Marieb Human Anatomy & Physiology: Chapter 27 - The Reproductive System

Overview of the Reproductive System

Functional Timing: The reproductive system does not become active or functional until puberty.
Primary Purpose: To produce offspring by executing four specific tasks: * Gamete Formation: Production of specialized cells for sexual reproduction called gametes. In males, these are sperm; in females, they are ova (eggs). * Copulation: Bringing male and female gametes together through sexual intercourse. * Fertilization: Combining genetic information from gametes through the fusion of sperm and egg to form a zygote. This zygote is the first cell of a new organism from which all body cells arise. * Development and Birth: Supporting the development of the fetus (gestation) and the birth of the baby (parturition).

Gonads and Reproductive Organs

Homologous Structures: Many male and female reproductive structures share a common origin and often shared functions. Examples include: * Male testes and female ovaries. * Male penis and female clitoris.
Primary Sex Organs (Gonads): These include the testes (male) and ovaries (female). * Gamete Production: Gametes are formed through a specialized cell division called meiosis. * Sex Hormone Secretion: Steroid hormones produced include testosterone in males, and estrogens and progesterone in females. These are vital for the development and function of reproductive and other organs, as well as sexual behavior and drives.
Accessory Reproductive Organs: Include ducts, glands, and external genitalia.

Reproductive Hormone Secretion and the HPG Axis

Hypothalamic-Pituitary-Gonadal (HPG) Axis: A series of hormonal interactions that regulate gamete and hormone production. * Gonadotropin-releasing hormone (GnRH): Secreted by the hypothalamus; reaches the anterior pituitary via the hypophyseal portal system to stimulate the release of gonadotropins. * Gonadotropins (FSH and LH): Released from the anterior pituitary. * Follicle-stimulating hormone (FSH): Named for actions in females; stimulates gamete production. * Luteinizing hormone (LH): Named for actions in females; stimulates sex hormone secretion. * Sex Hormones: Released by the gonads. They act on target tissues and exert negative feedback on the hypothalamus and anterior pituitary. * Inhibin: Secreted by the gonads (stimulated by $FSH$ ); exerts negative feedback specifically on $FSH$ release from the anterior pituitary.
Puberty Activation: * Prior to puberty, low levels of sex hormones suppress $GnRH$ secretion. * As puberty approaches, the hypothalamus becomes less sensitive to inhibition. * $GnRH$ is released in a pulse-like manner, causing the anterior pituitary to release $FSH$ and $LH$ . * The threshold for inhibition continues to rise until adult hormonal patterns are achieved.

Meiosis and Genetic Variability

Nature of Meiosis: A unique nuclear division occurring only in the gonads. It reduces the chromosome count by half to prevent the doubling of chromosomes during fertilization.
Key Terms: * Sister Chromatids: Identical copies of a chromosome joined by a centromere, resulting from $DNA$ replication during S-phase of interphase. * Homologous Pairs: One paternal and one maternal chromosome that carry genes for the same characters (e.g., eye color). Traits are variants of these characters (e.g., blue vs. brown eyes). * Nonsister Chromatids: Chromatids within a homologous pair that exchange $DNA$ through crossing over (at sites called chiasmata) to increase variability.
Chromosome Numbers: * Diploid ( $2n$ ): Human diploid number is $46$ ( $23$ pairs). * Haploid ( $n$ ): Human haploid number found in gametes is $23$ .
Sequence of Events: Meiosis involves one round of $DNA$ replication followed by two nuclear divisions: * Meiosis I (Reduction Division): Homologous pairs separate, reducing the number from $2n$ to $n$ . * Prophase I: Synapsis occurs where homologous chromosomes form tetrads ( $4$ chromatids) and cross over. * Metaphase I: Tetrads line up randomly via independent assortment on the metaphase plate. * Anaphase I: Homologous chromosomes separate; sister chromatids remain attached. * Telophase I and Cytokinesis: Two haploid daughter cells form. * Meiosis II (Equational Division): Identical to mitosis except cells are haploid and no replication occurs beforehand. Sister chromatids separate, resulting in four unique haploid daughter cells.

