Reproductive System - Male Anatomy and Physiology
Reproductive System
The reproductive system differs from other organ systems as it doesn't function continuously and becomes active at puberty.
Male and female reproductive organs have different structures but share four main tasks:
Forming specialized cells for sexual reproduction called gametes.
Sperm: male gametes.
Ova (eggs): female gametes.
Bringing gametes together through sexual intercourse (copulation).
Combining genetic information through fertilization.
Zygote: formed by the fusion of sperm and egg, the first cell of a new individual.
Supporting the development of the fetus (gestation) and birth of the baby (parturition).
Male and Female Reproductive System Similarities
Male and female reproductive structures are homologous, sharing a common origin during development. Examples include:
Male testes and female ovaries.
Male penis and female clitoris.
Gonads and Accessory Reproductive Organs
Primary sex organs (gonads):
Testes (male).
Ovaries (female).
Gonads produce:
Gametes: sperm (male) and ova (female), formed by meiosis.
Sex hormones (steroid hormones): testosterone (males), estrogens, and progesterone (females).
These hormones are vital for the development and function of reproductive organs and other tissues, influencing sexual behavior and drives.
Accessory reproductive organs: ducts, glands, and external genitalia.
Hypothalamic-Pituitary-Gonadal (HPG) Axis
The production of gametes and sex hormones is regulated by the hypothalamic-pituitary-gonadal (HPG) axis.
The HPG axis involves interacting hormones:
Gonadotropin-releasing hormone (GnRH).
Follicle-stimulating hormone (FSH).
Luteinizing hormone (LH).
Testosterone.
Inhibin.
Hormones Involved in the HPG Axis
Gonadotropin-releasing hormone (GnRH):
Released from the hypothalamus and reaches the anterior pituitary cells via the hypophyseal portal system.
Follicle-stimulating hormone (FSH) and Luteinizing hormone (LH):
Gonadotropins released from the anterior pituitary.
Sex hormones:
Testosterone (males), estrogen, and progesterone (females) act on target tissues in the body and exert negative feedback on the hypothalamus and anterior pituitary.
Inhibin:
Released from the gonads of both males and females and exerts negative feedback on FSH release from the anterior pituitary.
Activation of the HPG Axis at Puberty
Puberty: the period when reproductive organs grow to adult size and become functional, marking the earliest time reproduction is possible.
Before puberty, low levels of circulating steroidal sex hormones suppress GnRH secretion by the hypothalamus.
As puberty approaches, the hypothalamus becomes less sensitive to inhibition by sex hormones, leading to the release of GnRH in a pulsatile manner.
The anterior pituitary releases FSH and LH in response to increased GnRH levels.
FSH and LH stimulate the gonads to release more sex hormones.
The threshold for inhibition of GnRH rises, resulting in increased release of sex hormones until an adult pattern of hormone interaction is achieved.
Meiosis
Meiosis is a unique type of nuclear division that occurs only in the gonads.
It is the same process for males and females.
Sexual reproduction depends on meiosis because it reduces the number of chromosomes in gametes by half, preventing the zygote from having twice as many chromosomes.
Terminology of Meiosis
Sister chromatids: cell's DNA is replicated during interphase.
Each chromosome has two identical sister chromatids joined by a centromere.
Homologous pairs of chromosomes: one from the father (paternal chromosome) and one from the mother (maternal chromosome).
They look alike and carry the same genes coding for the same traits.
Nonsister chromatids: chromatids of one chromosome within a homologous pair are nonsister chromatids to the other chromosome in the pair.
Nonsister chromatids exchange DNA in a process called crossing over, increasing genetic variability.
Diploid: normal number of chromosomes in most body cells, consisting of two sets (one from each parent), symbolized as 2n.
Humans have a diploid number of 46; diploid cells contain 23 pairs of homologous chromosomes.
Haploid: number of chromosomes in a gamete, symbolized as n.
Gametes contain only one member of each homologous pair.
Humans have a haploid number of 23.
When sperm and egg fuse, they form a diploid (2n) zygote.
Sequence of Events in Meiosis
Meiosis involves two consecutive nuclear divisions: meiosis I and meiosis II, with DNA replication occurring only once.
