Functional Anatomy of Male Reproductive System PDF

# Anatomy of Male Reproduction ## E.S.E. HAFEZ **Chapter 1** The male gonads, the testes lie outside the abdomen within the scrotum, which is a purselike structure derived from the skin and fascia of the abdominal wall. Each testis lies within the vaginal process, a separate extension of the per...

# Anatomy of Male Reproduction ## E.S.E. HAFEZ **Chapter 1** The male gonads, the testes lie outside the abdomen within the scrotum, which is a purselike structure derived from the skin and fascia of the abdominal wall. Each testis lies within the vaginal process, a separate extension of the peritoneum, which passes through the abdominal wall at the inguinal canal. The deep and superficial inguinal rings are the deep and superficial openings of the inguinal canal. Blood vessels and nerves reach the testis in the spermatic cord, which lies within the vaginal process; the ductus deferens accompanies the vessels but leaves them at the orifice of the vaginal process to join the urethra. Besides permitting the passage of the vaginal process and its contents, the inguinal canal also gives passage to vessels and nerves supplying the external genitalia. The spermatozoa leave the testis by efferent ductules that lead into the coiled duct of the epididymis, which continues as the straight ductus deferens. Accessory glands discharge their contents into the ductus deferens or into the pelvic portion of the urethra. The urethra origins at the neck of the bladder. Throughout its length, it is surrounded by cavernous vascular tissue. Its pelvic portion, which is enclosed by striated urethral muscle and receives secretions from various gland, leads into a second penile portion at the pelvic outlet. Here, it is joined by two more cavernous bodies to make up the body of the penis, which lies beneath the skin of the body wall. A number of muscles grouped around the pelvic outlet contribute to the root of the penis. The apex or free part of the penis is covered by modified skin- the penile integument; in the resting condition it is enclosed within the prepuce. The topographic features of the organs of the important farm species are shown in Figure 1-1. The testis and epididymis are supplied with blood from the testicular artery, which originates from the dorsal aorta near the embryonic site of the testes. The internal pudendal artery supplies the pelvic genitalia and its branches leave the pelvis at the ischial arch to supply the penis. The external pudendal artery leaves the abdominal cavity via the inguinal canal to supply the penis, scrotum, and prepuce. Lymph from the testis and epididymis passes to the lumbar aortic lymph nodes. Lymph from the accessory glands, urethra, and penis passes to the sacral and medial iliac nodes. Lymph from the scrotum, prepuce, and peripenile tissues drains to the superficial inguinal lymph nodes. Afferent and efferent (sympathetic) nerves accompany the testicular artery to the testis. The pelvic plexus supplies autonomic (sympathetic and parasympathetic) fibers to the pelvic genitalia and to the smooth muscles of the penis. Sacral nerves supply motor fibers to the striated muscles of the penis and sensory fibers to the free part of the penis. Afferent fibers from the scrotum and prepuce travel mainly in the genitofemoral nerve. ## Development ### Prenatal Development The testes develop in the abdomen, medial to the embryonic kidney (mesonephros). The plexus of ducts within the testis becomes connected to mesonephric tubules and so to the mesonephric duct, to form the epididymis, ductus deferens, and vesicular gland. The prostate and bulbourethral glands form from the embryonic urogenital sinus and the penis forms by tubulation and elongation of a tubercle that develops at the orifice of the urogenital sinus. Two agents produced by the fetal testes are responsible for this differentiation and development (1). Fetal androgen causes development of the male reproductive tract. "Müllerian inhibiting substance," a glycoprotein, is responsible for suppression of the paramesonephric (Müllerian) ducts from which the uterus and vagina develop (2). Abnormalities in differentiation and development of gonads and ducts can result in varying degrees of intersexuality (3). ## Descent of the Testis During testicular descent (4), the gonad migrates caudally within the abdomen to the deep inguinal ring. It then traverses the abdominal wall to emerge at the superficial inguinal ring, which is, in fact, the much-enlarged foramen of the genitofemoral nerve (L3, L4). The testis completes its migration by passing fully into the scrotum. Descent is preceded by the formation of the vaginal process, a peritoneal sac extending through the abdominal wall and enclosing the inguinal ligament of the testis. The inguinal ligament of the gonad is often called the gubernaculum testis, and