Male Reproductive System Anatomy and Physiology

Reproductive System

  • Primary sex organs (gonads): testes and ovaries
    • Produce gametes (sex cells): sperm and ova
    • Secrete steroid sex hormones
      • Androgens (males)
      • Estrogens and progesterone (females)
  • Accessory reproductive organs: ducts, glands, and external genitalia

Male Reproductive System

  • Testes (within scrotum) produce sperm
  • Sperm delivered to exterior through system of ducts
    • Epididymis → ductus deferens → ejaculatory duct → urethra

Anatomy of Male Reproductive System

  • Testes: sperm-producing male gonads within the scrotum.
  • Sperm is delivered through ducts: epididymis, ductus deferens, ejaculatory duct, and urethra.
  • Accessory sex glands: seminal glands, prostate, and bulbo-urethral glands.

Scrotum and Testes

  • Scrotum: sac of skin and superficial fascia.
    • Hangs outside the abdominopelvic cavity.
    • Contains paired testes.
      • Temperature is 3°C lower than core body temperature; necessary for sperm production.
    • Midline septum divides the scrotum into two compartments, one for each testis.

Scrotum Temperature Regulation

  • Temperature kept constant by two sets of muscles:
    • Dartos muscle: smooth muscle; wrinkles scrotal skin, pulls scrotum close to body.
    • Cremaster muscles: bands of skeletal muscle that elevate testes.

Testes

  • Each testis surrounded by two tunics:
    • Tunica vaginalis: outer layer derived from peritoneum.
    • Tunica albuginea: inner layer forms fibrous capsule.
  • Septa divide testis into ~250 lobules, each containing one to four seminiferous tubules.
    • Seminiferous tubules: site of sperm production.

Seminiferous Tubules

  • Thick, stratified epithelium surrounds central fluid-containing lumen.
    • Epithelium contains spheroid spermatogenic cells (sperm-forming cells) embedded in sustentocytes (support cells).
  • Myoid cells: surround each seminiferous tubule.
    • Smooth muscle-like cells that may squeeze sperm and testicular fluids out of testes.
  • Tubules of each lobule converge to form the straight tubule.

Sperm Conveyance

  • Sperm conveyed from seminiferous tubules to straight tubule → rete testis → efferent ductules → epididymis.
    • Epididymis: head, body, and tail.
    • Sperm stored in the tail until ejaculation.
  • Interstitial endocrine cells (Leydig cells): located in soft tissue surrounding seminiferous tubules.
    • Produce androgens, such as testosterone.
    • Secrete testosterone into interstitial fluid.

Testes Blood Supply and Spermatic Cord

  • Blood supply
    • Testicular arteries: arise from the abdominal aorta
    • Testicular veins: arise from pampiniform venous plexus surrounding each testicular artery.
      • Cooler; absorb heat from testicular arteries.
      • Keep testes cool.
  • Spermatic cord: encloses nerve fibers, blood vessels, and lymphatics that supply testes.

Clinical - Homeostatic Imbalance: Testicular Cancer

  • Rare, but most common cancer in men age 15–35.
    • Risk factors include mumps that lead to orchitis (inflammation of testis).
    • Cryptorchidism is most common risk factor (nondescent of testes).
  • Sign: painless, solid mass in testis.
  • 90% cured by surgical removal of testis and often radiation or chemotherapy.

Penis

  • Penis: male copulatory organ.
  • External genitalia: scrotum and penis.
  • Consists of:
    • Root and shaft that ends in glans penis.
    • Prepuce (foreskin): cuff of loose skin covering glans.
      • Circumcision: surgical removal of foreskin.
        • 60% of males in the U.S. circumcised, but only 15% in other parts of the world.
        • Studies show a 60% reduction in HIV risk and reduced risk for other reproductive infections.

Penis Internal Structure

  • Internally, the penis is made up of the spongy urethra and three cylindrical bodies of erectile tissue, which is a spongy network of connective tissue and smooth muscle with vascular spaces.
  • Corpus spongiosum: surrounds urethra and expands to form glans and bulb.
  • Corpora cavernosa: paired dorsal erectile bodies.

Erection and Crura

  • Erection: erectile tissue fills with blood, causing the penis to enlarge and become rigid.
  • Crura: proximal ends of corpora cavernosa surrounded by ischiocavernosus muscle; anchors penis to pubic arch.

Male Perineum

  • Diamond-shaped region between pubic symphysis, coccyx, and ischial tuberosities.
  • Suspends scrotum.
  • Contains the root of the penis and anus.

