Physiology Of The Reproductive System

The Reproductive Physiology

Sexual Reproduction is the biological process through which organisms produce offspring by combining genetic material from two distinct gametes: the male gamete (sperm cell) and the female gamete (secondary oocyte).

Fertilization: The union of a sperm cell with a secondary oocyte results in a zygote containing a complete set of chromosomes—half from each parent.
Anatomical Distinctions: Males and females possess distinct reproductive organs tailored for:
- Producing gametes
- Facilitating fertilization
- Sustaining embryonic and fetal growth (in females)

Reproductive Systems Overview

Male Reproductive System

Components:

Testes (Male Gonads)
- Produce sperm (gametes)
- Secrete hormones (e.g., testosterone)
Duct System
- Epididymis: Stores and matures sperm
- Ductus Deferens: Transports sperm from epididymis
- Ejaculatory Ducts: Convey sperm to urethra
- Urethra: Pathway for sperm to exit the body
Accessory Sex Glands
- Seminal Vesicles: Produce seminal fluid rich in fructose
- Prostate Gland: Secretes a fluid that nourishes and protects sperm
- Bulbourethral Glands: Produce pre-ejaculate fluid
Supporting Structures
- Scrotum: Houses and protects the testes; regulates temperature for optimal sperm production
- Penis: Facilitates the delivery of sperm into the female reproductive tract

Female Reproductive System

Components:

Ovaries (Female Gonads)
- Produce oocytes (gametes)
- Secrete hormones (e.g., estrogen, progesterone)
Uterine (Fallopian) Tubes/Oviducts
- Transport oocytes from ovaries to the uterus
- Site of fertilization
Uterus
- Houses and nurtures the developing embryo and fetus
Vagina
- Receives sperm during copulation
- Serves as the birth canal

Spermatogenesis: Formation of Sperm

Location: Seminiferous Tubules within the testes

Process Duration: 65–75 days in humans

Stages of Spermatogenesis

Spermatogonia (Stem Cells)
- Type: Diploid (2n; 46 chromosomes)
- Function: Undergo mitosis to produce more spermatogonia or differentiate into primary spermatocytes
- Location: Near the basement membrane of seminiferous tubules
Primary Spermatocytes
- Type: Diploid (2n; 46 chromosomes)
- Process:
  - DNA replication occurs prior to meiosis
  - Undergo Meiosis I to form secondary spermatocytes
Secondary Spermatocytes
- Type: Haploid (n; 23 chromosomes)
- Structure: Each chromosome consists of two chromatids
- Process:
  - Undergo Meiosis II to form spermatids
Spermatids
- Type: Haploid (n; 23 chromosomes)
- Process: Undergo Spermiogenesis to transform into mature sperm cells
Spermiogenesis
- Description: Differentiation of spermatids into elongated, motile spermatozoa
- Outcome: Formation of mature sperm with distinct head and tail structures
- Event: Spermiation – release of mature sperm into the lumen of seminiferous tubules

Sperm Structure

Head
- Size: Approximately 4–5 μm long
- Components:
  - Nucleus: Contains 23 condensed chromosomes
  - Acrosome: Cap-like vesicle filled with enzymes (e.g., hyaluronidase, proteases) essential for penetrating the secondary oocyte
Tail
- Components:
  - Neck: Contains centrioles essential for tail formation
  - Middle Piece: Packed with mitochondria arranged in a spiral to provide ATP for motility
  - Principal Piece: Longest segment, responsible for the whip-like movement
  - End Piece: Tapers off at the terminal end

Daily Sperm Production: Approximately 300 million sperm are produced each day.

Hormonal Control of the Testes

Regulatory Axis: Hypothalamus → Anterior Pituitary → Testes

Gonadotropin-Releasing Hormone (GnRH)
- Source: Hypothalamic neurosecretory cells
- Function: Stimulates the anterior pituitary to secrete gonadotropins
Gonadotropins
- Luteinizing Hormone (LH)
  - Target: Leydig cells in testes
  - Function: Stimulates secretion of testosterone
- Follicle-Stimulating Hormone (FSH)
  - Target: Sertoli cells in testes
  - Function: Promotes spermatogenesis and secretion of androgen-binding protein (ABP)
Testosterone
- Source: Leydig cells
- Function:
  - Development and maintenance of male secondary sexual characteristics
  - Supports spermatogenesis
  - Acts via negative feedback to inhibit GnRH and LH secretion
Androgen-Binding Protein (ABP)
- Source: Sertoli cells
- Function: Binds testosterone to maintain high local concentrations necessary for spermatogenesis
Inhibin
- Source: Sertoli cells
- Function: Inhibits FSH secretion via negative feedback

Feedback Mechanism

Negative Feedback: Elevated testosterone levels inhibit GnRH and LH secretion, maintaining hormonal balance.

