Myometrium: Thick middle layer of smooth muscle, responsible for contractions during childbirth.

  3. Endometrium: Innermost mucosal layer, which is shed during menstruation if pregnancy does not occur. It consists of a functional layer (stratum functionalis) and a basal layer (stratum basalis).

• Vagina: A muscular tube extending from the cervix to the exterior of the body. It serves as:

  • The female organ of copulation.

  • The birth canal.

  • The passageway for menstrual flow.

Part 2: Physiology of the Female Reproductive System

The physiology revolves around two interconnected cycles: the Ovarian Cycle (changes in the ovary) and the Uterine Cycle (changes in the uterus). These are tightly regulated by hormones.

A. Oogenesis: Ovum Production

Oogenesis is the process of egg cell formation, occurring in the ovaries.

• Fetal Development: Oogonia (diploid stem cells) undergo mitosis to produce primary oocytes. These begin meiosis I but arrest in prophase I until puberty. A female is born with her lifetime supply of primary oocytes (around 1-2 million).

• From Puberty to Menopause: Each month, a few primary oocytes are stimulated to complete meiosis I, forming a large secondary oocyte and a small first polar body. The secondary oocyte then arrests in metaphase II and is ovulated.

• Fertilization: If a sperm penetrates the secondary oocyte, it quickly completes meiosis II, forming a large ovum (egg) and a small second polar body.

B. The Ovarian Cycle

This describes the monthly events in the ovaries that lead to ovulation. It has three phases:

1. Follicular Phase (Day 1-14):

   • Primordial follicles (primary oocyte surrounded by a single layer of flat follicle cells) are stimulated to develop.

   • They grow into primary, secondary, and finally a mature Graafian (vesicular) follicle.

   • Granulosa cells of the developing follicle produce estrogen.

   • The dominant follicle secretes a large amount of estrogen, leading to an LH surge.

2. Ovulation (Around Day 14):

   • The LH surge triggers the rupture of the Graafian follicle, expelling the secondary oocyte into the peritoneal cavity (where it is swept into the uterine tube).

3. Luteal Phase (Day 14-28):

   • The ruptured follicle transforms into the Corpus Luteum under the influence of LH.

   • The corpus luteum secretes large amounts of progesterone and some estrogen.

   • If no pregnancy occurs, the corpus luteum degenerates into a corpus albicans after about 10 days, leading to a drop in hormone levels and menstruation.

   • If pregnancy occurs, the corpus luteum persists, maintained by hCG (human chorionic gonadotropin) from the embryo, continuing to produce hormones until the placenta takes over.

C. The Uterine (Menstrual) Cycle

This describes the cyclical changes in the endometrial lining of the uterus, responding to ovarian hormones. It also has three phases:

1. Menstrual Phase (Days 1-5):

   • Triggered by the drop in ovarian hormones (estrogen and progesterone) as the corpus luteum degenerates.

   • The stratum functionalis of the endometrium detaches from the uterine wall, causing bleeding (menstruation).

2. Proliferative Phase (Days 6-14):

   • Rising estrogen levels from the developing ovarian follicles stimulate the regeneration and thickening of the stratum functionalis.

   • Endometrial glands enlarge, and spiral arteries increase in number.

3. Secretory Phase (Days 15-28):

   • Rising progesterone levels from the corpus luteum prepare the endometrium for implantation.

   • Endometrial glands secrete nutrients (glycogen), and the spiral arteries become highly coiled and elaborate.

   • If fertilization and implantation occur, the embryo will implant into this highly vascularized and nutrient-rich endometrium.

   • If no pregnancy, progesterone levels drop, causing vasoconstriction of the spiral arteries, leading to cell death and the start of the next menstrual phase.

Part 3: Hormonal Regulation (The HPG Axis)

The female reproductive cycle is exquisitely controlled by the Hypothalamic-Pituitary-Gonadal (HPG) axis and local ovarian factors.

1. Hypothalamus: Releases Gonadotropin-Releasing Hormone (GnRH) in a pulsatile fashion.

2. Anterior Pituitary: GnRH stimulates the release of two gonadotropins:

   • Follicle-Stimulating Hormone (FSH): Stimulates follicle growth and estrogen production by granulosa cells.

   • Luteinizing Hormone (LH): Triggers ovulation and promotes the formation and maintenance of the corpus luteum (and thus progesterone production).

3. Ovaries (Gonads):

   • Estrogen: Produced primarily by developing follicles. Responsible for:

     • Promoting oogenesis and follicle growth.

     • Developing and maintaining female secondary sexual characteristics.

     • Stimulating proliferation of the endometrium.

     • Inhibiting FSH and LH release (negative feedback) at low levels, but causing a massive surge of LH (positive feedback) at high, sustained levels just before ovulation.

   • Progesterone: Produced primarily by the corpus luteum. Responsible for:

     • Maintaining the secretory endometrium, preparing it for implantation.

     • Inhibiting uterine motility.

     • Exerting negative feedback on the hypothalamus (GnRH) and anterior pituitary (LH and FSH), preventing further follicle development and ovulation during the luteal phase and pregnancy.

   • Inhibin: Produced by granulosa cells and the corpus luteum. Selectively inhibits FSH release from the anterior pituitary.

Part 4: Female Glands and Their Functions

Beyond the ovaries, several other glands play supportive roles.

1. Greater Vestibular Glands (Bartholin's Glands):

   • Location: Pea-sized glands located on either side of the vaginal opening.

   • Function: Secrete mucus into the vestibule during sexual arousal, providing lubrication for intercourse.

2. Lesser Vestibular Glands (Skene's Glands or Paraurethral Glands):

   • Location: Located on either side of the urethral opening.

   • Function: Secrete mucus to lubricate the opening of the urethra and vestibule. Some researchers believe these glands may be homologous to the male prostate gland and contribute to "female ejaculation."

3. Mammary Glands (Breasts):

   • Location: Modified sweat glands located in the superficial fascia of the anterior thorax.

   • Structure: Consist of 15-25 lobes, which contain smaller lobules with alveoli (milk-producing glands). The milk drains via lactiferous ducts which open at the nipple.

   • Function: Produce milk (lactation) to nourish an infant after childbirth.

     • Estrogen and progesterone stimulate breast development during puberty and pregnancy.

     • Prolactin (from the anterior pituitary) stimulates milk production.

     • Oxytocin (from the posterior pituitary) stimulates milk ejection (let-down reflex) in response to suckling.