178 The Male Reproductive System
Anatomy of the Male Reproductive System
The male reproductive system includes external (penis, scrotum, epididymus, and testes) and internal (accessory) organs.
Learning Objectives
Distinguish among the parts and functions of the male reproductive system
Key Takeaways
Key Points
- The functions of the male reproductive system include producing and transporting sperm, ejaculating sperm into the female reproductive tract, and producing and secreting male hormones.
- Most of the male reproductive system is located outside of the body. These external structures are the penis, scrotum, epididymis, and testes.
- The internal organs of the male reproductive system are called accessory organs. They include the vas deferens, seminal vesicles, prostate gland, and bulbourethral glands.
Key Terms
- semen: Contains spermatozoa, proteolytic and other enzymes, and
fructose that promotes spermatozoa survival. It also provides a medium for sperm
motility. - spermatogenesis: The process of sperm production within the seminiferous tubules in the testes.
- testosterone: Steroid hormone produced primarily in the male testes and responsible for the development of male secondary sex characteristics.
The organs of the male reproductive system are specialized for three primary functions:
- To produce, maintain, transport, and nourish sperm (the male reproductive cells), and protective fluid ( semen ).
- To discharge sperm within the female reproductive tract.
- To produce and secrete male sex hormones.
External Male Sex Organs
Most of the male reproductive system is located outside of the man’s body. These external structures are the penis, scrotum, epididymis, and testes.
Male Reproductive System: Lateral view of male reproductive system with organs labeled.
The penis is the male organ for sexual intercourse and urination. Semen and urine leave the penis through the urethra. The scrotum is a loose, pouch-like sack of skin that hangs behind the penis, containing the testes.
The scrotum has a protective function, including the maintenance of optimal temperatures for sperm survival and function. For sperm development, the testes must maintain a temperature slightly cooler than normal body temperature. Special muscles in the wall of the scrotum contract and relax in order to move the testes near the body.
The epididymus is located at the back of the testis and connects it to the vas deferens. Its function is to store and carry sperm. The testis is the location for testosterone production. The coiled collection of tubes within the testes are the seminiferous tubules. Within these tubules, spermatogenesis takes place.
Accessory Sex Organs
The internal organs of the male reproductive system are called accessory organs. They include the vas deferens, seminal vesicles, prostate gland, and bulbourethral (Cowper’s) glands.
- Vas deferens: Transports mature sperm to the urethra in preparation for ejaculation.
- Seminal vesicles: Sac-like pouches that attach to the vas deferens near the base of the bladder. The vesicles produce molecules such as fructose that serve as energy sources for sperm. The seminal vesicle fluid makes up most of the volume of a man’s ejaculate.
- Prostate gland: A walnut-sized structure located below the urinary bladder in front of the rectum. It contributes additional fluid to the ejaculate that serves as nourishment for sperm.
- Bulbourethral (Cowper’s) glands: Pea-sized structures located on the sides of the urethra just below the prostate gland. These glands produce a clear, slippery fluid that empties directly into the urethra. Fluid produced by these glands lubricates the urethra and neutralizes acidity associated with residual urine.
Scrotum
The purpose of the scrotum is to provide the testes with a chamber of appropriate temperature for optimal sperm production.
Learning Objectives
Describe the functions of the scrotum
Key Takeaways
Key Points
- Moving the testes away from the abdomen and increasing the exposed surface area allow a faster dispersion of excess heat.
- If testes were in the abdomen, constant pressure from abdominal muscles would possibly empty the testes and epididymis before sperm were sufficiently mature for fertillization.
- The function of the scrotum appears to be to keep the temperature of the testes slightly lower than that of the rest of the body.
Key Terms
- scrotum: The bag of skin and muscle that contains the testes in mammals.
- testosterone: A steroid hormone that plays a key role in male reproductive development including the promotion of secondary sexual characteristics.
