OBJECTIVES

This chapter should help you to:

· Name and give the location of the glands, ducts, and external genitalia of the male reproductive system .
· Describe the general organization of the testis (coverings, scpta, mediastinum, scminifcrous tubules, intratesticular ducts) as they appear in a midsagittal section.
· Trace the life cycle of the male gametcs (spermatozoa) beginning with their embryonic origin, continuing with their structural and positional changes in the walls of the seminiferous tubules during spermatogenesis, and ending with a detailed description of the path they follow from the seminiferous tubules through the intratesticular and excretory genital ducts; describe the differences in wall structure of these ducts and changes in the composition of semen that occur along the way.
· Distinguish between spermatogenesis, spermatocytogenesis, and spermiogenesis and describe the changes in the number of chromosomes and amount of DNA that occur in the spermatogenic cells during the process.
· Describe Sertoli (supporting) cells in terms of their structure, function, location, and embryonic origin.
· Describe their role in the function of the blood-testis barrier. · Describe the structure, function, and location of the interstitial (Leydig) cells of the testis.
· Describe the roles of temperature, the pituitary, and cells within the testis itself in regulating spermatogenesis.
· Compare the seminal vesicles, prostate, and bulbourethral (Cowper's) glands in terms of general organization, epithelial lining, secretory products, and the point(s) at which their secretions enter the excretory pathway
· Describe the 3 erectile bodies of the penis in terms of their histologic structure and relative positions and indicate which of them contains the urethra and forms the glans.
· Describe the blood supply to the erectile tissue of the penis and the factors that control the transitions between the flaccid and erectile states.

SYNOPSIS

I. GENERAL FEATURES OF THE MALE REPRODUCTIVE SYSTEM

This system (Fig 22-1) consists of the external genitalia and a series of glands and ducts that produce and transport the male gametes (spermatozoa) and the seminal fluid. Together the seminal fluid and spermatozoa constitute the semen.

A. Glands: The glands of this system include the paired testes and several accessory glands.

1. Testes. These are the male gonads, located in the scrotum; they are the primary glands of this system, with both exocrine and endocrine functions. a. Exocrine component. This includes the seminiferous tubules, where spermatozoa are produced (spermatogenesis), and the intratesticular genital ducts into which the semi- niferous tubules deliver their products for transport to the excretory genital ducts (III.B). b. Endocrine component. This consists of nests of tcstosterone-secreting interstitial cells in the connective tissue between the seminiferous tubules. c. Role of temperature. Although testosterone production can occur at the normal core body temperature (37 "C), normal production of spermatozoa occurs only at 35 "C. To maintain this lower temperature, the testes are held away from the body in the scrotum. In addition to sweat evaporating from the scrotal surface, cooling is aided by the pam piniform plexus of veins that surround each testicular artery. The plexus contains cooler blood returning from the scrotum and cools the testes' blood supply before it reaches them .
2. Accessory glands. Located along the excretory genital duct system, these include the semi nal vesicles, prostate glands, bulbourethral glands, and glands of Littre. a. Seminal vesicles. These are paired glands whose secretions increase the volume of the seminal fluid and raise its pH. They also add fructose, providing a source of nourishment and energy for the gametes. b. Prostate gland. More fluid, rich in citric acid (another nutrient) and acid phosphatase, is added by this gland. c. Bulbourethral glands and glands of Littre. These mucous glands help to lubricate the distal part of the duct system.

B. Ducts: The ducts of the male reproductive system are described in terms of their location, number, length, diameter, wall structure, and related functions.


1. Intratesticular ducts. Located within the testes, these are continuous with the seminiferous tubules and include the tubuli recti, rcte testis, and ductuli efferentes.
2. Excretory genital ducts. These are located outside the testes and include the ductus epi didymis, ductus (or vas) deferens, ejaculatory duct, and urethra. In these larger ducts the secretions of the testes and the accessory glands combine to form the semen.

C. External Genitalia: These include the penis and scrotum. The penis contains the most dietal element of the duct system, the penile urethra. The scrotum contains the testes. The histology and reproductive function of the penis are covered with emphasis on erectile tissue, blood flow, and autonomic innervation.

II. TESTES

A. Embryonic Origin: Primordial germ cells, originating from yolk sac endoderm, migrate into the dorsal wall of the abdominal cavity and invade the mesoderm of the genital ridge, where they collect to form the primitive sex cords. The primordial germ cells form the spermatogonia, and the mesoderm forms the Sertoli cells, interstitial cells, and connective tissue between the cords. The solid sex cords form seminiferous tubules, developing central lumens and anastomosing with adjacent cords. Later, the seminiferous tubules anastomose with remnants of the meso nephric tubules and the excretory duct system. The entire developing testis becomes encapsu lated by connective tissue, separates from the dorsal wall, and descends into the scrotum.

