The Reproductive System

Four Basic Component Categories

The Testes

Hang within the scrotum, a fleshy pouch suspended inferior to the perineum. Each testis is a flattened egg roughly 5 cm long, 3 cm wide, 2.5 cm thick, weighing 10–15 g. A serous membrane lines the scrotal cavity, reducing friction.

• Dartos muscle — smooth muscle in scrotal skin; sustained contraction wrinkles the scrotal surface.
• Cremaster muscle — skeletal muscle beneath the dermis; contracts to pull testes closer to the body when cold.

Normal sperm development requires temperatures approximately 1.1°C (2°F) lower than core body temperature. When warm, cremaster relaxes and testes drop away from body. When cold, cremaster contracts and testes pull closer.

Internal Testicular Structure

• Tunica albuginea — dense fibrous capsule wrapping each testis. Collagen septa extend inward, subdividing each testis into ~250 lobules.
• Seminiferous tubules — ~800 per testis, each ~80 cm long (~half a mile total per testis). Site of sperm production.
• Interstitial cells (Leydig cells) — located between seminiferous tubules. Produce androgens, primarily testosterone.
• Sperm exit tubules → rete testis (network) → efferent ductules → epididymis.

The Penis

Three regions: (1) root (fixed, attaches to body wall), (2) body/shaft (tubular, contains erectile tissue), (3) glans (expanded distal portion surrounding the external urethral orifice). The prepuce (foreskin) covers the glans. Circumcision = surgical removal of the prepuce.

Three columns of erectile tissue:

• Two corpora cavernosa — dorsal columns, bases bound to pubis and ischium, extend to glans.
• One corpus spongiosum — ventral column, surrounds the urethra, expands at tip to form the glans.

Erection mechanism: Parasympathetic neurons release nitric oxide (NO) → smooth muscle in arterial walls relaxes → vessels dilate → erectile tissue fills with blood → erection.

Spermatogenesis — Three Processes

Meiosis Details

During meiosis I prophase, maternal and paternal chromosomes pair in synapsis, forming 23 tetrads (matched sets of four chromatids). Crossing-over can exchange genetic material between chromatids, increasing genetic variation. Meiosis I produces two secondary spermatocytes. Meiosis II separates the duplicate chromatids, producing four spermatids.

Nurse Cells (Sustentacular / Sertoli Cells)

Large cells extending from tubule perimeter to lumen. Functions:

• Nourish developing sperm cells
• Control the chemical environment inside seminiferous tubules (no blood vessels inside tubules — nutrients must diffuse)
• Provide chemical stimuli promoting differentiation of spermatozoa
• Produce inhibin — regulates spermatogenesis via feedback on FSH

Anatomy of a Spermatozoon

Mature spermatozoa lack an endoplasmic reticulum, Golgi apparatus, lysosomes, peroxisomes, and glycogen reserves. They must absorb fructose from seminal gland secretions as their primary energy source.

The Reproductive Tract

The spermatic cord is a sheath enclosing the ductus deferens plus blood vessels, nerves, and lymphatics serving the testis.

Capacitation: Spermatozoa leaving the epididymis are physically mature but immobile. They become motile and fully functional only after (1) mixing with seminal gland secretions and (2) exposure to conditions inside the female reproductive tract.

Accessory Glands & Semen Composition

Semen = sperm + accessory gland secretions. A typical ejaculation: 2–5 mL. Normal sperm count: 20–100 million per mL. Key enzymes in semen: protease (dissolves vaginal mucus), seminalplasmin (kills bacteria), clotting enzyme + liquefying enzyme.

The Three-Level Hormonal Axis

Negative Feedback Loops

Peripheral Effects of Testosterone

• Maintains libido (sexual drive) and related behaviors
• Stimulates bone and muscle growth
• Establishes and maintains male secondary sex characteristics
• Maintains accessory glands and organs of the male reproductive system

Developmental Timeline

Testosterone production begins at week 7 of fetal development and peaks at roughly six months of development. This early surge stimulates differentiation of the male duct system, accessory organs, and affects CNS development. Levels decline after the fetal peak, then surge again at puberty, initiating sexual maturation and secondary sex characteristics. Negative feedback keeps levels stable until late in life (ages 50–60).

Principal Organs

Ovaries, uterine tubes, uterus, vagina, and external genitalia. Accessory glands release secretions into the tract.

The Ovaries

Small, lumpy, almond-shaped organs near the lateral pelvic walls (~5 cm long, 2.5 cm wide, 8 mm thick). Three functions:

• Production of female gametes (ova)
• Secretion of female sex hormones (estrogens and progestins)
• Secretion of inhibin (negative feedback on pituitary FSH)

Stabilized by the broad ligament (mesentery enclosing tubes and uterus) and supporting ligaments. The suspensory ligament contains the ovarian artery and ovarian vein.

The Uterine Tubes (Fallopian Tubes / Oviducts)

~13 cm long. The ovarian end forms the infundibulum — an expanded funnel with fingerlike fimbriae. Both fimbriae and infundibulum surfaces are carpeted with cilia that beat toward the tube entrance.

Oocytes are transported by ciliary movement + peristaltic contractions. Transit time: 3–4 days from infundibulum to uterine cavity. Fertilization must occur within the first 12–24 hours of passage. Peg cells secrete fluid that completes capacitation of spermatozoa and supplies nutrients.