- epididymis: A narrow, tightly-coiled tube where sperm are stored during maturation. It connects the efferent ducts from the rear of each testicle to its vas deferens.
Anatomical Considerations
The scrotum is a dual-chambered suspended sack of skin and smooth muscle that contains the testes, and is homologous to the labia majora in females. It is an extension of the perineum, and is located between the penis and anus. In humans and some other mammals, increased testosterone secretion during puberty causes the darkening of the skin and development of pubic hair on the scrotum. The left testis is usually lower than the right, which may function to avoid compression in the event of impact. This asymmetry may also allow more effective cooling of the testes.
Function
The function of the scrotum appears to be to keep the temperature of the testes slightly lower than that of the rest of the body. For human beings, the temperature should be one or two degrees Celsius below body temperature (around 35 degrees Celsius or 95 degrees Fahrenheit); higher temperatures may be damaging to sperm count.
The temperature is controlled by scrotal movement of the testes away or towards the body depending on the environmental temperatures. Moving the testes away from the abdomen and increasing the exposed surface area allow a faster dispersion of excess heat. This is done by means of contraction and relaxation of the cremaster muscle and the dartos fascia in the scrotum.
However, temperature regulation may not be the only function of the scrotum. It has been suggested that if testes were situated within the abdominal cavity, they would be subjected to the regular changes in abdominal pressure that are exerted by the abdominal muscles, resulting in the more rapid emptying of the testes and epididymis of sperm before the spermatozoa were matured sufficiently for fertilization. Some mammals (elephants and marine mammals, for example) do keep their testes within the abdomen where there may be mechanisms to prevent this inadvertent emptying.
The scrotum: Image of the external, muscle, and deep tissue views of the scrotum.
Testes
The testis is homologous to the ovary in that it produces the male gamete (sperm) while the ovary produces the female gamete (egg).
Learning Objectives
Describe the function of testes and the development of sperm
Key Takeaways
Key Points
- The testes produce the hormones testosterone and other androgens.
- Sperm are produced within seminiferous tubules.
- Leydig cells produce and secrete male hormones.
- Sertoli cells help in the process of spermatogenesis.
Key Terms
- Leydig cells: Also known as interstitial cells of Leydig, these are found adjacent to the seminiferous tubules in the testicle and produce testosterone in the presence of luteinizing hormone.
- follicle-stimulating hormone: Stimulates the growth and recruitment of immature ovarian follicles in females. In males, it is critical for spermatogenesis as it stimulates primary spermatocytes to form secondary spermatocytes.
- sertoli cells: Part of the seminiferous tubule that helps in the process of spermatogenesis.
- Luteinizing hormone: A hormone produced by gonadotropic cells of the anterior pituitary gland. It triggers ovulation and development of the corpus luteum in females and stimulates Leydig cell production of testosterone in males.
- testes: Also referred to as testicles, the male gonads in animals.
Testicle: A diagram of the major components of an adult human testis, including the following numbered items: 1. Tunica albuginea, 2. Septula testis, 3. Lobulus testis, 4. Mediastinum testis, 5. Tubuli seminiferi contorti, 6. Tubuli seminiferi recti, 7. Rete testis, 8. Ductuli efferentes testis, 9a. Head of epididymis, 9b. Body of epididymis, 9.c Tail of epididymis,10. Vas deferens, 11a. Tunica vaginalis (parietal lamina), 11b. Tunica vaginalis (visceral lamina), and 12. Cavity of tunica vaginalis.
The testis is the male gonad in animals. Like the ovaries to which they are homologous, testes are components of both the reproductive system and the endocrine system. Thee testes produce sperm (spermatogenesis) and androgens, primarily testosterone. Both functions of the testis are influenced by gonadotropic hormones produced by the anterior pituitary gland. Luteinizing hormone results in testosterone release. The presence of both testosterone and follicle-stimulating hormone (FSH) is needed to support spermatogenesis.