it terminates in the region of the scrotal rudiments. Descent follows the line of the gubernaculum testis. The time of descent varies (Table 1-1). In the horse, the epididymis commonly enters the inguinal canal before the testis, and that part of the inguinal ligament connecting testis and epididymis (proper ligament of testis) remains extensive until after birth. ## Testis and Scrotum The testis is secured to the wall of the vaginal process along the line of its epididymal attachment. The position in the scrotum and the orientation of the long axis of the testis differ with the species (Figure 1-1). The arrangement of tubules and ducts within the testis in the bull is shown in Figure 1-2. The histologic and cytologic characteristics of the cellular components of the seminiferous tubules are summarized in Table 1-2 The rete testis is lined by a nonsecretory cuboidal epithelium. Testicular size varies throughout the year in seasonal breeders (ram, stallion, camel). Removal of one testis results in considerable enlargement of the remaining gonad (up to 80% increase in weight). In the unilateral cryptorchid, removal of the descended testis may be followed by descent of the abdominal testis as it enlarges. The interstitial (Leydig) cells, which lie between the seminiferous tubules, secrete male hormones into the testicular veins and lymphatic vessels. The spermatogenic cells of the tubule divide and differentiate to form spermatozoa. Just before puberty, the sustentacular (Sertoli) cells of the tubule form a barrier (6), which isolates the differentiating germ cells from the general circulation. These sustentacular cells contribute to fluid production by the tubule and may produce the Müllerian-inhibiting factor found in the rete fluid of adult males (2). The sustentacular cells do not increase in numbers after puberty is attained. This may limit spermiogenesis. Sperm production increases with age in the postpubertal period and is subject to seasonal changes in many species. Castration of prepubertal males suppresses sexual development. Regressive changes in behavior and structure take place following castration of adult males. Castration is a standard procedure in animal husbandry to modify aggressive male behavior and to eliminate undesirable carcass qualities, e.g., boar taint. Spermatogenesis disorders are monitored by changes in sperm parameters in the ejaculate or by infertility. Turner et al. (7) conducted extensive studies to identify the proteins which play major roles in spermatogenesis and are subsequently transported into the bloodstream. Autonomic innervation of the testis plays a major role in regulating the functions of the male genitourinary tract. Adrenergic, cholinergic, and nonadrenergic noncholinergic (NANC) mechanisms operate in a highly orchestrated fashion to ensure reliable storage and release of urine from the bladder to regulate the transport and storage of sperm in the reproductive tract and coordinate the emission/ejaculation of the sex accessory glands (8). The adrenergic innervation may play a role in mediating epididymal function. The sympathetic innervation within the epididymis is necessary for neuromuscular events required for the transport of sperm. The neuronal input may play an important role in the maintenance of epididymal function (8). ## Thermoregulation of the Testis For effective functioning, the mammalian testes must be maintained at a temperature lower than that of the body. Anatomic features of the testis and scrotum permit the regulation of testicular temperature. Temperature receptors in the scrotal skin can elicit responses that tend to lower whole body temperature and provoke panting and sweating (9). The scrotal skin is richly endowed with large adrenergic sweat glands, and its muscular (dartos) component enables it to alter the thickness and surface area of the scrotum and vary the closeness of the contact of the testes with the body wall. In the horse, this action may be supported by the smooth muscle within the spermatic cord and tunica albuginea, which can lower or raise the testis. In cold conditions, these smooth muscles contract, elevating the testes and wrinkling and thickening the scrotal wall. In hot conditions the muscles relax, lowering the testes within the thin-walled pendulous scrotum. The advantages offered by these mechanisms are enhanced by the special relationship of the veins and arteries. In all farm animals, the testicular artery is a convoluted structure in the form of a cone, the base of which rests on the cranial or dorsal pole of the testis. These arterial coils are intimately enmeshed by the so-called pampiniform plexus of testicular veins (10). In this countercurrent mechanism, arterial blood entering the testis is cooled by the venous blood leaving the testis. In the ram, blood in the testicular artery falls 4°C in its course from the