The product is four haploid daughter cells.
Each nuclear division has four steps: prophase, metaphase, anaphase, and telophase.
Overview of meiosis:
During interphase, all chromosomes are replicated.
During meiosis I, homologous pairs separate, reducing the chromosome number from 2n to n (reduction division).
During meiosis II, sister chromatids separate, distributing an equal share of chromosomes to each daughter cell (equational division).
Detailed Look at Meiosis I
Chromosome number is reduced from 2n to n.
Prophase I includes events not seen in mitosis or meiosis II:
Synapsis: homologous chromosomes pair up to form tetrads consisting of four chromatids.
Crossover (chiasmata): exchange of genetic material between male and female chromatids, resulting in unique chromosomes that are mixtures of maternal and paternal chromosomes.
Metaphase I: tetrads line up randomly at the spindle equator, allowing either the paternal or maternal homologous chromosome to be on either side of the equator.
Anaphase I: sister chromatids of one homologous chromosome separate from sister chromatids of the other homologous pair.
Two joined sister chromatids of each homologous pair are distributed to opposite ends of the cell.
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.
A haploid chromosomal number, because the still-united sister chromatids are considered one chromosome (with twice the amount of DNA).
Meiosis II
Meiosis II mirrors mitosis except for the following:
Chromosomes are not replicated before the process begins.
Cells undergoing meiosis II are haploid, not diploid.
Meiosis Compared with Mitosis
Mitosis:
Ensures all cells have the same DNA for normal growth and repair.
Meiosis:
Reduces the chromosome number by half (2n to n).
Introduces genetic diversity through random alignment of homologous pairs and crossing over, making daughter cells genetically different from the original cell.
Male Reproductive Anatomy
Testes: sperm-producing male gonads within the scrotum.
Sperm delivery system:
Epididymis.
Ductus deferens.
Ejaculatory duct.
Urethra.
Accessory sex glands: seminal glands, prostate, and bulbo-urethral glands.
Scrotum
Sac of skin and superficial fascia hanging outside the abdominopelvic cavity at the root of the penis.
Contains paired testes kept at lower than core body temperature, necessary for sperm production.
The midline septum divides the scrotum into two compartments, one for each testis.
Temperature changes affect the scrotum:
When cold, testes are pulled closer to the warm body wall by the dartos muscle (wrinkles skin) and cremaster muscles (elevate testes).
Testes
Each testis is surrounded by two tunics:
Tunica vaginalis: outer layer derived from the peritoneum.
Tunica albuginea: inner layer forming a fibrous capsule.
Septa divide the testis into approximately 250 lobules, each containing one to four seminiferous tubules (site of sperm production).
Sperm travel from seminiferous tubules to the straight tubule → rete testis → efferent ductules → epididymis.
The epididymis consists of the head, body, and tail, where sperm are stored until ejaculation.
Blood supply:
Testicular arteries arise from the abdominal aorta.
Testicular veins form the pampiniform venous plexus, which cools and absorbs heat from the testicular arteries to keep the testes cool.
The spermatic cord encloses nerve fibers, blood vessels, and lymphatics that supply the testes.
Clinical Imbalance: Testicular Cancer
Rare, but the most common cancer in men aged 15–35.
Risk factors: mumps leading to orchitis (inflammation of the testis) and cryptorchidism (nondescent of testes).
Sign: painless, solid mass in the testis.
90% cure rate via surgical removal of the testis, often combined with radiation or chemotherapy.
Male Duct System
Accessory ducts transport sperm from the testes to the body exterior:
Epididymis
Ductus deferens
Ejaculatory duct
Urethra
Epididymis
Sperm mature in the epididymis.
Head: contains efferent ductules that empty into the highly coiled duct of the epididymis.
Body and tail: make up the remainder of the duct of the epididymis, approximately 6 m (20 ft) in length.
Pseudostratified epithelial cells of the duct mucosa contain microvilli (stereocilia).
Large surface area allows absorption of excess testicular fluid and transfers nutrients to stored sperm.