Almost all healthy male vertebrates have two testes. In mammals, the testes are often contained within an extension of the abdomen called the scrotum. In mammals with external testes, it is most common for one testicle to hang lower than the other. While the size of the testis varies, it is estimated that 21.9% of men have one higher-positioned testis, while 27.3% of men have reported equally-positioned testicles.
The tough membranous shell called the tunica albuginea contains very fine coiled tubes called seminiferous tubules. These are lined with a layer of germ cells that develop into sperm cells (also known as spermatozoa or male gametes) from puberty into old age. The developing sperm travels through the seminiferous tubules to the rete testis located in the mediastinum testis, to the efferent ducts, and then to the epididymis where newly-created sperm cells mature. The sperm moves into the vas deferens and is eventually expelled through the urethra, via the urethral orifice through muscular contractions.
Leydig cells located between seminiferous tubules produce and secrete testosterone and other androgens important for sexual development and puberty, including secondary sexual characteristics such as facial hair and sexual behavior. They also support libido, spermatogenesis, and erectile function. In addition, testosterone controls testicular volume. The sertoli cells are the testes’ somatic cells, necessary for testis development and spermatogenesis.
Inside the Human Testes: Diagram illustrates the scrotum with a portion of the covering removed to display the testis.
Penis
In human males, the penis serves as both a reproductive organ and as a urinal duct.
Learning Objectives
Describe the structure and function of the penis
Key Takeaways
Key Points
- The major structure of the penis is formed by columns of corpus cavernosum and spongiosum tissue.
- The head of the penis, called the glans, contains the opening for the urethral duct, the passage for urine and seminal fluid.
- The penis and clitoris (found in women) are homologous organs.
- Blood engorgement of penile tissue causes the penis to become erect, facilitating sexual intercourse.
- Ejaculation is the release of sperm, which propels the sperm into the vaginal canal when it occurs during intercourse.
Key Terms
- glans: The vascular body which forms the apex of the penis.
- penis: The male sexual organ for copulation and urination; the tubular portion of the male genitalia (excluding the scrotum).
- intromittent organ: A term for a male external organ that delivers sperm during copulation.
- clitoral glans: Highly innervated part of the clitoris that exists at the tip of the clitoral body as a fibrovascular cap.
- ejaculation: The forcible ejection of semen from the mammalian urethra, a reflex in response to sexual stimulation.
The penis is an intromittent organ of male animals with reproductive and urinary functions. Unlike many other species, the human penis has no baculum, or erectile bone. Instead, it relies entirely on engorgement with blood to achieve erection. The human penis cannot be withdrawn into the groin, and
is larger than that of any other primate, particularly in regards to proportion to body mass.
Human Penis: This is a diagram of a human penis with its parts labeled.
Parts of the Penis
The human penis is made up of three columns of tissue: two corpora cavernosa that lie next to each other on the dorsal side and a corpus spongiosum that lies between the corpora cavernosa on the ventral side. The glans penis is the bulbous end of the penis formed by the corpus spongiosum. It supports the foreskin (prepuce) that retracts to expose the glans. The area on the underside of the penis, where the foreskin is attached, is called the frenum (or frenulum). The rounded base of the glans is called the corona. The perineal raphe is the noticeable line along the underside of the penis.
The urethra is the last part of the urinary tract and traverses the corpus spongiosum. The urethral opening is called the meatus and lies on the tip of the glans penis. It serves as a passage both for urine and semen. Sperm are produced in the testes and stored in the attached epididymis. Sperm are propelled through the vas deferens during ejaculation. Fluids are added by the seminal vesicles before the vas deferens carries the sperm to the ejaculatory ducts, which join the urethra inside the prostate gland. The prostate as well as the bulbourethral glands add further secretions, then the semen is expelled through the penis. The raphe is the noticeable ridge between the halves of the penis. It is located on the ventral aspect of the penis and runs from the meatus and across the scrotum to the perineum (area between scrotum and anus).