Anatomy of the Male Reproductive System

Testes: The sperm-producing gonads located in the scrotum.
Duct System (Proximal to Distal): Epididymis → Ductus deferens → Ejaculatory duct → Urethra.
Accessory Glands: Seminal glands, prostate, and bulbo-urethral glands secrete fluids that form semen.
The Scrotum: * A skin pouch hanging outside the abdominopelvic cavity. Divided by a midline septum. * Maintains a temperature roughly $3^{\circ} ext{C}$ lower than core body temperature, required for sperm production. * Dartos Muscle: Smooth muscle that wrinkles scrotal skin to reduce surface area and heat loss. * Cremaster Muscles: Skeletal muscle bands that elevate the testes for warmth.
Structure of the Testes: * Tunica Vaginalis: Outer serous layer. * Tunica Albuginea: Inner fibrous capsule; its septa divide the testis into $250$ to $300$ lobules. * Seminiferous Tubules: $1$ to $4$ per lobule; the actual site of sperm production. * Path of Sperm: Seminiferous tubules → Straight tubule → Rete testis → Efferent ductules → Epididymis (head, body, then tail).
Testicular Blood Supply: * Testicular Arteries: Arise from the abdominal aorta. * Pampiniform Venous Plexus: Surrounds the arteries; acts as a heat exchanger to cool arterial blood before it reaches the testes.
Clinical—Testicular Cancer: Most common in males aged $15-35$ . Cryptorchidism (undescended testes) is the major risk factor. Most common sign is a painless solid mass. Healing rate is over $96\%$ via orchiectomy, radiation, or chemo.

Male Accessory Ducts and Penis

Epididymis: * Comma-shaped; contains a duct approximately $6\,m$ ( $20\,ft$ ) long. * Lined with pseudostratified epithelium containing stereocilia (nonmotile microvilli) to absorb fluid and nourish sperm. * Sperm take $20$ days to pass through, during which they gain the ability to swim. Can be stored for months.
Ductus Deferens (Vas Deferens): * Passes through the inguinal canal into the pelvic cavity. Forms an ampulla behind the bladder. * Vasectomy: Surgical cutting/ligating of the ductus deferens for birth control.
Urethra: Three regions: Prostatic, Intermediate (membranous), and Spongy (penile).
Penis: The copulatory organ. * Glans Penis: Enlarged tip; covered by the prepuce (foreskin). Circumcision is the surgical removal of the prepuce. * Erectile Tissue: * Corpus spongiosum: Surrounds the urethra; expands to form the bulb and glans. * Corpora cavernosa: Paired dorsal bodies; proximal ends are the crura (anchors to pubic arch).

Male Accessory Glands and Semen

Seminal Glands (Vesicles): Produce an alkaline fluid containing fructose, citric acid, and prostaglandins. Accounts for $70\%$ of semen volume. Fluoresces under UV light.
Prostate: Approximately the size of a peach pit. Secretes a milky, slightly acidic fluid containing citrate and prostate-specific antigen (PSA). Accounts for $30\%$ of semen volume.
Bulbo-urethral Glands: Produce thick, clear mucus that neutralizes acidic urine traces in the urethra and lubricates the glans.
Semen Characteristics: * Volume: $2-5\,ml$ per ejaculation. * Sperm Count: $20-150$ million sperm per $ml$ . * pH: $7.2-8.0$ (alkaline to neutralize the acidic vagina). * Functions: Fructose for $ATP$ ; prostaglandins to thin cervical mucus and trigger reverse uterine peristalsis; relaxin for motility; antibiotic chemicals; clotting factors (to make it stick) and subsequent liquefying enzymes.

