9.2 & 9.3

Anatomy and Physiology of the Scrotum

Definition and Gross Anatomy:

The scrotum is a skin-covered, muscular sack that extends externally from the body, situated posterior to the penis.
It houses the male gonads (testes) outside the abdominopelvic cavity.

Thermoregulation and Spermatogenesis:
- Sperm production (spermatogenesis) occurs within the testes and proceeds most efficiently when the testes are maintained at a temperature a few degrees below normal core body temperature ( $37\,^\circ\text{C}$ ).
- Dartos Muscle:
  - A layer of smooth muscle situated in the outer portion of the scrotum.
  - Cold Response: Contracts in cold temperatures, causing the scrotal skin to wrinkle. Wrinkling reduces the surface area of the scrotal skin, diminishing heat escape and retaining heat within the scrotum.
  - Warm Response: Relaxes in warm temperatures, smoothing out the scrotal skin, which increases surface area to promote heat loss via evaporation and radiation.
- Cremaster Muscle:
  - Skeletal and smooth muscle fibers located deeper within the scrotum.
  - Cold Response: Reflexively contracts in cold temperatures, elevating the testes and pulling them closer to the warm pelvic floor. This movement decreases scrotal surface area and absorbs body heat.
  - Warm Response: Relaxes as environmental temperature rises, allowing the testes to descend farther away from the body core, increasing scrotal surface area and promoting heat dissipation.
Spermatic Cord:
- A protective bundle of connective tissues, nerve fibers, blood vessels, and lymphatic vessels that runs from the abdominal region down to each testicle.
- Serves as a conduit for the ductus deferens (vas deferens).
- Plays a pivotal role in the thermoregulation of the testes to ensure proper spermatogenesis.
Pampiniform Plexus:
- A dense network of small venous vessels surrounding the testicular artery inside the spermatic cord.
- Functions as a countercurrent heat exchange system:
  - Warm arterial blood descending from the abdominal cavity toward the testes transfers its heat to the cooler venous blood ascending from the testes back toward the core body circulation.
  - This heat transfer cools the incoming arterial blood prior to its entry into the testes, guaranteeing that testicular temperature remains consistently below core body temperature.

Anatomy and Histology of the Testes

Histological section through seminiferous tubules

Overview of the Male Gonads:
- The testes (singular: testis) are the primary male reproductive organs (gonads).
- They produce male gametes (sperm) and androgen hormones (primarily testosterone) throughout the male reproductive lifespan.
Protective Connective Tissue Coverings:
- Tunica Vaginalis: The outer serous membrane surrounding each testis, derived from the peritoneum. Consists of a parietal layer and a thin visceral layer.
- Tunica Albuginea: A tough, white, dense connective tissue layer positioned beneath the tunica vaginalis, directly enveloping the testis tissue.
  - Invaginates inward to form fibrous septa.
  - Divides the interior of each testis into $300\text{ to }400$ internal compartments called lobules.
Intratesticular Duct Pathways:
- Seminiferous Tubules:
  - Tightly coiled tubules forming the functional bulk of each testis lobule.
  - Site where germ cells develop surrounding a central hollow space called the lumen.
- Straight Tubules (Tubuli Recti): Receive newly formed sperm from the lumens of the seminiferous tubules.
- Rete Testes: A fine meshwork of interconnected tubules that receives sperm from the straight tubules.
- Efferent Ductules: $15\text{ to }20$ delicate ducts that carry sperm out of the rete testes, crossing the tunica albuginea to enter the epididymis.
Primary Cell Types within the Testes:
- Nurse Cells (Sertoli Cells):
  - Large, branching structural cells extending from the peripheral basement membrane of the seminiferous tubule inward to the lumen.
  - Secrete signaling molecules that regulate sperm production and determine germ cell survival or apoptosis.
  - Adjacent Sertoli cells form tight junctions near the basement membrane, creating the blood--testis barrier (BTB).
    - Blood--Testis Barrier Function: Prevents bloodborne toxins, pathogens, and chemicals from reaching developing germ cells. Simultaneously prevents surface antigens of developing haploid germ cells from entering systemic circulation, avoiding autoimmune destruction of sperm.
  - Phagocytize excess cytoplasm shed by developing spermatids during differentiation.
  - Secrete testicular fluid into the tubule lumen to facilitate passive transport of immotile sperm toward the duct system.
- Interstitial Endocrine Cells (Leydig Cells):
  - Located in the interstitial connective tissue spaces surrounding the seminiferous tubules.
  - Synthesize and secrete male androgenic hormones, primarily testosterone.
- Germ Cells (Spermatogonia):
  - Stem cells lined along the basement membrane of the seminiferous tubules, capable of continuous differentiation into primary spermatocytes, secondary spermatocytes, spermatids, and mature sperm.