Homology to the Clitoris
The various parts of the male penis are homologous to parts of the female clitoris:
- Glans of the penis: homologous to the clitoral glans
- Corpora cavernosa: homologous to the body of the clitoris
- Corpus spongiosum: homologous to vestibular bulbs beneath the labia minora
- Scrotum: homologous to the labia minora and labia majora
- Foreskin: homologous to the clitoral hood
Penile and Clitoral Structure: This diagram compares the structure of the penis to the clitoris.
The Penis As a Sexual Organ
An erection is the stiffening and rising of the penis that facilitates sexual arousal, though it can also happen in nonsexual situations. The primary physiological mechanism that brings about an erection is the autonomic dilation of arteries supplying blood to the penis. This allows more blood to fill the three spongy erectile tissue chambers in the penis, causing it to lengthen and stiffen.
The engorged erectile tissue presses against and constricts the veins that carry blood away from the penis. More blood enters than leaves the penis until an equilibrium is reached where an equal volume of blood flows into the dilated arteries and out of the constricted veins; a constant erectile size is achieved at this equilibrium. Although many erect penises point upwards, it is common and normal for the erect penis to point nearly vertically upwards, nearly vertically downwards, or even horizontally forward, depending on the tension of the suspensory ligament that holds it in position.
Ejaculation is the ejection of semen from the penis and is usually accompanied by orgasm. A series of muscular contractions delivers semen, containing sperm cells or spermatozoa, from the penis. It is usually the result of sexual stimulation, including prostate stimulation. Ejaculation may occur spontaneously during sleep (known as a nocturnal emission or “wet dream”).
Ejaculation has two phases: emission and ejaculation proper. The emission phase of the ejaculatory reflex is under control of the sympathetic nervous system, while the ejaculatory phase is under the control of a spinal reflex at the level of spinal nerves S2 to S4 via the pudendal nerve. A refractory period succeeds the ejaculation, and sexual stimulation precedes it. Anejaculation is the condition of being unable to ejaculate.
Testes Ducts
Testes ducts, which include the seminiferous tubules and vas deferens, are involved in the creation or transportation of sperm.
Learning Objectives
Differentiate among the ducts of the testes
Key Takeaways
Key Points
- Creation of sperm occurs in the seminiferous tubules of the testes.
- The efferent ducts maintain proper fluid concentration in the testes and propel sperm to the epididymus.
- The vas deferens carry the sperm from the epididymus to the urethra.
- A vasectomy cuts the vas deferens to prevent sperm from entering the urethra and being ejaculated.
Key Terms
- vasectomy: The surgical removal of all or part of the vas deferens, usually as a means of male sterilization.
- peristalsis: A radially symmetrical contraction and relaxation of muscles that propagates in a anterograde (forward) wave down a tube.
- vas deferens: The duct in the testes that carries semen from the epididymis to the ejaculatory duct.
- rete testis: An anastomosing network of tubules located in the hilum of the testicle (mediastinum testis) that carries sperm from the seminiferous tubules to the efferent ducts.
Inside the Human Testes: Diagram of cross-section of testes: 1: Testicular septa 2: Convoluted seminiferous tubules 3: Testicular lobules 4: Straight seminiferous tubules 5: Efferent ductules 6: Rete testis.
Seminiferous tubules, located in the testes, are where meiosis occurs and the gametes (spermatozoa) are created. The seminiferous tubules are formed from primitive sex cords from the gonadal ridge. The epithelium of the tubule consists of tall, columnar cells called Sertoli cells. Between the Sertoli cells are spermatogenic cells, which differentiate through meiosis to become sperm cells. There are two types of seminiferous tubules: convoluted, located toward the lateral side, and straight, as the tubule comes medially to form ducts that will exit the testis.
Efferent Ducts
The efferent ducts connect the rete testis with the initial section of the epididymis. There are two basic types of efferent ductule structure.
- Multiple entries into the epididymis: This type is seen in most large mammals. In humans and other large mammals, there are approximately 15–20 efferent ducts which occupy nearly one-third of the head of the epididymis.