B. General Organization:


1. External coverings a. Tunica vaginalis. This is a double-layered mesothelial sac that covers the anterior surface of each testis. This extension of the peritoneum is picked up during the descent of the testes and into the scrotum. b, Tunica albuginea. This dense fibrous connective tissue capsule thickens along the pos- terior surface to form the mediastinum testis.
2. Internal structure a. Septa. These extensions of the tunica albuginea penetrate each testis and divide it into about 250 compartments, or lobules. b. Lobules, Each lobule includes 1-4 seminifcrous tubules (the exocrine component) and loose vascular connective tissue between the tubules that contains discrete clumps of testosterone-sccreting interstitial cells of Leydig (the endocrine component).

C. Seminiferous Tubules:

1. General structure. Each long (40-70-cm) narrow (0.2-mm) tubule is highly convoluted and packed into a small space. The walls of each tubule, from the exterior to the lumen, are composed of the following 3 layers: a. The tunica propria is a thin tunic of fibrous connective tissue comprising several layers of fibroblasts. The innermost layer includes contractile myoid cells that attach to the basal lamina. b. A well-defined basal lamina lies between the tunica propria and the seminiferous epithelium. c. The stratificd seminiferous epithclium consists of 2 cell lineages: spermatogenic cells and supportive (Sertoli) cells.
2. Spermatogenic cells. Deriving from embryonic yolk sac endodcrm, these cells undergo spermatogenesis (II.D)--a multistcp process of diffcrentiation--that begins with the cells closest to the basal lamina (spermatogonia). The process ends with the release of spermatozoa into the tubule lumen. Cells at different steps in the process are identified according to their size, nuclear morphology, and position in the epithelium. a. Spermatogonia are small round cells near the basal lamina. They are the least differenti ated and are the only spermatogenic cell type present before puberty. They have a round nucleus with patchy heterochromatin. Like most of the body's cells, they are diploid for chromosome number (46, 2n) and diploid for DNA (2N) until stimulated to divide. b. Primary spermatocytes are closer to the lumen than are the spermatogonia. They are the largest germ cells present; each has a large round nucleus with dark strands of hetero chromatin resembling tangled string. They are usually seen in the prophase of meiosis I, the longest phase of meiosis (up to 22 days). They are diploid for chromosome number (46, 2n) and tetraploid for DNA (4N) in preparation for the first meiotic division. c. Secondary spermatocytes are closer to the lumen than are the primary spermatocytes. The product of the first meiotic division, they are about half the size of the primary spermatocytes that divide to form them. Secondary spermatocytes are rare in histologic section because they undergo the second meiotic division almost immediately after formation. They are haploid for chromosome number (23, n) and diploid for DNA (2N). d. Spermatids are products of the second meiotic division of secondary spermatocytes and are located next to the lumen. Spermatids are small cells with dark heterochromatic nuclei. They may exhibit a range of nuclear morphology, depending on the stage of spermiogenesis. They are haploid for both chromosome number (23, n) and DNA (N). e. Spermatozoa are located in the lumen. They are the result of spermiogenesis, the differentiation of spermatids (II.D). They are recognizable by their long flagella (II.E). They are haploid for both chromosome number (23, n) and DNA (N).
3. Supporting (Sertoli) cells. These derive from the mesoderm of the embryonic genital ridge. a. Structure. These elongated, branched, pyramidal epithelial cells extend from the basal lamina to the luminal surface of each seminiferous tubule. They exhibit deep cytoplasmic infoldings that embrace the developing spermatogenic cells. Their large pale nuclei are ovoid and indented and contain a prominent nucleolus. Sertoli cells have a well developed SER and Golgi complex, numerous mitochondria, and some RER. The mar gins of the cells are bound tightly to neighboring supporting cells by occiuding junctions, forming a continuous sheath around the tubule lumen. b. Function. The functions of the cells include (1) physical support for the spermatogenic cells, which attach to one another by cytoplasmic bridges; (2) nutritional regulation of the developing spermatozoa, which are isolated from the blood supply by the occluding junctions between the supporting cells. Spermatozoa therefore depend on these cells to mediate the exchange of nutrients and metabolites with the blood; (3) protection from autoimmune attack by immunoglobulins in the blood; (4) phagocytosis of residual bodies shed by the maturing spermatozoa; and (5) secretion of fluid for sperm transport; androgen-binding protein (ABP), which combines with the testosterone produced by interstitial cells and is released into the tubule lumen (ABP secretion increases in re sponse to increased levels of FSH and testosterone); and inhihin, which acts on the pituitary to decrease FSH production. They may also secrete estrogen.