Fetal Testicular Descent and Clinical Anomalies

Embryonic Descent of the Testes:
- During early fetal development, the testes develop within the abdominal cavity adjacent to the kidneys.
- They are anchored to the peritoneum by a fibrous connective tissue cord termed the gubernaculum.
- During the $7\text{th month}$ of fetal gestation, testosterone secretion stimulates contraction of the gubernaculum.
- The contracting gubernaculum pulls each testis inferiorly through the abdominal wall musculature via the inguinal canal, depositing them into the scrotal cavity alongside peritoneal extensions.
Cryptorchidism:
- Clinical condition defined as the failure of one or both testes to descend into the scrotal cavity prior to birth.
- Occurs with higher frequency in premature infants (where descent frequently resolves within several weeks post-birth).
- Pathophysiological Consequences: An undescended testis exposed to core body temperature cannot carry out spermatogenesis, leading to infertility.
- Clinical Management: Requires surgical repositioning into the scrotum prior to puberty. If left uncorrected, cryptorchidism causes permanent sterility and significantly increases the risk of testicular cancer.

Sperm Transport and Physiology of the Epididymis

Sequence of Sperm Transport:
- Seminiferous tubule lumens → Straight tubules → Rete testes → Efferent ductules → Epididymis (head, body, tail) → Ductus (vas) deferens → Ampulla → Ejaculatory duct → Prostatic urethra → Membranous urethra → Spongy (penile) urethra → External urethral orifice.
Structure of the Epididymis:
- A highly coiled, single tube structure attached to the posterior border of each testis.
- Measures approximately $20\text{ feet}$ ( $\approx 6\,\text{meters}$ ) in length when fully uncoiled.
- Divided into three anatomical regions: the head (caput), body (corpus), and tail (cauda).
Physiological Functions of the Epididymis:
- Sperm Transit & Maturation: Takes an average of $14\,\text{days}$ for sperm to pass through the coils of the epididymis.
- Acquisition of Motility: Newly formed sperm entering the head of the epididymis are immotile; as smooth muscle contractions move them along the tube, they mature and gain independent flagellar motility required to navigate the female reproductive tract.
- Fluid Absorption and Concentration: Epididymal epithelial cells reabsorb excess testicular fluid, increasing sperm concentration within semen.
- Protection: Cells of the epididymis protect sperm from immune recognition and secrete antioxidant enzymes that guard against oxidative cell damage.
- Storage and Reabsorption: Fully mature sperm are stored in the tail of the epididymis until ejaculation. Sperm that are not ejaculated are broken down and reabsorbed by epididymal epithelial cells.