Male Duct System

  • Ducts carry sperm from testes to the body exterior:
    • Epididymis
    • Ductus deferens
    • Ejaculatory duct
    • Urethra

Epididymis

  • Head: contains efferent ductules and is located on the superior aspect of the testis.
  • Body and tail: located on the posterolateral area of the testis.
  • Duct of the epididymis: ~6 m in length.
    • Microvilli (stereocilia) absorb testicular fluid and pass nutrients to stored sperm.
  • Nonmotile sperm enter, pass slowly through (~20 days), and become motile.
    • Can be stored for several months.
  • During ejaculation, the epididymis contracts, expelling sperm into the ductus deferens.

Ductus Deferens and Ejaculatory Duct

  • Ductus deferens (vas deferens): ~45 cm long.
    • Passes through the inguinal canal to the pelvic cavity.
    • Expands to form the ampulla.
    • Joins the duct of the seminal vesicle to form the ejaculatory duct.
  • Smooth muscle in walls propels sperm from epididymis to urethra.
  • Vasectomy: cutting and ligating the ductus deferens.
    • Nearly 100% effective form of birth control.

Urethra

  • Conveys both urine and semen (at different times).
  • Has three regions:
    • Prostatic urethra: surrounded by the prostate.
    • Intermediate part of the urethra (membranous urethra): in the urogenital diaphragm.
    • Spongy urethra: runs through penis; opens at external urethral orifice.

Male Accessory Glands

  • Seminal glands (seminal vesicles)
    • On posterior bladder surface
    • Contains smooth muscle that contracts during ejaculation
    • Produces viscous alkaline seminal fluid
      • Fructose, citric acid, coagulating enzyme (vesiculase), and prostaglandins
      • Yellow pigment fluoresces with UV light
      • Comprises 70% volume of semen
    • Duct of seminal gland joins ductus deferens to form ejaculatory duct
  • Prostate
    • Encircles urethra inferior to bladder
    • Size of peach pit
    • Consists of smooth muscle that contracts during ejaculation
    • Secretes milky, slightly acid fluid
      • Contains citrate, enzymes, and prostate-specific antigen (PSA)
      • Plays a role in sperm activation
      • Enters prostatic urethra during ejaculation
      • Makes up one-third of semen volume
  • Prostate disorders
    • Prostatitis: inflammatory disorders
      • Bacterial infection; acute and chronic; treated with antibiotics
      • Chronic prostatitis: also called pelvic pain syndrome is most common form; two types:
        • Inflammatory type: urinary tract infection symptoms; pain in external genitalia and lower back; leukocytes in urine
        • Noninflammatory type: same as inflammatory but no leukocytes or bacteria in urine
    • Benign prostatic hyperplasia
      • May be age-related
      • Distorts urethra
      • Treated with surgery, but newer options include:
        • Using microwaves or drugs to shrink prostate
        • Balloon compression
        • Radio-frequency radiation
    • Prostate cancer
      • Second most common cause of cancer death in males
      • Digital exam screening, PSA levels
        • Biopsy if abnormal
      • Treated with surgery and sometimes radiation, castration, drugs
      • In clinical trials: cryosurgery, chemotherapy, ultrasound, proton beam therapy
  • Bulbo-urethral glands
    • Pea-sized glands inferior to prostate
    • Produce thick, clear mucus during sexual arousal
      • Lubricate glans penis
      • Neutralize traces of acidic urine in urethra

Semen

  • Milky-white mixture of sperm and accessory gland secretions.
    • 2–5 ml semen are ejaculated containing 20–150 million sperm/ml.
  • Contains fructose for ATP production, protects and activates sperm, and facilitates sperm movement.
  • Alkaline fluid neutralizes acidity of male urethra and female vagina and enhances motility.
  • Characteristics of semen
    • Contain prostaglandins that decrease viscosity of mucus in cervix and stimulate reverse peristalsis in uterus.
    • Contains the hormone relaxin and other enzymes that enhance sperm motility.
    • Contains ATP for energy.
    • Can suppress female immune response.
    • Antibiotic chemicals destroy some bacteria.
    • Clotting factors coagulate semen initially to prevent draining out, then liquefy it by fibrinolysin so sperm can finish the journey.

Male Sexual Response

  • Erection: Enlargement and stiffening of penis.
  • Arterioles are normally constricted.
    • Sexual excitement causes CNS activation of parasympathetic neurons, which releases nitric oxide (NO).
      • NO release causes relaxation of local vascular smooth muscle.

Erection Mechanism

  • When smooth muscles relax, arterioles dilate.
  • Corpora cavernosa expands and retards venous drainage, leading to engorgement of erectile tissues with blood and enlargement and stiffening of penis.
  • Initiated by sexual stimuli such as touch; mechanical stimulation of penis; erotic sights, sounds, and smells.
  • Can be induced or inhibited by emotions or higher mental activity.
  • Longitudinal and circular collagen fibers around penis prevent kinking/buckling of erect penis.
  • Corpus spongiosum keeps urethra open.