D. Spermatogenesis: This is the entire multistep process from spermatogonia through sper matozoa. It is testosterone-dependent and can be divided into 3 phases: spermatocytogenesis, meiosis, and spermiogenesis.

1. Spermatocytogenesis. This is the production of primary spermatocytes from spermatogonia through a series of standard mitotic divisions. Daughter cells of early divisions form 2 types of spermatogonia, A and B. Spermatogonia A remain undifferentiated stem cells able to produce more A and B cells. Spermatogonia B may undergo further mitoses to form more spermatogonia B or enter meiosis to form primary spermatocytes. 2. Meiosis. This process involves 2 successive cell divisions that yield 4 haploid spermatids from one diploid primary spermatocyte. a. Meiosis I. The first meiotic division involves the production of secondary spermatocytes from primary spermatocytes. (1) During the S phase (DNA synthesis) prior to division, DNA doubles (as in mitosis), becoming tetraploid. (2) During an extended prophase, the 23 pairs of homologous chromosomes (22 pairs of autosomes f XY) thicken by coiling. They pair up point-for-point (synapsis) and form bridges (chiasma) allowing the trading of DNA between paired chromosomes ("crossing over"). The nuclear membrane disintegratcs late in prophase. (3) During metaphase, the 23 pairs line up at the equatorial plate. (4) During anaphase, the pairs separate, with one member of each pair moving toward the opposite pole. (5) During telophase, the nuclear membranes reform in the daughter cells. Each of these secondary spermatocytes is now haploid, containing only one member of each ho mologous chromosome pair (22 + X or 22 f Y). The equatorial constriction tightens, but leaves the daughter cells connected by a narrow cytoplasmic bridge. Since DNA doubled prior to the first meiotic division, each secondary spermatocyte has a diploid amount of DNA. b. Meiosis II. The second meiotic division involves the production of spermatids from the secondary spermatocytes. During this division, the chromosome number in each cell remains the same (haploid), but the amount of DNA is halved (as during a standard mitosis), resulting in spermatids that are haploid for both chromosome number and the amount of DNA.
3. Spermiogenesis, This is the complex process of cytodifferentiation by which spermatids become spermatozoa. Because many structural changes are involved, spermatids exhibit stage-dependent variations in appearance. Spermiogenesis includes the following processes. a. Acrosome formation. Proacrosomal granules form in the Golgi complex and coalesce to form a large membrane-bound acrosomal vesicle that moves next to the nucleus and attaches to the nuclear envelope. The vesicle membrane spreads over the surface of the nucleus, covering its anterior two-thirds and forming the head cap. The head cap's contents redistribute to form the mature acrosome (acrosomal cap), a large specialized lysosome. The acrosome contents are rich in carbohydrate and hydrolytic enzymes such as hyaluronidase, neuraminidase, acid phosphatase, and a trypsinlike protease. Together, these substances aid in penetrating the egg's corona radiata and zona pellucida (Chapter 23) during fertilization. b. Migration of the centrioles and formation of the flagellum, The centrioles migrate to the spermatid's posterior pole. A flagellum emerges from one, perpendicular to the cell surface, and forms the tail. The other centriole forms a collar around the flagellum base. c. Shift of cytoplasm toward the flagellum. The anterior plasma membrane now contacts the acrosome, and the excess cytoplasm forms a residual body. d. Migration of mitochondria. The mitochondria move toward the flagellum and form a spiral collar around the proximal part of the tail (the middle piece), concentrating at the future site of high energy consumption. The fructose and citric acid in semen aid in sperm motility, serving as mitochondrial metabolites. e. Condensation of nuclear chromatin. The chromatin forms a dense mass with no visible substructure. A cylindric band of microtubules (manchette) surrounds the nucleus, associating with the posterior border of the acrosome. This causes flattening and elonga tion of the nucleus. f. Sloughing of residual bodies. Upon their release into the tubule lumen, the spermatozoa release their residual cytoplasm. Much of this is phagocytosed by the supporting Sertoli cells.