- Single entry: This type is seen in most small animals such as rodents. This is characterized by three to six ductules that merge into a single small ductule prior to entering the epididymis.
The ductuli are unilaminar and composed of columnar ciliated and nonciliated (absorptive) cells. The ciliated cells stir the luminal fluids, which may help ensure homogeneous absorption of water from the fluid produced by the testis. This results in an increase in the concentration of luminal sperm. The epithelium is surrounded by a band of smooth muscle that helps to propel the sperm toward the epididymis.
Histiological Representation of Seminferious Tubules: Seminiferous tubule in cross-section (large tubular structure: center of image) with sperm (black, tiny, ovoid bodies furthest from the outer edge of the tubular structure).
The ductus (vas) deferens, also called the sperm duct, extend from the epididymis on each side of the scrotum into the abdominal cavity through the inguinal canal, an opening in the abdominal wall for the spermatic cord. The spermatic cord is a connective tissue sheath that contains the vas deferens, testicular blood vessels, and nerves.
The smooth muscle layer of the vas deferens contracts in waves of peristalsis during ejaculation. Two ducts connect the left and right epididymis to the ejaculatory ducts in order to move sperm. In humans, each tube is about 30 centimeters (a foot) long,
3 to 5 mm in diameter, and surrounded by smooth muscle. The sperm are transferred from the vas deferens into the urethra, collecting secretions from the male accessory sex glands such as the seminal vesicles, prostate gland, and bulbourethral glands, which provide the bulk of semen.
The procedure of deferentectomy, also known as a vasectomy, is a method of contraception in which the vas deferens are permanently cut, though in some cases it can be reversed. A modern procedure that does not include cutting the ducts involves injecting an obstructive material into the ductus to block the flow of sperm. Research in male contraception has focused primarily on the vas deferens with the use of the intra-vas device and reversible inhibition of sperm under guidance.
Accessory Sex Glands
The accessory sex glands produce seminal fluid and clean and lubricate the urethra.
Learning Objectives
Describe the functions of male accessory sex glands
Key Takeaways
Key Points
- Seminal glands contain seminal vesicles that produce 50–70% of the seminal fluid.
- The excretory duct of the seminal gland opens into the vas deferens as it enters the prostate gland.
- Sperm are not in contact with the seminal fluid produced by the seminal vesicles, possibly to block the progress of sperm from other males.
- The prostate secretes 20–30% of the seminal fluid which carries the sperm in the ejaculate.
- The prostate surrounds the urethra below the bladder and can be felt in a rectal exam.
- Bulbourethral glands produce a pre-ejaculate secretion which lubricates and flushes out the urethra in preparation for the sperm.
Key Terms
- bulbourethral gland: An exocrine gland which secretes a clear fluid upon sexual arousal as pre-ejaculate (or Cowper’s fluid).
- Lipofuscin: Yellow-brown pigment granules composed of lipid-containing residues of lysosomal digestion.
- prostate: A compound tubuloalveolar exocrine gland of the male reproductive system in most mammals.
- seminal gland: A pair of simple tubular glands located within the pelvis
that secrete fluid that partly composes semen.
The accessory sex glands, including the seminal, prostate glands, and bulbourethral glands, produce seminal fluid and clean and lubricate the urethra.
Seminal Gland Anatomy
Each seminal gland forms as an outward growth of the wall of ampulla of each vas deferens. They are curled and folded within the gland and can spread out to approximately 5 cm, but the unfolded length is approximately 10 cm. The excretory duct of the seminal gland opens into the vas deferens as it enters the prostate gland.