Anatomy and Morphological Structure of Sperm

Quantitative Features and Lifespan:
- Sperm are exceptionally small cells; cell volume is $85{,}000\times$ smaller than that of a female oocyte.
- Production rate: Approximately $100 \times 10^6\text{ to }300 \times 10^6\,\text{sperm/day}$ (contrasting with female ovulation of $1\,\text{oocyte/month}$ ).
- Survival in Female Tract: Sperm can live up to $5\,\text{days}$ inside the female reproductive tract without direct vascular oxygen or nutrient delivery, expanding the fertility window.
Anatomical Subdivisions of Formed Sperm:
- Head:
  - Contains an extremely compact haploid nucleus ( $n = 23$ chromosomes) with minimal cytoplasm.
  - Acrosome: A specialized, cap-like vesicle covering the anterior head region. Packed with lysosomal enzymes necessary for penetrating the outer cellular layers of the female egg during fertilization.
- Midpiece:
  - Densely packed with spiral-wrapped mitochondria.
  - Produces ATP via cellular respiration to supply kinetic energy for flagellar movement.
- Tail (Flagellum):
  - Extends from the neck and midpiece down the length of the cell.
  - Driven by an inner central structure called the axial filament, which develops from one centriole during late spermatogenesis.

Anatomy of the Male Duct System and Vasectomy

Ductus Deferens (Vas Deferens):
- A thick, highly muscular tube that receives sperm from the tail of the epididymis during ejaculation.
- Travels inside the scrotum bundled with connective tissues, blood vessels, and nerves within the spermatic cord.
- Passes superiorly into the pelvic cavity through the inguinal canal in the anterior abdominal wall.
- Courses posteriorly along the base of the urinary bladder, expanding into a swollen terminal region called the ampulla.
- Unites with the duct of the seminal vesicle to form the short ejaculatory duct, which empties into the prostatic urethra.
Vasectomy:
- A surgical sterilization procedure performed by making a small incision in the scrotum, cutting, and sealing a small section of each ductus deferens.
- Blocks sperm from traveling from the testes to the ejaculate.
- Considered a permanent form of birth control by medical practitioners, requiring full certainty prior to surgery.

Accessory Glands and Semen Composition

Physiological Role and Volume of Semen:
- Ejaculated semen is a thick, milky fluid composed of sperm ( $5\%$ of volume) and seminal plasma ( $95\%$ of volume derived from accessory glands).
- Normal ejaculate volume: $2\text{ to }5\,\text{mL}$ , containing $20 \times 10^6\text{ to }150 \times 10^6\,\text{sperm/mL}$ .
- Alkalinity: Semen maintains an alkaline pH ranging from $7.2\text{ to }8.0$ . This neutralizes the acidic environment of the female vagina ( $\text{pH } 3.5\text{ to }4.5$ ), protecting sperm from microbial-defense acidity and optimizing flagellar motility.
Seminal Vesicles (Seminal Glands):
- Paired glands situated posterior to the urinary bladder.
- Contribute approximately $60\%$ of total semen volume.
- Secretion Components:
  - Fructose: High concentrations used by sperm mitochondria to synthesize ATP for locomotion.
  - Ascorbic Acid.
  - Prostaglandins: Stimulate uterine muscle contraction to assist sperm motility.
  - Coagulating Enzymes.
Prostate Gland:
- A single, walnut-sized structure located inferior to the bladder, anterior to the rectum, surrounding the prostatic urethra.
- Composed of muscular tissue and glandular tubuloalveolar glands (containing elongated tubular and sac-like alveolar secretory units).
- Secretes a milky, alkaline fluid into passing seminal fluid.
- Secretion Components: Citric acid (nutrient substrate), zinc, antimicrobial compounds, and prostate-specific antigen (PSA).
- Semen Coagulation and Liquefaction Cycle:
  1. Upon ejaculation, proteins from seminal vesicle fluid cause semen to rapidly clot into a gel-like state for $15\text{ to }20\,\text{minutes}$ .
  2. Coagulation prevents semen from washing out of the vagina, allowing time for sperm to absorb fructose.
  3. PSA (a proteolytic enzyme) breaks down coagulating proteins, liquefying the gel so sperm can swim freely into the cervix.
- Liquid → coagulates → liquid
Bulbourethral Glands (Cowper's Glands):
- Paired, pea-sized glands positioned inferior to the prostate.
- Secrete a clear, alkaline mucus (pre-ejaculate) during sexual arousal prior to ejaculation.
- Secretion Components: Rich in sodium, potassium, chloride, and bicarbonate ions.
- Function: Cleanses urine residues and neutralizes urethral acidity within the penile urethra, while lubricating the glans penis.
- Clinical Point: Pre-ejaculate can pick up viable residual sperm stored in the urethra from past ejaculations, making pregnancy possible without full ejaculation.