Ejaculation

  • Propulsion of semen from the male duct system.
  • Sympathetic spinal reflex
    • Bladder sphincter muscle constricts, preventing expulsion of urine.
    • Ducts and accessory glands contract and empty their contents.
    • Bulbospongiosus muscles undergo rapid series of contractions that cause expulsion of semen at ~ 500500 cm/s (close to 11 mph).
    • The Ejaculatory event is called climax (orgasm).

Clinical - Homeostatic Imbalance: Erectile Dysfunction

  • Parasympathetic nerves of the penis release too little NO.
  • Possible causes: alcohol, drugs, hormones, blood vessel or nervous system problems, incompetent venous valves that fail to retain blood in penis.
  • New drugs (Viagra, Cialis) potentiate existing NO effects.

Spermatogenesis

  • Spermatogenesis: production of sperm (spermatozoa) in seminiferous tubules.
  • Most body cells have 46 chromosomes:
    • Two sets (23 pairs) of chromosomes
      • One maternal, one paternal: homologous chromosomes
      • Referred to as diploid chromosomal number (2n)
  • Gametes have only 23 chromosomes: haploid chromosomal number (n)
    • Only one member of the homologous pair

Meiosis Compared to Mitosis

  • Gamete formation involves meiosis, which differs from mitosis.
    • Involves two consecutive cell divisions (meiosis I and II) but only one round of DNA replication.
      • Produces four daughter cells.
  • Functions of meiosis
    • Number of chromosomes is cut in half (2n to n).
    • Introduces genetic diversity, as all daughter cells are genetically different from the original cell.

Meiosis I

  • Reduction division of meiosis: reduces chromosome number from 2n→n2n → n.
  • Prophase I has events not seen in mitosis or meiosis II
    • Synapsis: homologous chromosomes pair up, forming tetrads consisting of 4 chromatids.
    • Crossover (chiasmata): exchange of genetic material between male and female chromatids.
      • Results in unique chromosomes that are mixtures of maternal and paternal chromosomes.
  • During metaphase I, tetrads line up randomly at the spindle equator.
  • During anaphase I, sister chromatids of one homologous chromosome are separated from sister chromatids of the other.
  • At the end of meiosis I, each daughter cell has:
    • Two copies (sister chromatids) of one member of each homologous pair (either maternal or paternal) and none of the other.
    • Haploid chromosomal number because still-united sister chromatids are considered one chromosome (twice the amount of DNA in each chromosome).

Meiosis II

  • Equational division of meiosis.
    • Events are similar to mitosis, except there is no chromosome replication before the process begins.
    • Sister chromatids from meiosis I are separated and pulled toward opposite poles.
      • Results in one per cell.
  • Meiosis I accomplishes two important tasks:
    • Reduces chromosomal number by half.
    • Introduces genetic variability.
      • Random alignment of homologous pairs in meiosis I leads to the variability of gametes.
      • Crossover → variability of gametes.
        • Results in no two gametes being exactly alike.
        • All are different from the original mother cells.

Spermatogenesis: Summary of Events in the Seminiferous Tubules

  • Occurs in seminiferous tubules of testis.
  • Spermatogenic cells give rise to sperm.
  • Overview of three steps of spermatogenesis
    1. Mitosis of spermatogonia (stem cell) forms two spermatocytes.
    2. Meiosis
      • Spermatocytes form secondary spermatocytes, which form spermatids.
    3. Spermiogenesis
      • Spermatids become sperm.

Mitosis of Spermatogonia

  • Spermatogenesis begins at puberty.
  • Begins with spermatogonia.
    • Stem cells that are in contact with the epithelial basal lamina.
    • Each mitotic division yields one type A daughter cell and one type B daughter cell.
      • Type A cells maintain germ cell line at the basal lamina.
      • Type B cells move toward the lumen and develop into primary spermatocytes.

Meiosis: Spermatocytes to Spermatids

  • Meiosis I
    • Primary spermatocyte (2n) undergoes meiosis I, forming two secondary spermatocytes (n).
  • Meiosis II
    • Each secondary spermatocyte (n) rapidly undergoes meiosis II to become two spermatids (n).
  • Spermatids: small, nonmotile cells found close to the lumen of the tubule.

Spermiogenesis: Spermatids to Sperm

  • Spermatids
    • Contain correct haploid chromosome number needed for fertilization (n).
    • Nonmotile
  • Spermiogenesis
    • Streamlining process where the spermatid elongates, loses excess cytoplasm, and forms a tail to become a spermatozoon (sperm).