E. Structure of MatureSpermatozoa:


1. Head. In frontal view the head has an oval outline. On its side, it appears as a 4-5-um-long spearhead. It is mostly nucleus, with the anterior two-thirds of the nucleus covered by the acrosome and the posterior region covered by the manchettc.
2. Tail, About 55 um long, the tail is enveloped by plasma membrane. It has 4 named parts. The neck includes the proximal centriole, connecting piece, capitulum, flagellar base, and an occasional mitochondrion. The middle piece contains many mitochondria arranged end to end in a helical sheath around the flagellum. It is 5-7 um long and 1 um thick, with the annulus at its posterior end. In this region the ffagellum has 9 outer dense fibers and a 9 + 2 core pattern of microtubules. The flagellum in the 50-um-long principal piece is surrounded by an outer fibrous sheath with dorsal and ventral longitudinal columns connected by circum ferential ribs. The flagellum itself has 7 dense outer fibers collected in 2 compartments and a 9 x 2 core pattern of microtubules. The end piece lacks the fibrous sheath but otherwise has the same structure as the principal piece. As it tapers toward the tip, the 9 doublets dissociate to form 18 single microtubules.

F. Interstitial (Leydig) Cells: Derived from embryonic genital-ridge mesoderm, these cells secrete testosterone upon stimulation by pituitary LH (ICSH). They occur as vascular nests of pale acidophilic cells in the loose connective tissue between the seminiferous tubules. Their large pale nuclei contain one or 2 prominent nucleoli. The cytoplasm contains the extensive SER typical of steroid-secreting cells, a well-developed Golgi complex, and lipid droplets.

G. Blood -Testis Barrier: Spermatogenesis involves the appearance of new sperm-specific pro teins and glycoproteins on the differentiating spermatogenic cells. Since this process begins at puberty, well after development of the immune system, these surface molecules may be recog nized as nonself antigens by the immune system. The blood-testis barrier protects the developing sperm from damage by an autoimmune response. The barrier consists of a continuous belt of junctional complexes joining the Sertoli cells at their lateral surfaces. It separates the semi nifcrous tubule lining into 2 functionally different compartments.


1. The basal compartment houses the spermatogonia. It lies between the basal lamina and the junctional belt and is accessible to any blood-borne substance that can penetrate the basal lamina.
2. The adluminal compartment extends from the junctional belt inward to the lumen. It is inaccessible to blood-borne substances except those taken up by the supporting cells and passed through their cytoplasm to this privileged space.

III. DUCT SYSTEM

Extending from the tubuli recti in the testis through the ejaculatory duct in the prostate and into the urethra, the duct system has roles in the maturation, storage, and transport of spermatozoa. All parts require adequate circulating testosterone for maintenance of normal function. The secretory epithelial lining provides spermatozoa with nutrients. Two complete sets of ducts (one per testis) empty into a common urethra.

A. Intratesticular Genital Ducts:


1. Tubuli recti (straight tubules). These connect the seminiferous tubules to the rete testis. They begin with epithelium similar to that of the seminiferous tubules, gradually losing the spermatogenic cells until only Sertoli cells remain. The main segment is lined by simple cuboidal epithelium supported by a dense connective tissue sheath.
2. Rete testis. An anastomosing network of tubules lying in the mediastinum testis, the rete testis is lined by low cuboidal epithelium.
3. Ductuli efferentes (efferent ductules). These are 10 to 20 4-6-mm-long ducts connecting the rete tcstis with the epididymis. The walls contain smooth muscle, and the epithelium has alternating groups of simple cuboidal and ciliated columnar cells. The cuboidal cells absorb much of the fluid secreted by seminiferous tubules, while the cilia sweep the spermatozoa toward the epididymis. Together the ductules form the head of the epididymis, and they converge to form a single ductus epididymis.

B. Excretory Genital Ducts:


1. Ductus epididymis. This single, highly coiled 4-6-m-long tube comprises the body and tail of the epididymis. It is lined by pseudostratified columnar epithelium resting on a basal lamina. Its cells have abundant apical stereocilia (long, irregular, nonmotile microvilli) and secrete glycerophosphocholine (a possible capacitation inhibitor) and a spermatozoon binding glycoprotein of unknown function. The epithelial cells also phagocytose and digest residual bodies sloughed during spermatogenesis. A sheath of circular smooth muscle under lies the basal lamina, gradually thickening along the length of the tube. Peristaltic contrac tions of this muscle propel sperm toward the ductus deferens. Sperm move slowly through this long coiled tube and are often seen in its lumen in tissue sections.
2. Ductus deferens (vas deferens). A single straight tube with thick muscular walls, it begins in the scrotum at the termination of the epididymis. It ascends within the spermatic cord through the inguinal canal into the abdomen, joining with the duct of the seminal vesicle in the pelvic cavity near the prostate. The lumen is narrowed by longitudinal mucosal folds. The pseudostratified columnar epithelial lining has fewer stereocilia than the epididymis. The 3 layers of smooth muscle in the wall (inner and outer longitudinal, middle circular) are capable of powerful peristaltic contractions during ejaculation. The diameter of the duct increases near the termination to form the ampulla, which is characterized by a highly folded mucosa.
3. Ejaculatory duct. This short duct, lined by pseudostratified columnar epithelium, is formed by the junction of the ductus deferens and the duct of the seminal vesicle. It penetrates the prostate to empty into the prostatic urethra.
4. Urethra. The male urethra serves as a genitourinary passageway shared by the urinary and reproductive systems (its divisions, structure, and epithelial lining are described in Chapter 19). It contains small, mucus-secreting glands of Littre in its wall.