Nonmotile sperm enter and slowly pass through over approximately 20 days, gaining the ability to swim.
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): approximately 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 the walls propels sperm from the epididymis to the urethra.
Vasectomy: cutting and ligating the ductus deferens, a nearly 100% effective form of birth control.
Urethra
Conveys both urine and semen (at different times).
Three regions:
Prostatic urethra: surrounded by the prostate.
Intermediate part of the urethra (membranous urethra): located in the urogenital diaphragm.
Spongy urethra: runs through the penis and opens at the external urethral orifice.
Penis
Penis: male copulatory organ.
External genitalia: scrotum and penis.
Consists of:
Root and shaft ending in the glans penis.
Prepuce, or foreskin: a cuff of loose skin covering the glans.
Circumcision: surgical removal of the foreskin (60% of males in the U.S. are circumcised).
Studies show a 60% reduction in HIV risk and reduced risk for other reproductive infections.
Internally, the penis consists of the spongy urethra and three cylindrical bodies of erectile tissue (spongy network of connective tissue, smooth muscle, and vascular spaces).
Corpus spongiosum: surrounds the urethra and expands to form the glans and bulb of the penis.
Corpora cavernosa: paired dorsal erectile bodies.
Erection: erectile tissue fills with blood, causing the penis to enlarge and become rigid.
Crura: proximal ends of the corpora cavernosa surrounded by the ischiocavernosus muscle; anchors the penis to the pubic arch.
Male Perineum
Diamond-shaped region between the pubic symphysis, coccyx, and ischial tuberosities.
Suspends the scrotum.
Contains the root of the penis and the anus.
Male Accessory Glands
Seminal glands (seminal vesicles):
Located on the posterior bladder surface.
Contain smooth muscle that contracts during ejaculation.
Produce viscous alkaline seminal fluid:
Fructose, citric acid, coagulating enzyme (vesiculase), and prostaglandins.
Yellow pigment fluoresces with UV light.
Comprises 70% of the semen volume.
The duct of the seminal gland joins the ductus deferens to form the ejaculatory duct.
Prostate:
Encircles the urethra inferior to the bladder.
Size of a peach pit.
Consists of smooth muscle that contracts during ejaculation.
Secretes a milky, slightly acidic fluid:
Contains citrate, enzymes, and prostate-specific antigen (PSA).
Plays a role in sperm activation.
Enters the prostatic urethra during ejaculation.
Makes up one-third of semen volume.
Bulbo-urethral glands:
Pea-sized glands inferior to the prostate.
Produce thick, clear mucus during sexual arousal to:
Lubricate the glans penis.
Neutralize traces of acidic urine in the urethra.
Clinical Imbalance: Benign Prostatic Hyperplasia and Prostate Cancer
Benign prostatic hyperplasia:
Common, age-related disease affecting approximately 50% of men aged 50 and 80% of men aged 70.
Benign growth of the stroma and glands constricts the urethra, making urination difficult and leading to incomplete emptying.
Can result in urinary tract infections or kidney damage.
Treatment: drugs to relax smooth muscle and inhibit growth, surgical transurethral resection to widen the prostatic urethra.
Prostate cancer:
The third most common cause of cancer death in males, affecting 1 in 6 men in the U.S.
Screening: digital exam and PSA levels, though these have a high rate of false positives.
Treatment: surgery and sometimes radiation.
Metastatic prostate cancer is treated with drugs that block testosterone synthesis or action.
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 the male urethra and female vagina, enhancing motility.
Semen characteristics:
Contains prostaglandins to decrease mucus viscosity in the cervix and stimulate reverse peristalsis in the uterus.
Contains the hormone relaxin and other enzymes that enhance sperm motility.
Contains ATP for energy.
Can suppress the female immune response.
Antibiotic chemicals destroy some bacteria.
Clotting factors coagulate semen initially to prevent draining out, then liquefy it via fibrinolysin so sperm can finish their journey.
Male Sexual Response
Erection: enlargement and stiffening of the penis.
Arterioles are normally constricted.
Sexual excitement causes CNS activation of parasympathetic neurons innervating internal pudendal arteries, leading to local release of nitric oxide (NO).