Male Sexual Response

Erection: A parasympathetic reflex. Sexual excitement triggers the release of nitric oxide (NO), causing vasodilation of penile arterioles. Engorgement of corpora cavernosa compresses veins to maintain stiffness.
Ejaculation: A sympathetic spinal reflex. * Emission: Contraction of ducts and glands to empty contents into the urethra. The internal urethral sphincter constricts to block urine. * Expulsion: A somatic reflex causes rhythmic contractions of bulbospongiosus muscles, propelling semen at speeds up to $500\,cm/s$ (nearly $11\,mph$ ).
Resolution: Period of relaxation. Sympathetic activity constricts arteries.
Latent (Refractory) Period: Time during which a male cannot achieve another orgasm; lengthens with age.
Clinical—Erectile Dysfunction (ED): Affects $50\%$ of men over $40$ . Often linked to diabetes, atherosclerosis, or psychological factors. Sildenafil (Viagra) enhances $NO$ effects.

Spermatogenesis

Location: Seminiferous tubules.
Timing: Begins at age $14$ ; males produce $400$ million sperm daily.
Cell types in Tubules: * Sustentocytes (Nurse Cells): Support cells that form the blood-testis barrier via tight junctions. This barrier protects haploid cells from the immune system. * Spermatogenic Cells: Stem cells (spermatogonia) and developing sperm. * Myoid Cells: Contract to move fluid and sperm. * Interstitial Endocrine Cells (Leydig Cells): Found between tubules; produce testosterone.
Stages of Spermatogenesis: 1. Mitosis of Spermatogonia: Forms Type A (stays at basement membrane) and Type B (becomes primary spermatocyte) cells. 2. Meiosis: Primary spermatocyte ( $2n$ ) → two secondary spermatocytes ( $n$ ) → four spermatids ( $n$ ). 3. Spermiogenesis: Transformation of a round spermatid into a mobile sperm. * Head: Genetic region containing the nucleus and acrosome (enzymes for egg penetration). * Midpiece: Metabolic region full of mitochondria. * Tail (Flagellum): Locomotor region.

Anatomy of the Female Reproductive System

Ovaries: Female gonads producing ova and hormones (estrogens and progesterone). Estrogens include estradiol, estrone, and estriol.
Ligaments: * Ovarian ligament: Anchors ovary to uterus. * Suspensory ligament: Anchors ovary to pelvic wall. * Mesovarium: Part of the broad ligament.
Ovarian Follicles: Sac-like structures containing an immature egg (oocyte). * Primordial Follicle: Single layer of squamous pre-granulosa cells. * Primary Follicle: Single layer of cuboidal cells. * Secondary Follicle: Multiple layers of granulosa cells. * Vesicular (Antral) Follicle: Contains a large fluid-filled cavity called an antrum.
The Uterine Tubes (Fallopian Tubes): * Regions: Infundibulum (with ciliated fimbriae), Ampulla (site of fertilization), and Isthmus. * Oocytes are carried toward the uterus via peristalsis and ciliary action.
The Uterus: * Regions: Body, Fundus, Isthmus, and Cervix. * Cervix: Secretes thick mucus that blocks sperm and bacteria, except at ovulation. * Uterine Wall Layers: * Perimetrium: Outermost layer. * Myometrium: Smooth muscle for childbirth contraction. * Endometrium: Mucosal lining. Has two layers: Stratum functionalis (shed during menses) and Stratum basalis (rebuilds the functional layer). * Blood Supply: Spiral arteries serve the functional layer; their spasms cause the layer to shed.

The Vagina and External Genitalia

Vagina: Thin-walled birth canal and organ of copulation. * Layers: Adventitia, muscularis, and mucosa (with stimulated rugae). * Acidity: Adult vaginal pH is acidic to prevent infection; adolescent pH is more alkaline, increasing STI risk.
External Genitalia (Vulva): * Mons pubis: Adipose cushion. * Labia majora: Homologue of male scrotum. * Labia minora: Homologue of male spongy urethra. * Vestibule: Contains urethral and vaginal openings and the greater vestibular glands (homologue of bulbo-urethral glands). * Clitoris: Richly innervated erectile tissue; homologous to the penis. Contains corpora cavernosa and a glans.