Roles of Androgens

Prenatal Development
- Testosterone: Stimulates male duct development and testicular descent
- Dihydrotestosterone (DHT): Promotes external genitalia formation
Pubertal Development
- Testosterone & DHT: Induce growth and enlargement of male reproductive organs and development of secondary sexual characteristics (e.g., facial hair, deepening voice)
Adult Functions
- Sexual Behavior: Influences libido and sexual drive
- Anabolic Effects: Promote protein synthesis and muscle growth

Female Reproductive Physiology

Female Reproductive Cycle

Phases:

Ovarian Cycle
- Events: Maturation of oocytes and hormone secretion
Uterine (Menstrual) Cycle
- Events: Preparation of the endometrium for implantation

Cycle Duration: Approximately one month (28 days)

Phases of the Ovarian Cycle

Menstrual Phase (Days 1–5)
- Events:
  - Shedding of the stratum functionalis of the endometrium if fertilization does not occur
  - Menstrual flow: 50–150 mL of blood and cellular debris
- Hormonal Changes:
  - Declining progesterone and estrogen levels
  - Release of prostaglandins causing vasoconstriction and cell death in the endometrium
Preovulatory Phase (Days 6–14) – Proliferative Phase
- Events:
  - Follicular growth stimulated by FSH
  - Selection of a dominant follicle
  - Maturation of the dominant (graafian) follicle
  - Proliferation and thickening of the endometrium under estrogen influence
- Hormonal Changes:
  - Increased estrogen and inhibin from the dominant follicle
  - Negative feedback reduces FSH levels, inhibiting growth of other follicles
Ovulation (Around Day 14)
- Events:
  - Rupture of the mature graafian follicle
  - Release of the secondary oocyte into the pelvic cavity
  - Possible pain (mittelschmerz) due to follicle rupture
- Hormonal Changes:
  - Positive feedback from high estrogen levels triggers a surge in LH and GnRH
  - LH surge induces ovulation
Postovulatory Phase (Days 15–28) – Luteal Phase
- Events:
  - Transformation of the ruptured follicle into the corpus luteum
  - Secretion of progesterone, estrogen, relaxin, and inhibin by the corpus luteum
  - Preparation of the endometrium for potential implantation (secretory phase)
- Hormonal Changes:
  - Progesterone and estrogen exert negative feedback on GnRH, LH, and FSH
  - If fertilization does not occur, corpus luteum degenerates into corpus albicans
  - Decline in progesterone and estrogen leads to menstruation

Hormonal Regulation of the Female Reproductive Cycle

Gonadotropin-Releasing Hormone (GnRH)
- Source: Hypothalamus
- Function: Stimulates anterior pituitary to release FSH and LH
Follicle-Stimulating Hormone (FSH)
- Function: Initiates follicular growth in ovaries
- Mechanism: Granulosa cells convert androgens to estrogens under FSH influence
Luteinizing Hormone (LH)
- Function: Triggers ovulation and stimulates corpus luteum formation
- Mechanism: Promotes secretion of progesterone and estrogen from the corpus luteum
Estrogens
- Sources: Ovarian follicles and corpus luteum
- Functions:
  - Development and maintenance of female reproductive structures
  - Promotion of secondary sex characteristics (e.g., breast development)
  - Enhancement of protein anabolism and bone strength
  - Inhibition of GnRH, LH, and FSH at high levels
Progesterone
- Source: Corpus luteum
- Functions:
  - Prepares and maintains the endometrium for implantation
  - Inhibits GnRH and LH secretion at high levels
  - Prepares mammary glands for milk secretion
Relaxin
- Source: Corpus luteum and placenta (during pregnancy)
- Functions:
  - Relaxes uterine smooth muscle to facilitate implantation and pregnancy
  - Increases flexibility of pubic symphysis and dilates the cervix during labor
Inhibin
- Source: Granulosa cells and corpus luteum
- Function: Inhibits FSH secretion through negative feedback

Ovarian and Uterine Cycle Coordination

Ovarian Cycle: Involves the maturation of the oocyte and hormonal changes in the ovary.
Uterine (Menstrual) Cycle: Involves cyclical changes in the endometrium to prepare for potential implantation.