NO release causes relaxation of local vascular smooth muscle.
When smooth muscles relax, arterioles dilate.
The corpora cavernosa expands and retards venous drainage, causing engorgement of erectile tissues with blood, resulting in enlargement and stiffening of the penis.
The corpus spongiosum keeps the urethra open.
Initiated by sexual stimuli (touch, mechanical stimulation of the penis, erotic sights, sounds, and smells).
Can be induced or inhibited by emotions or higher mental activity.
Ejaculation
Propulsion of semen from the male duct system.
A spinal reflex initiates it over sympathetic nerves serving the genital organs, resulting in:
Ductus deferens, prostate, and seminal glands contract and empty contents into the prostatic urethra.
The bladder's internal sphincter muscle constricts, preventing expulsion of urine or reflux of semen into the bladder.
Semen in the urethra triggers a spinal reflex through somatic neurons.
Bulbospongiosus muscles undergo rapid series of contractions, causing expulsion of semen at approximately 500 cm/s (close to 11 mph).
Ejaculatory event accompanied by intense pleasure (climax/orgasm) and increased heart rate and blood pressure.
Ejaculation is followed by resolution, a period of muscular and psychological relaxation.
Arteries and arterioles are constricted by signals from sympathetic nerves, reducing blood flow to the penis.
Latent (refractory) period: the time during which a man is unable to achieve another orgasm.
Can last minutes to hours and lengthens with age.
Clinical Imbalance: Erectile Dysfunction (ED)
Parasympathetic nerves of the penis release too little NO, leading to an inability to attain an erection.
Affects approximately 50% of American men over 40.
Temporary ED can be caused by psychological factors, alcohol, or drugs (antihypertensives, antidepressants, etc.).
Chronic ED results from problems with blood vessels (atherosclerosis, varicose veins) or the nervous system (stroke, penile nerve damage, multiple sclerosis).
Diabetes mellitus is often an underlying cause.
New drugs like sildenafil (Viagra) enhance the effect of existing NO.
Spermatogenesis
Spermatogenesis: the process of forming male gametes.
Occurs in the seminiferous tubules.
Begins at puberty, around 14 years of age.
Adult males make approximately 90 million sperm daily.
Histology of the Seminiferous Tubules
Consist of a thick stratified epithelium surrounding a central fluid-filled lumen containing four important types of cells:
Sustentocytes: large columnar cells act as supporting cells and play a role in sperm formation.
Spermatogenic cells: cells that are surrounded by sustentocytes and give rise to sperm.
Myoid cells: smooth muscle-like cells surrounding the seminiferous tubule that contract to squeeze sperm and testicular fluid through the tubules.
Interstitial endocrine cells (Leydig cells): produce androgens and some estrogen.
Summary of Events in Spermatogenesis
Occurs in seminiferous tubules of the testis.
Spermatogenic cells give rise to sperm through three main steps:
Mitosis of spermatogonia (stem cell) forms two spermatocytes.
Meiosis:
Spermatocytes form secondary spermatocytes, which form spermatids.
Spermiogenesis:
Spermatids become sperm.
Mitosis of Spermatogonia
Spermatogenesis begins at puberty with spermatogonia (stem cells) in contact with the epithelial basal lamina, dividing more or less continuously via mitosis.
After puberty, each division produces:
Type A daughter cells: remain at the basal lamina to maintain the pool of dividing germ cells.
Type B daughter cells: move toward the lumen and develop into primary spermatocytes.
Meiosis: Spermatocytes to Spermatids
Meiosis I:
Primary spermatocytes (2n) undergo meiosis I, forming two secondary spermatocytes (n).
Meiosis II:
Secondary spermatocytes (n) rapidly undergo meiosis II to become two spermatids (n).
Spermatids: small, round cells with large nuclei, found close to the lumen of the tubule.
Nearly all genes are turned off, and DNA is compacted into dense pellets.
Spermiogenesis: Spermatids to Sperm
Spermatids contain the correct haploid chromosome number needed for fertilization (n) but are still nonmotile.
Spermiogenesis is the streamlining process where spermatids elongate, lose excess cytoplasm, and form a tail.