Mammary Glands

Anatomy: Modified sweat glands with $15-25$ lobes radiating from the nipple.
Support: Suspensory ligaments attach the breast to muscle fascia.
Milk Path: Glandular alveoli → lactiferous ducts → lactiferous sinuses → nipple.
Breast Cancer: Second most common cancer in U.S. females. Risk factors: lifetime estrogen exposure, BRCA1/BRCA2 genetic mutations. Screening starts at age $40$ via mammography.

Oogenesis and the Ovarian Cycle

Oogenesis Timeline: Begins in the fetus with oogonia. At birth, all primary oocytes ( $2n$ ) are present and arrested in Prophase I.
Resumption: From puberty to menopause, a few follicles are rescued from atresia (cell death) each month. One dominant follicle completes Meiosis I to form a secondary oocyte and a first polar body.
Second Arrest: The secondary oocyte arrests in Metaphase II and is ovulated. It only completes Meiosis II if penetrated by sperm, resulting in one ovum and a second polar body.
Ovarian Cycle Phases: 1. Follicular Phase (Days 1–14): Growth of follicles, dominant follicle selection, estrogen levels rise. 2. Ovulation (Midcycle): Secondary oocyte is expelled. Mittelschmerz is the twinge of pain felt by some. 3. Luteal Phase (Days 15–28): Ruptured follicle becomes a corpus luteum (produces progesterone/estrogen). If no pregnancy, it becomes the corpus albicans (scar) after $10$ days.

Regulation and the Uterine Cycle

Hormonal Control: $GnRH$ triggers $FSH$ and $LH$ . Rising estrogen levels initially inhibit the pituitary, but at high concentration, they provide positive feedback causing an LH surge (triggers ovulation).
Uterine (Menstrual) Cycle Phases: 1. Menstrual Phase (Days 0–4): Low hormones cause the functional layer to shed. 2. Proliferative Phase (Days 5–14): Estrogen levels rise, rebuilding the functional layer and thinning cervical mucus. Ovulation occurs at Day $14$ . 3. Secretory Phase (Days 15–28): Progesterone levels rise, making the uterus receptive to implantation. Cervical plug re-forms.
Clinical—Amenorrhea: Cessation of menses often due to low body fat and low leptin levels (typical in athletes), which informs the hypothalamus to stop reproductive cycles.

Sexually Transmitted Infections (STIs)

Chlamydia: Most common bacterial STI; frequently asymptomatic. Can cause PID and sterility.
Gonorrhea: Bacterial; symptoms include painful urination and penile/vaginal discharge. Increasingly antibiotic-resistant.
Syphilis: Bacterial; progresses through primary (chancre), secondary (rash), and tertiary (CNS/vascular damage) stages.
HPV: Most common STI overall; causes genital warts and serves as the primary cause of cervical cancer.
Genital Herpes: Caused by Herpes Simplex Virus; no cure, only remission via antivirals.

Developmental Aspects

Genetic Sex: Determined by the sperm (X or Y). Female is $XX$ ; Male is $XY$ . The SRY gene on the Y chromosome initiates testes development.
Nondisjunction Disorders: * Turner's Syndrome ( $45$ , XO): Females with one X; short stature, often sterile. * Klinefelter's Syndrome ( $47$ , XXY): Males with extra X; usually sterile, most common sex chromosome abnormality ( $1$ in $500$ births).
Differentiation: Embryos are sexually indifferent until Week $7$ (males via testosterone) or Week $8$ (females).
Menopause: Occurs between ages $46-54$ when a year passes without menstruation. Estrogen decline leads to organ atrophy, bone loss, and increased cardiovascular risk.