superficial inguinal ring to the surface of the testis; the blood in the veins is warmed to a similar degree between the testis and the superficial ring. The position of the arteries and veins close to the surface of the testis tends to increase direct loss of heat from the testis. In the boar, the scrotum is less pendulous (Figure 1-1) and sweating is less efficient. This may explain the smaller difference between scrotal and rectal temperatures (3.2°C) (11). ## Epididymis and Ductus Deferens Three anatomic parts of the epididymis are recognized (Figure 1-2). The caput epididymidis (head), in which a variable number of efferent ductules (13 to 20) (12) join the duct of the epididymis. It forms a flattened structure applied to one pole of the testis. The narrow corpus epididymidis (body) terminates at the opposite pole in the expanded cauda epididymidis (tail). The middle region of each efferent duct shows marked secretory activity (13). The convoluted duct of the epididymis is very long (bull, 36 m; boar, 54 m). The wall of the duct of the epididymis has a prominent layer of circular muscle fibers and a pseudostratified epithelium of columnar cells. Three segments of the duct of the epididymis can be distinguished histologically; these do not coincide with the gross anatomic regions (14). There is a progressive decrease in the height of the epithelium and stereocilia and a widening of the lumen throughout the three segments. The first two segments are concerned with sperm maturation, whereas the terminal segment is for sperm storage. The lumen of the epididymal tubules is lined with epithelium made of a basal layer of small cells and a surface layer of tall columnar ciliated cells. The mucosa of the ductus deferens is thrown into longitudinal folds. Near the epididymal end, the epithelium resembles that of the epididymis: the nonciliated cells have little secretory activity. The lumen is lined with pseudostratified epithelium. The ampulla of the ductus deferens is furnished with branched tubular glands, which, in the stallion, are highly developed and contribute ergothioneine to the ejaculate. The ejaculatory duct enters the urethra. Fluid uptake and spermiophagy take place in the epithelium of the ejaculatory duct (15). Scanning electron microscopy has been used to evaluate functional ultrastructure of male reproductive organs with emphasis on spermatogenesis (Figure 1-3). Large volumes of fluid (up to 60 ml in the ram) leave the testis daily, and most of this is absorbed in the caput epididymidis by the initial segment of the duct of the epididymis. Transport of sperm through the epididymis takes about 9 to 13 days. Maturation of sperm occurs during transmit through the epididymis; motility increases as sperm enter the corpus epididymidis. The environment of the sperm in the cauda epididymidis provides factors that enhance fertilizing ability. Sperm from this region give higher fertility than those from the corpus epididymidis (14). Spermatozoa stored in the epididymis retain fertilizing capacity for several weeks; the cauda epididymidis is the principal storage organ, and it contains about 75% of the total epididymal spermatozoa. The special ability of the cauda epididymidis to store sperm depends on low scrotal temperatures and on the action of male sex hormone (16). Sperm stored in the ampullae constitute only a small part of the total extra-gonadal sperm reserves. Small numbers of nonmotile sperm appear in ejaculates collected weeks or even months after castration. ## Accessory Glands The prostate and bulbourethral glands pour their secretions into the urethra, where at the time of ejaculation, they are mixed with the fluid suspension of sperm and ampullary secretions from the ductus deferens. Weber et al (17) have demonstrated volumetric changes in the accessory glands of the stallion resulting from sexual stimulation (increased volume) and ejaculation (reduced volume). ### Comparative Anatomy * **The Seminal Vesicles:** These lie laterally to the terminal parts of each ductus deferens. In ruminants, they are compact lobulated glands. In the boar, they are large and less compact. In the stallion, they are large pyriform glandular sacs. The duct of the seminal vesicles and the ductus deferens may share a common ejaculatory duct that opens into the urethra. * **The Prostate Gland:** A distinct lobulated external part of body lies outside the thick urethral muscle, and a second internal or disseminated part surrounds the pelvic urethra. The disseminate prostate extends caudally as far as the ducts of the bulbourethral glands. The body of the prostate is small in the bull and large in the boar. In the stallion, the prostate gland is wholly external. * **The Bulbourethral Glands:** These are dorsal to the urethra near the termination of its pelvic portion. In the bull they are almost hidden by the bulbospongiosus