Seminal Gland Physiology and Function
The seminal vesicles secrete a significant proportion of the fluid that ultimately becomes semen. Lipofuscin granules from dead epithelial cells give the secretion its yellowish color. About 50–70% of seminal fluid in humans originates from the seminal vesicles, but is not expelled in the first ejaculate fractions which are dominated by spermatozoa and zinc-rich prostatic fluid. Seminal vesicle fluid is alkaline, resulting in human semen with a mildly alkaline pH. This helps neutralize the acidity of the vaginal tract, prolonging the lifespan of sperm. Acidic ejaculate (pH <7.2) may be associated with ejaculatory duct obstruction. The vesicle produces a substance that causes the semen to become sticky after ejaculation, thought to help keep the semen near the cervix.
The thick seminal vesicle secretions contain proteins (including enzymes), mucus, fructose, vitamin C, flavins, phosphorylcholine, and prostaglandins. The high fructose concentrations provide nutrient energy for the spermatozoa when stored in semen in the laboratory. Spermatozoa ejaculated into the vagina are not likely to have contact with seminal vesicular fluid, but transfer directly from the prostatic fluid into the cervical mucus as the first step on their travel through the female reproductive system.
Seminal vesicle fluid is expelled under sympathetic contraction of the muscularis muscle coat. In vitro studies have shown that sperm expelled together with seminal vesicular fluid show poor motility and survival and less-protected chromatin. Thus, the exact physiological importance of seminal vesicular fluid is unclear. It may be a developmental rest, such as in some rodents where the last part of the ejaculate forms a spermicidal plug to reduce the chances for sperm from a later-arriving male to proceed to the oocyte.
Prostate Gland Anatomy
Prostate: Prostate with seminal vesicles and seminal ducts, viewed from the front and above, including the urethra, seminal vesicle, vas deferens, ampulla, ejaculatory duct, and isthmus.
The prostate surrounds the urethra just below the urinary bladder and can be felt during a rectal exam. It is the only exocrine organ located in the midline in humans and similar animals. Within the prostate, the urethra coming from the bladder is called the prostatic urethra and merges with the two ejaculatory ducts. The prostate is sheathed in the muscles of the pelvic floor, which contract during the ejaculatory process.
Prostate Gland Physiology and Function
The prostate secretes a slightly acidic fluid, milky or white in appearance, that usually constitutes 20–30% of the volume of the semen along with spermatozoa and seminal vesicle fluid. The prostatic fluid is expelled in the first ejaculate fractions, together with most of the spermatozoa. In comparison with the few spermatozoa expelled in seminal vesicular fluid, those expelled in prostatic fluid have better motility, longer survival, and better protection of the genetic material. The prostate also contains some smooth muscles that help expel semen during ejaculation.
To work properly, the prostate needs male hormones (e.g., testosterone), which are produced mainly by the testes. Some male hormones are produced in small amounts by the adrenal glands. However, dihydrotestosterone regulates the prostate. A healthy human prostate is slightly larger than a walnut in adult males, with a weight ranging between 7 and 16 grams.
Bulbourethral Glands
Bulbourethral Gland: Image shows internal view of penis and male sexual anatomy. The bulbourethral gland is labeled at center left.
Bulbourethral glands are located posterior and lateral to the membranous portion of the urethra at the base of the penis, between the two layers of the fascia of the urogenital diaphragm in the deep perineal pouch. They are enclosed by transverse fibers of the sphincter urethrae membranacea muscle. The bulbourethral glands are compound tubulo-alveolar glands, each approximately the size of a pea. They are composed of several lobules held together by a fibrous covering. Each lobule opens into a duct that joins with the ducts of other lobules to form a single excretory duct. This duct is approximately 2.5 cm long and opens into the urethra at the base of the penis. The glands gradually diminish in size with advancing age.
During sexual arousal, each gland produces a clear, salty, viscous secretion known as pre-ejaculate. This fluid helps to lubricate the urethra for spermatozoa to pass through, neutralizes traces of acidic urine in the urethra, and helps flush out any residual urine or foreign matter. It is possible for this fluid to pick up sperm remaining in the urethral bulb from previous ejaculations and carry them out prior to the next ejaculation.