The Uterus

Muscular chamber (~7.5 cm long, ~5 cm diameter, 30–40 g). Provides mechanical protection and nutritional support for the developing embryo (weeks 1–8) and fetus (week 9 to delivery). Contractions eject the fetus at birth.

Two regions: body (largest division; fundus = rounded portion superior to uterine tube attachment; ends laterally at the isthmus) and cervix (inferior tubular portion projecting into the vagina). The external os opens into the vagina; the internal os opens into the uterine cavity.

Three wall layers (inside to outside):

The Vagina

Elastic muscular tube, 7.5–9 cm long, highly distensible. The fornix is the shallow recess surrounding the cervical protrusion. The hymen partially blocks the entrance. Bulbospongiosus muscles constrict the vaginal entrance and cover the vestibular bulbs (erectile tissue).

Three functions: (1) passageway for menstrual fluids, (2) receives penis and holds spermatozoa, (3) lower portion of birth canal. Resident bacteria create an acidic environment that restricts pathogen growth.

External Genitalia (Vulva / Pudendum)

The Mammary Glands

Specialized integumentary organs controlled by reproductive and placental hormones. Lactation = milk production. Each breast contains a mammary gland within subcutaneous tissue of the pectoral fat pad.

• Glandular tissue = separate lobes containing lobules with milk glands
• Ducts converge into a single lactiferous duct per lobe
• Each duct expands near the nipple into a lactiferous sinus
• 15–20 lactiferous sinuses open onto the nipple
• Reddish-brown skin surrounding the nipple = areola
• Suspensory ligaments originate in the dermis and support the gland

Oogenesis — Key Differences from Spermatogenesis

• Oogonia complete mitotic divisions before birth. Primary oocytes begin meiosis during fetal months 3–7 but arrest at prophase of meiosis I until puberty.
• Of ~2 million oocytes at birth, only ~400,000 survive to puberty (rest undergo atresia). Only ~400 will actually ovulate.
• Cytoplasm is unequally distributed — produces ONE functional ovum + up to THREE nonfunctional polar bodies.
• The ovary releases a secondary oocyte, NOT a mature ovum. Meiosis II completes only if fertilization occurs.

Follicle Development Stages

The 28-Day Ovarian Cycle

The Uterine (Menstrual) Cycle

Menarche = first menstrual period (typically ages 11–12). Menopause = last menstrual cycle (ages 45–55). Normal cycle length: 21–35 days.

Estrogen Functions

• Stimulates bone and muscle growth
• Establishes and maintains female secondary sex characteristics (body hair/fat distribution)
• Affects CNS activity (hypothalamus — increases sexual drive)
• Maintains functional accessory reproductive glands and organs
• Initiates repair and growth of the endometrium

Body Temperature and Ovulation

During the follicular phase (estrogen dominant): basal body temperature is ~0.3°C lower. At ovulation: temperature declines sharply. During the luteal phase (progesterone dominant): temperature rises. Progesterone stimulates metabolic activity and elevates basal body temperature. Tracking this pattern can identify the day of ovulation.

Male Sexual Function

Impotence (erectile dysfunction) = inability to achieve or maintain an erection.

Female Sexual Function

Largely comparable to male events:

• Arousal (parasympathetic): Engorgement of clitoris and vestibular bulbs; increased secretion from cervical mucous glands and greater vestibular glands (lubrication); nipple blood vessel engorgement (increased sensitivity); vaginal wall vessels fill with blood, surfaces moistened by fluid from underlying connective tissues.
• Orgasm: Peristaltic contractions of uterine and vaginal walls + rhythmic contractions of bulbospongiosus and ischiocavernosus muscles (the latter produce the sensations of orgasm).

Menopause (Female)

Defined as the time when ovulation and menstruation cease. Typically occurs at ages 45–55. Perimenopause (transition from normal cycles to none) normally begins around age 40.

Underlying cause: shortage of follicles. Of ~2 million at birth and ~400,000 at puberty, by age 50 often NO secondary follicles remain to respond to FSH. In premature menopause, this depletion occurs before age 40.

Hormonal Changes at Menopause

• Estrogen and progesterone decline (no follicles or corpus luteum to produce them)
• GnRH, FSH, and LH rise sharply and remain high (loss of negative feedback)

Consequences of Estrogen Decline

• Reduction in size of uterus and breasts
• Thinning of urethral and vaginal walls
• Development of osteoporosis (bone deposition slows)
• Neural effects: hot flashes (associated with LH surges), anxiety, depression
• Increased risk of atherosclerosis and cardiovascular disease

The Male Climacteric (Andropause)

Changes are more gradual than in females. Testosterone levels begin declining between ages 50 and 60. FSH and LH levels increase. Sperm production continues — men in their eighties can still father children — but sexual activity gradually decreases. Testosterone replacement therapy may enhance libido but may increase prostate disease risk.

The reproductive system secretes hormones affecting growth and metabolism of ALL body systems. Sex hormones affect brain development, behavioral drives, muscle mass, bone mass and density, body proportions, hair and body fat distribution.

Normal reproduction requires the participation of multiple systems — digestive, endocrine, nervous, cardiovascular, and urinary must all function properly.

Reproductive Hormones Summary