Detailed Phases of the Female Reproductive Cycle

1. Menstrual Phase (Days 1–5)

Follicular Development:
- Under FSH influence, primordial follicles develop into primary and then secondary follicles.
- Note: Follicular maturation can span several menstrual cycles; not all developing follicles reach ovulation.
Endometrial Changes:
- Decline in progesterone and estrogen leads to vasoconstriction of uterine spiral arterioles.
- Resulting hypoxia causes cell death and shedding of the stratum functionalis.
- Menstrual Flow: Composed of blood, tissue fluid, mucus, and epithelial cells.

2. Preovulatory Phase (Days 6–14) – Proliferative Phase

Dominant Follicle Selection:
- Among developing follicles, one becomes dominant due to better responsiveness to FSH.
Hormonal Influence:
- Dominant follicle secretes estrogens and inhibin, inhibiting FSH and preventing further follicular growth.
Endometrial Proliferation:
- Estrogens stimulate mitosis in the stratum basalis, regenerating the stratum functionalis.
- Development and thickening of endometrial glands and blood vessels.
Corpus Luteum Formation:
- As the dominant follicle matures, it prepares for potential ovulation.

3. Ovulation (Around Day 14)

Process:
- LH surge induced by positive feedback from high estrogen levels.
- LH causes rupture of the mature graafian follicle, releasing the secondary oocyte.
Post-Ovulation:
- Oocyte enters the uterine tube, surrounded by zona pellucida and corona radiata cells.
- Potential for fertilization occurs within the uterine tube.
Clinical Note:
- Mittelschmerz: Ovulation pain due to follicle rupture.
- Ovulation Prediction: Home LH tests can predict ovulation by detecting LH surge.

4. Postovulatory Phase (Days 15–28) – Luteal Phase

Corpus Luteum Formation:
- Ruptured follicle transforms into corpus luteum (via corpus hemorrhagicum)
- Hormones Secreted: Progesterone, estrogen, relaxin, inhibin
Endometrial Preparation:
- Secretory Phase: Progesterone and estrogen enhance endometrial glands to secrete nutrients (e.g., glycogen)
- Endometrial Thickening: Up to 12–18 mm to support potential implantation
If Fertilization Occurs:
- Corpus Luteum Maintenance: Rescued by human chorionic gonadotropin (hCG) produced by the embryo
- Pregnancy Support: Corpus luteum continues hormone secretion to sustain the endometrium
If Fertilization Does Not Occur:
- Corpus Luteum Degeneration: Transforms into corpus albicans
- Hormonal Decline: Decreased progesterone and estrogen lead to menstrual phase initiation

Summary of Hormonal Regulation

Hypothalamus secretes GnRH → stimulates anterior pituitary to release FSH and LH
FSH: Promotes follicular growth and estrogen secretion
LH: Triggers ovulation and corpus luteum formation; stimulates testosterone production in males
Estrogens and Progesterone: Regulate endometrial changes and exert feedback on GnRH, FSH, and LH
Inhibin: Specifically inhibits FSH secretion

Key Concepts

Negative Feedback: Maintains hormonal balance by regulating hormone secretion based on current levels (e.g., testosterone in males, estrogens and progesterone in females)
Positive Feedback: Occurs during ovulation where high estrogen levels lead to a surge in LH
Anabolic Effects of Androgens: Promote growth and protein synthesis, contributing to secondary sexual characteristics
Role of ADH and Relaxin: While ADH is more associated with kidney function, relaxin plays a critical role in the female reproductive system by preparing the uterus for pregnancy and childbirth

Important Terms

Gamete: Reproductive cell (sperm in males, oocyte in females)
Meiosis: Type of cell division that reduces chromosome number by half, producing haploid cells
Spermiogenesis: Process of sperm maturation without cell division
Corpus Luteum: Temporary endocrine structure involved in the production of progesterone
Corpus Albicans: Degenerated corpus luteum post non-fertilization
hCG (Human Chorionic Gonadotropin): Hormone produced during pregnancy, detected by pregnancy tests

Clinical Relevance

Ovulation Prediction: LH surge detection helps in family planning and fertility treatments
Menstrual Disorders: Imbalances in hormonal regulation can lead to irregular cycles, amenorrhea, or dysmenorrhea
Infertility: Disruptions in spermatogenesis or oogenesis, hormonal imbalances, or anatomical issues can affect fertility
Hormonal Therapies: Used in contraceptives, hormone replacement therapies, and treatment of reproductive disorders