Reproductive System (Male)

 
Reproductive System
     The reproductive system of both the male and female have the same basic components:
  1. Gonads or sexual organs - produce the gametes (sex cells) and hormones.
  2. Reproductive tract - ducts and organs that receive, store and transport gametes.
  3. Accessory glands - secrete fluids that support the transmission of gametes from the male to the female.
  4. External genitalia - external organs that enable transmission of male gametes from the male to the female reproductive tract.
 
Male Reproductive System
 
Testes (sing. testis)
     The testes are ovoid organs located in an external pouch called the scrotum.
  Descent of the Testes
     The testes first appear in the body cavity at a location where the kidneys will come to lie but during development the testes come to lie within the scrotum. The pathway of descent is indicated by a fold of peritoneum called the gubernaculum that attaches the inferior pole of the testis to the location where the scrotum will develop.
     The actual descent of the testes is the result of two processes:
  1. The testes remain in position while the surrounding structures grow rapidly.
  2. The inferior part of the gubernaculum develops into a fibromuscular cord that contracts during the last two months of gestation and pulls the testes into the out pocketing of the peritoneal cavity that becomes the scrotum.
  Spermatic Cords
     The spermatic cords form during the descent of the testis. The cords contain the ductus deferens, the testicular artery, the pampiniform plexus, the genitofemoral nerves and lymphatic vessels. In addition, the spermatic cord is held together by layers of fascia, connective tissue and muscle.
     The spermatic cord enters the scrotum through the inguinal canal passing through the deep inguinal ring to enter the canal from the peritoneal cavity and exiting through the superficial inguinal ring to enter the scrotum.
  Scrotum and Position of the Testes
     Each testis is within a separate scrotal cavity. The wall between the two scrotal cavities is indicated on the exterior of the scrotum by the perineal raphe. The scrotal cavities are lined by a serous membrane called the tunica vaginalis.
     The placement of the testes in the scrotum provides an environment that is 2 degrees centigrade less warm than that inside the body. Keeping the testes at this cooler temperature is essential for the proper function of the testes. Two muscles help to ensure that the testes remain at the appropriate temperature:
  1. Dartos
     The dartos is smooth muscle that is found in the superficial fascia under the thin skin of the scrotum. Contraction of the dartos gives the scrotal skin its wrinkled appearance and decreases the surface area by which heat is lost.
  2. Cremaster Muscle
     The cremaster muscle is a continuation of the internal oblique muscle through the inguinal canal. It lies between the superficial fascia and the parietal layer of the tunica vaginalis. Contraction of the muscles draw the scrotum closer to the body and increases the testicular temperature while relaxation of the muscle allows the scrotum to hang more loosely and decreases testicular temperature.
  Histology of Testis
     The testis is surrounded by a thick connective tissue capsule called the tunica albuginea. On the posterior aspect of the testis the connective tissue extends into the testis and forms the mediastinum. The interior of the testis is divided into compartments called lobules by connective tissue partitions called septa that extend from the tunica albuginea.
     Each lobule contains one to three tightly coiled seminiferous tubules. The seminiferous tubules are u-shaped with each end of the tubule connected to a straight tubule that enters the mediastinum. Within the mediastinum, the straight tubules open into a complex maze of interconnected tubules called the rete testis. The rete testis is connected to the epididymis by 20 efferent ductules.
     A thin capsule forms the wall of the seminiferous tubules. The connective tissue between the tubules contain blood vessels and interstitial cells. The interstitial cells produce the male sex hormones, the androgens, of which the most important is testosterone.
  Spermatogenesis
     Spermatogenesis is the process by which stem cells called spermatogonia (sing. spermatogonium) become sperm cells or spermatozoa. This process involves meiosis in which a cell containing a pair of each type of chromosome (diploid) divides twice to produce four cells containing a single chromosome of each type (haploid).
     The spermatogonium divides by mitosis to produce cells that commit to meiosis while retaining some cells as future stem cells. The spermatogonia undergo a number of mitotic divisions until the cells that will undergo meiosis, primary spermatocytes, are produced. The primary spermatocyte undergoes the first division of meiosis (reduction division) and produces two secondary spermatocytes which contain a single chromosome of each type (haploid). Each chromosome has two copies (chromatids) connected by a centromere. Each secondary spermatocyte quickly undergoes the second division of meiosis (replication division) to produce four spermatids.
  Spermiogenesis
     The process by which the spermatid becomes a mature male gamete called a spermatozoon (plural: spermatozoa) is called spermiogenesis.
     The spermatozoon has three distinct regions:
  1. Head