Major regions of sperm:
Head: genetic region including the nucleus and helmet-like acrosome containing hydrolytic enzymes that enable sperm to penetrate the egg.
Midpiece: metabolic region containing mitochondria that produce ATP to move the tail.
Tail: locomotor region including the flagellum.
Spermatogenesis takes 64–72 days under hospitable conditions.
Sperm are initially unable to swim, but the pressure of testicular fluid pushes immotile sperm into the epididymis, where they gain motility and fertilizing power.
Role of Sustentocytes (Sertoli Cells)
Large supporting cells extending from the basal lamina to the tubule lumen, surrounding developing spermatogonia.
Sustentocytes contain tight junctions that divide the tubule into two compartments:
Basal compartment: basal lamina to tight junctions; spermatogonia and primary spermatocytes are located here.
Adluminal compartment: internal to the tight junction; the area where meiotically active cells and the tubule lumen are located.
Tight junctions form the blood-testis barrier:
Prevents sperm antigens from escaping into the blood and causing an immune system activation.
Sperm are not formed until puberty, so they are absent during immune system development.
Results in sperm not being recognized as "self" and needing separation from the rest of the body to avoid immune attack.
Sustentocytes also:
Provide nutrients and signals to dividing cells.
Move spermatocytes and spermatids along to the lumen.
Secrete testicular fluid (rich in androgens and metabolic acid) into the lumen for sperm transport.
Phagocytize faulty germ cells and excess cytoplasm.
Produce 2 mediators to regulate spermatogenesis:
Androgen-binding protein (ABP): keeps testosterone levels high to stimulate spermatogenesis.
Inhibin: inhibits spermatogenesis by inhibiting FSH release by the anterior pituitary.
Clinical Imbalance: Infertility
Infertility affects approximately 1 in 7 couples in America and is often caused by problems with sperm quality or quantity.
A gradual decline in male fertility has occurred in the past 50 years, potentially linked to environmental toxins, phthalates (oily solvents from plastics), pesticides, herbicides, etc.
Other possibilities include:
Estrogen-like compounds blocking the action of male sex hormones.
Antibiotics such as tetracycline suppressing sperm formation.
Radiation, lead, marijuana, and excessive alcohol causing abnormal sperm production (two-headed, multiple-tailed).
Defects in calcium channels, hormonal imbalances, and oxidative stress.
Thermal-related events (e.g., hot tubs) inhibiting sperm maturation.
Regulation of the Male Reproductive System
Gamete and sex hormone production is regulated by the hypothalamic-pituitary-gonadal (HPG) axis.
Involves interacting hormones: GnRH, FSH, LH, testosterone, and inhibin.
Before birth, male infants have testosterone levels two-thirds of the adult level.
After a brief rise in early infancy, blood levels recede and remain low through childhood.
As puberty nears, higher testosterone levels are required to suppress hypothalamic GnRH release, and the adult pattern is established.
The balance among interacting hormones of the HPG axis determines the amount of testosterone and sperm produced by the testes.
This balance takes 3 years to achieve, after which testosterone and sperm production are fairly stable throughout life.
Without GnRH and gonadotropins, the testes atrophy, and sperm and testosterone production ceases.
Mechanism and Effects of Testosterone Activity
Testosterone, synthesized from cholesterol, is transformed at some target cells.
Converted to dihydrotestosterone (DHT) in the prostate and estradiol in some brain neurons.
Prompts spermatogenesis and targets all accessory organs.
Has multiple anabolic effects throughout the body.
Deficiency leads to atrophy of accessory organs, semen volume declines, and erection/ejaculation are impaired; treatment involves testosterone replacement.
Male Secondary Sex Characteristics
Features induced in non-reproductive organs by male sex hormones (mainly testosterone).
Appearance of pubic, axillary, and facial hair.
Enhanced hair growth on the chest or other areas.
Larynx enlargement, causing deepening of the voice.
Skin thickens and becomes oily.
Bones grow and increase in density.
Skeletal muscles increase in size and mass.
Boosts basal metabolic rate.
Basis of sex drive (libido) in males.