Major Regions of Sperm

  • Head: genetic region that includes the nucleus and helmet-like acrosome containing hydrolytic enzymes that enable sperm to penetrate egg.
  • Midpiece: metabolic region containing mitochondria that produce ATP to move the tail.
  • Tail: locomotor region that includes flagellum.

Role of Sustentocytes

  • Large supporting cells (also called Sertoli cells).
    • Extend through the wall of the tubule and surround developing cells.
    • Provide nutrients and signals to dividing cells.
    • Move cells along to the lumen.
    • Secrete testicular fluid into lumen for sperm transport.
    • Phagocytize faulty germ cells and excess cytoplasm.
    • Produce chemical mediators to regulate spermatogenesis.
  • Sustenocytes contain tight junctions that divide tubule into two compartments
    • Basal compartment: area where spermatogonia and primary spermatocytes are located.
    • Adluminal compartment: area where meiotically active cells and tubule lumen are located.

Blood Testis Barrier

  • Tight junctions form the blood-testis barrier.
    • Prevents sperm antigens from escaping into the blood and causing activation of the immune system.
    • Sperm is not formed until puberty, so it is absent during immune system development.
      • Results in sperm not being recognized as "self."
      • Therefore, sperm needs to be kept separated from the rest of the body to avoid being attacked by the immune system.
  • Spermatogenesis takes 64–72 days if conditions are hospitable.
  • Sperm are unable to swim, but the pressure of testicular fluid pushes immotile sperm into the epididymis, where they gain motility and fertilizing power.

Regulation of Male Reproductive System

  • The Hypothalamic-Pituitary-Gonadal (HPG) Axis
  • Production of gametes and sex hormones is regulated by a sequence of hormonal events involving the hypothalamus, anterior pituitary gland, and testes.
    • Referred to as the hypothalamic-pituitary-gonadal (HPG) axis.
    • Involves interacting hormones: GnRH, FSH, LH, testosterone, and inhibin.

Sequence of Regulatory Events in HPG Axis

  1. Hypothalamus releases gonadotropin-releasing hormone (GnRH).
  2. GnRH binds to anterior pituitary gonadotropic cells, causing them to secrete:
    • Follicle-stimulating hormone (FSH) and
    • Lutenizing hormone (LH)
  3. FSH stimulates spermatogenesis indirectly by stimulating sustentocytes to release androgen-binding protein (ABP).
    • ABP keeps concentration of testosterone high near spermatogenic cells, promoting spermatogenesis.
  4. LH binds to interstitial endocrine cells, prodding them to secrete testosterone.
    • Rising testosterone levels trigger spermatogenesis.
  5. Testosterone entering blood stimulates sex organ maturation, development/maintenance of secondary sex characteristics, and libido.
  6. Rising testosterone levels feed back on the hypothalamus to inhibit GnRH and on the pituitary to inhibit gonadotropin release.
  7. Inhibin: released by sustenocytes when sperm count is high; inhibits GnRH and FSH release.

HPG Axis Balance

  • The Amount of testosterone and sperm produced by the testes reflects a balance among three interacting sets of hormones.
    • The balance takes 3 years to achieve, after which testosterone and sperm production are fairly stable throughout life.
    • Without GnRH and gonadotropins, testes atrophy, and sperm and testosterone production ceases.
  • Before birth, a male infant has testosterone levels two-thirds of an adult.
    • Soon after birth, levels recede and remain low through childhood until puberty.

Mechanism and Effects of Testosterone Activity

  • Testosterone, synthesized from cholesterol, is transformed at some target cells.
    • Converted to dihydrotestosterone (DHT) in prostate and estradiol in some brain neurons.
    • Prompts spermatogenesis and targets all accessory organs.
    • Has multiple anabolic effects throughout body.
  • Deficiency leads to atrophy of accessory organs, semen volume declines, and erection/ejaculation are impaired; treatment: testosterone replacement.

Male Secondary Sex Characteristics

  • Features induced in nonreproductive organs by male sex hormones (mainly testosterone).
    • Appearance of pubic, axillary, and facial hair.
    • Enhanced growth of chest hair; deepening of voice.
    • Skin thickens and becomes oily.
    • Bones grow, increase in density.
    • Skeletal muscles increase in size and mass.
    • Boosts basal metabolic rate.
    • Basis of sex drive (libido) in males.
  • Testosterone
    • Masculinizes embryonic brain.
    • Continues to exert effect well into adulthood.
    • Although adrenal glands also produce androgens in small amounts, production is insufficient to maintain normal testosterone-mediated functions.