  The head contains a flattened oval nucleus that contains densely packed chromosomes. The tip is covered by an enzyme-filled vesicle called the acrosomal cap.
  2. Middle Piece
  The middle piece is connected to the head by the neck that contains the centrioles that form the basal body of the flagellum. Within the middle piece, numerous mitochondria that provide the energy for motility are arranged in a spiral fashion.
  3. Tail
  The tail represents a continuation of the flagellum beyond the middle piece.
  Sustentacular Cells (Sertoli Cells)
     The sustentacular cells extend the full thickness of the wall of the seminiferous tubules from the basal lamina to the lumen. These cells serve a number of essential functions:
  1. Maintenance of the Blood-Testis Barrier
  This barrier separates the fluid within the lumen of the tubules from the surrounding interstitial fluid and thereby preserves its unique characteristics. The barrier is also essential for preventing cells of the immune system from attacking the unique cells that result from meiosis. These cells contain molecules that would be recognized as foreign.
  2. Support Spermatogenesis
  Sustentacular cells provide metabolic support for spermatogenesis.
  3. Support of Spermiogenesis
  The developing spermatids remain attached to sustentacular cells until they are mature enough to detach.
  4. Secretion of Inhibin
  Sustentacular cells moderate the rate of spermatogenesis by secreting inhibin which inhibits secretion of the hormones that stimulate the process (negative feedback).
  5. Secretion of Androgen-Binding Protein
  Secretion of androgen-binding protein increases the concentration of androgens in the tubules to levels necessary to stimulate sperm cell production.
Male Reproductive Tract
     The male reproductive tract stores, nourishes and transports the spermatozoa while promoting their maturation.
  Epididymis
     The epididymis lies along the posterior border of the testis. It is a long (7 m or 23 ft) tightly coiled tubule lined by a simple columnar epithelium with long microvilli called stereocilia. The epididymis is divided into three segments:
1. Head - This segment incorporates the efferent and lies on the superior pole of the testis.
2. Body - The body extends inferiorly from the last efferent ductule along the posterior surface of the testis.
3. Tail - The tail is located on the inferior pole and is the principal storage site for sperm.
     The three main functions of the epididymis are:
1. Adjusts the composition of the fluid produced by the seminiferous tubules.
2. Absorbs and recycles defective sperm cells and cellular debris.
3. Stores spermatozoa while facilitating their maturation.
  Ductus Deferens (Vas Deferens)
     The ductus deferens extends from the tail of the epididymis through the inguinal canal as part of the spermatic cord and enters the abdominal cavity. In the abdominal cavity the duct arches over the urinary bladder in between the ureters and reaches the prostate gland. The expanded portion of the duct before it reaches the prostate is called the ampulla.
     During ejaculation the thick muscular layer undergoes peristaltic contraction which propel the spermatozoa along the duct. At the prostate, the ductus deferens unites with the duct of the seminal vesicles to form the ejaculatory duct which is found within the prostate gland.
  Urethra
     The urethra in the male is the final passageway for the urinary and reproductive systems and was covered with the urinary system.
Accessory Glands
     The accessory glands account for about 95% of the volume of the semen.
  Seminal Vesicles
     The seminal vesicles are highly coiled tubular glands located on the posterior wall of the urinary bladder. The seminal vesicles contribute about 60% of the volume of the semen. The secretion of the gland includes prostaglandins, clotting proteins, and fructose.
  Prostate Gland
     The prostate gland is a small, muscular, round organ that encircles the urethra as it exits the urinary bladder. The gland produces a secretion that contributes 20-30 percent to the volume of the semen. The secretion includes seminalplasmin, an antibiotic substance that may prevent urinary tract infections.
  Bulbourethral Gland (Cowper's Gland)
     These are small paired organs found in the urogenital diaphragm at the base of the penis. These glands produce a thick, sticky, alkaline mucus that neutralizes the acidity of the urethra and lubricates the tip of the penis.
Penis
     The penis is a tubular organ that introduces semen into the female vagina during sexual intercourse. It can be divided into three regions:
1. Root - The fixed portion of the penis which is attached to the pelvic bone.
2. Body (Shaft) - This is the tubular, moveable portion of the penis.
3. Glans - This is the expanded distal end that surrounds the external urethral orifice. A fold of skin called the prepuce (foreskin) attaches to the glans of the penis.
     The penis contains three cylindrical bodies of erectile tissue:
  Corpora cavernosa
  These are a pair of cylindrical bodies in the upper portion of the shaft that are separated by a thin partition and are encapsulated by dense connective tissue. At the root of the penis the corpora cavernosa divide to form the crura (legs) that attach to the pelvic bone by ligaments.
  Corpus spongiosum
  This body is a single cylindrical body that surrounds the spongy, or penile, urethra. The proximal portion of the corpus spongiosum expands to form the bulb of the penis. The distal expanded end of the body is the glans.