Mechanics of Cell Division: Mitosis vs. Meiosis

Mitosis:
- Process of cell division wherein a single parent cell yields two genetically identical daughter cells.
- Preserves the original diploid ( $2n$ ) chromosome number of $46$ chromosomes.
- Essential for tissue repair, somatic growth, and germline stem cell maintenance.
Meiosis:
- Two-stage division process resulting in four genetically unique haploid ( $n$ ) gametes, each containing $23$ chromosomes.
- Meiosis I:
  - Homologous chromosome pairs align to form tetrads.
  - Crossing over exchanges genetic material between non-sister chromatids, enhancing genetic diversity.
  - Homologous chromosome pairs separate into two separate cells; sister chromatids remain joined.
- Meiosis II:
  - Resembles mitosis; sister chromatids within each of the two daughter cells are pulled apart.
  - Yields four total haploid gametes ( $n = 23$ chromosomes).
  - Fertilization restores the diploid chromosome number ( $2n = 46$ ) in the zygote.

Stages of Spermatogenesis

Overview and Dynamics:
- Spermatogenesis is the process of generating mature sperm cells from stem cells.
- Begins at puberty (ages $9\text{ to }14$ ) and continues uninterrupted throughout life, slowing after age $35$ .
- A single full cycle from spermatogonial stem cell to released sperm in the tubule lumen takes approximately $60\,\text{days}$ .
Phase 1: Proliferative Phase (Spermatogonial Mitosis):
- Spermatogonia are diploid stem cells ( $2n = 46$ ) positioned along the peripheral basement membrane of the seminiferous tubule.
- Undergo mitotic division to produce two identical diploid cells:
  - Type A Cell: Remains at the basement membrane to preserve the germline stem cell line.
  - Type B Cell: Moves toward the tubule lumen and differentiates into a primary spermatocyte ( $2n = 46$ ).
Phase 2: Meiotic Phase:
- The primary spermatocyte replicates its DNA and enters Meiosis I, dividing into two secondary spermatocytes ( $n = 23$ , each containing duplicated sister chromatids).
- Secondary spermatocytes immediately enter Meiosis II, separating sister chromatids to produce four haploid spermatids ( $n = 23$ ).
- Early spermatids are round cells with central nuclei and large amounts of cytoplasm.
Phase 3: Differentiation Phase (Spermiogenesis):
- A $24\text{-day}$ maturation process where round spermatids morphologically transform into formed sperm near the tubule lumen.
- Key Structural Changes:
  1. Excess cytoplasm is shed and phagocytized by Sertoli cells.
  2. The acrosomal vesicle expands over the nucleus to form the acrosome cap.
  3. The axial filament elongates from a centriole to form the tail flagellum.
  4. Mitochondria condense around the midpiece.
- Formed sperm are released into the lumen (spermiation) and carried by fluid into the epididymis.