IV. ACCESSORY GENITAL GLANDS

A. Seminal Vesicles: The paired seminal vesicles each consist of 2 highly coiled 15-cm-long tubes that develop as outgrowths of the ductus deferens. Their mucosa is highly folded, with primary, secondary, and tertiary branching. The pseudostratificd low columnar epithelium forms the secretory product. This thick, yellowish liquid is rich in fructose; it also contains citrate, inositol, prostaglandins, and several proteins. Seminal vesicle secretions make up 70% of the human ejaculate. The smooth muscle underlying the lamina propria contracts during ejacula tion. In each gland, the tubes converge to form a single duct that joins with the ductus deferens of each side to form the ejaculatory duct.

B. Prostate Gland: The prostate surrounds the urethra at its origin below the bladder. It consists of 30-50 compound tubuloalveolar glands arranged in 3 concentric groups--mucosal, sub mucosal, and main--whose ducts empty independently into the urethra. The mucosa is folded, and the epithelium varies from tall cuboidal to pseudostratified columnar; it produces prostatic fluid, which is rich in citric acid and acid phosphatase and also contains amylase, fibrinolysin, and lipids. The entire gland is surrounded by a fibroelastic capsule containing smooth muscle that contracts during ejaculation, expelling the prostatic fluid into the urethra. Extensions of the capsule form septa that penetrate the gland, divide it into indistinct lobes, and aid in expelling the prostatic fluid. Histologically, a characteristic feature of the prostate is the presence of corpora amylacea in the lumen of the gland. These small glycoprotein spheres become larger, more numerous, and calcified with age, but their significance is unknown. The prostate is a common site of disease in men over 50 years old.

C. Bulbourethral Glands (Cowper's Glands): These paired spheric tubuloalveolar glands are 3-5 mm in diameter and are lined by cuboidal epithelium. Their ducts empty clear lubricating mucus into the membranous urethra.

V PENIS

A. General Organization: The penis consists of 3 cylindric bodies of spongy erectile tissue surrounded by a common loose connective tissue sheath and covered by hairless thin skin (Fig 22-2).


1. Corpora cavernosa. Each of these 2 dorsal erectile cylinders is penetrated by a deep artery and enshcathed by a thick dense-connective-tissue tunica albuginea.
2. Corpus spongiosum (corpus carvernosum urethrae). This single, smaller, ventral cylinder is surrounded by a thinner connective tissue sheath. Its expanded distal tip is termed the glans penis. The corpus spongiosum is penetrated along its length by the cavernous (penile) urethra, whose lumen communicates with the exterior through an opening (the urethral meatus) in the glans.
3. Erectile tissue. Within each of the cylinders is an irregularly arranged network of fibrous connective tissue trabeculae containing smooth muscle fibers. The trabeculae form the sup porting framework between the numerous lacunae (vascular sinuses) that are lined by endo thelium.

B. Blood Supply: The blood supply of the penis depends on its functional state.


1. Flaccid. The peripheral dorsal arteries in the loose connective tissue sheath supply much of the arterial blood, which is drained by the superficial veins. In the flaccid state, arteriovenous shunts between the deep arteries in the corpora cavernosa and the superficial veins are open, and the branches of the deep arteries that feed the vascular spaces (the helicine arteries) are closed.
2. Erect. In this condition, the arteriovenous shunt closes down. The deep arteries in the corpora cavernosa force blood through the dilated helicine arteries into the vascular spaces in the erectile tissue. The sudden filling of the lacunae may block the veins draining them.

C. Innervation: Parasympathetic stimulation causes erection by affecting the arteriovenous shunts and helicine arteries. The sympathetic discharge accompanying ejaculation contributes to the subsequent decline of parasympathetic activity and return to the flaccid state.

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