Endocrine Regulation of Male Reproduction

Biosynthesis of Testosterone:
- Testosterone is a steroid androgen synthesized from cholesterol inside the Leydig (interstitial) cells.
- Embryonic Development: Leydig cells begin secreting testosterone by the $7\text{th week}$ of gestation, peaking in the second trimester to drive male anatomical sex differentiation.
- Puberty: Secretion ramps up between ages $9\text{ and }14$ , activating secondary sexual characteristics and triggering spermatogenesis.
Intratesticular vs. Systemic Testosterone Functions:
- Testicular Steroidogenesis: Intratesticular testosterone concentration is maintained $100\times$ higher than systemic circulation levels, which is strictly required to sustain spermatogenesis.
- Systemic Functions:
  - Stimulates skeletal muscle growth and muscle mass accumulation.
  - Promotes bone growth and density.
  - Drives facial, chest, body, and pubic hair growth.
  - Enlarges the larynx, thickening vocal cords to deepen the voice (forming the thyroid cartilage protrusion).
  - Shifts body fat distribution (reducing hip fat, increasing abdominal fat).
  - Promotes growth and maturation of the penis and testes.
  - Increases sebaceous gland activity (causing acne).
  - Maintains libido (sex drive) in both males and females.
Hypothalamic-Pituitary-Gonadal (HPG) Negative Feedback Loop:
- Hypothalamus: Releases pulsatile Gonadotropin-Releasing Hormone (GnRH).
- Anterior Pituitary: GnRH stimulates the release of two gonadotropic hormones:
  - Follicle-Stimulating Hormone (FSH): Binds to Sertoli cells in seminiferous tubules, causing them to release Androgen-Binding Protein (ABP) (which binds local testosterone to keep its concentration high) and Inhibin.
  - Luteinizing Hormone (LH): Binds to Leydig cells, upregulating testosterone synthesis.
- Feedback Controls:
  - High circulating testosterone inhibits GnRH release from the hypothalamus and LH release from the anterior pituitary.
  - When sperm count is high, Sertoli cells release Inhibin, which selectively acts on the anterior pituitary to stop FSH secretion, slowing down spermatogenesis.

Physiology of Male Sexual Response

Erectile Tissue Anatomy of the Penis:
- The penis consists of a shaft enclosing three cylindrical columns of erectile tissue:
  - Corpora Cavernosa: Paired, larger dorsolateral erectile chambers.
  - Corpus Spongiosum: A smaller midventral erectile column surrounding the spongy (penile) urethra, extending distally into the nerve-dense glans penis.
Mechanism of Erection (Vasocongestion):
- Resting State: Arterial branches inside erectile tissue are constricted under sympathetic tone, restricting blood flow.
- Erection Process:
  1. Sexual stimulation triggers parasympathetic nervous system activity.
  2. Parasympathetic neurons release Nitric Oxide (NO) into penile blood vessels.
  3. NO induces smooth muscle relaxation in penile arterial walls, causing vasodilation.
  4. Rapid arterial inflow fills the erectile spaces of the corpora cavernosa and spongiosum.
  5. High tissue pressure compresses thin-walled penile venules against the outer connective tissue sheath, blocking venous drainage and producing penile rigidity (turgidity).
Mechanism of Ejaculation:
- Ejaculation is a sympathetic nervous system reflex paired with somatic motor output:
  - Emission Phase: Peristaltic smooth muscle contractions sweep sperm from the epididymis and ductus deferens into the urethra, mixing with fluids from the seminal vesicles and prostate. The internal urethral sphincter constricts to prevent urine release or retrograde ejaculation into the bladder.
  - Expulsion Phase: Somatic motor neurons in the sacral/lumbar spinal cord trigger rhythmic contractions of pelvic floor muscles (bulbospongiosus), ejecting semen out of the external urethral orifice accompanied by elevated heart rate and blood pressure.
- Detumescence: Post-ejaculation, sympathetic activity constricts penile arteries, opening venous drainage pathways to return the penis to a flaccid state.

Male Reproductive Aging (Andropause)

Definition and Onset:
- Andropause refers to the gradual age-related decline in serum testosterone levels and sexual function in males, typically beginning in the late $40\text{s}$ to early $50\text{s}$ .
- Exacerbated by health factors such as obesity, elevated waist circumference, chronic disease, systemic stress, and specific medications.
Symptom Profile:
- Decreased libido, erectile dysfunction, mood fluctuations, physical fatigue, and reduced skeletal muscle mass.
Contrast with Female Menopause:
- Unlike the abrupt cessation of ovulation and fertility in female menopause, male reproductive decline is gradual and progressive, allowing many males to retain fertility into advanced age.