muscle. They are large in the boar and contribute the gel-like component of boar semen. In ruminants and the boar, the ducts of the bulbourethral glands open into urethral recesses (18). * **The Urethral Glands:** The bull lacks urethral glands comparable with those found in man (19). Glands of this name in the horse have been considered comparable to the disseminate prostate of ruminants. ### Function Apart from providing liquid vehicle for the transport of sperm, the function of the accessory glands is obscure although much is known about the specific chemical agents contributed by the glands to the ejaculate (20, 21). Fructose and citric acid are important components of seminal vesicle secretions of domestic ruminants. Citric acid alone is found in stallion seminal vesicle; boar seminal vesicle also contain little fructose and are characterized by a high content of ergothioneine and inositol. Spermatozoa from the cauda epididymidis are capable of fertilization when inseminated without the addition of semen. ## Penis and Prepuce ### Structure In the mammalian penis, three cavernous bodies are aggregated around the penile urethra. The corpus spongiosum penis - which surrounds the urethra - is enlarged. This bulb is covered by the striated bulbospongiosus muscle. The corpus cavernosum penis arises as a pair of crura from the ischial arch, which are covered by ischiocavernosus muscles. A thick covering (tunica albuginea) encloses the cavernous bodies. The retractor penis muscles in ruminants and swine control the effective length of the penis by their action on the sigmoid flexure. In the stallion, the cavernous bodies contain large cavernous spaces; during erection, considerable increases in size result from accumulation of blood in these spaces. In bull, ram, and boar the cavernous spaces of the corpus cavernosum penis is small, except in the crura and at the distal bend of the sigmoid flexure. In ruminants and swine, the orifice of the prepuce is controlled by the cranial muscle of the prepuce; a caudal muscle may also be present. In the boar there is a large dorsal diverticulum in which urine and epithelial debris accumulate. ### Erection and Protrusion Sexual stimulation produces dilatation of the arteries supplying the cavernous bodies of the penis (especially the crura). Stiffening and straightening of the penis in ruminants is caused by the ischiocavernosus muscle, which pumps blood from the cavernous spaces of the crura into the rest of the corpus cavernosum penis. Erection failures (impotence) arise from structural defects rather than from psychological causes (23). Rising pressure in the corpus cavernosum penis produces considerable elongation of the ruminant and porcine penis with little dilation (24). When the penis of the bull is protruded, the prepuce is everted and stretched over the protruded organ. In normal service, this occurs after intromission. If it occurs before the penis enters the vestibule, intromission cannot be achieved. Intromission in the bull lasts for about 2 seconds, and straightening of the penis after withdrawal often occurs abruptly as the dorsal apical ligament reasserts its action in keeping the penis straight. Withdrawal into the prepuce follows as the pressure in the cavernous spaces subsides. The fibrous architecture of the corpus cavernosum penis in the region of the sigmoid flexure tends to reform the flexure; this is assisted by shortening of the retractor penis muscle. The terminal 5 cm or so of the boar penis are spiraled (Figure 1-5), and during erection the whole visible length of the free end of the penis becomes spiraled (24). Intromission lasts for up to 7 minutes, during which time a large volume of semen is ejaculated. Spiral deviation does not occur in the ram or goat, and intromission is of short duration. In the horse intromission lasts for several minutes. ### Emission and Ejaculation Emission consists of movement of the spermatic fluid along the ductus deferens to the pelvic urethra, where it is mixed with secretions from the accessory glands. Ejaculation is the passage of the resultant semen along the penile urethra. Emission is brought about by smooth muscles, under the control of the autonomic nervous system. Electrical stimulation of ejaculation in farm animals is a crude imitation of the complex natural mechanisms. During natural service, the sensory nerve endings in the penile integument and the deeper penile tissues are essential to the process of ejaculation. Passage of semen along the ductus deferens is continual during sexual inactivity. Prinz and Zaneveld (25) suggest that during sexual rest a complex random or cyclic process of sperm removal from the cauda epididymidis may aid the regulation of sperm reserves. Sexual excitement and ejaculation are accompanied by contractions of the cauda epididymidis and ductus deferens, which increase the rate of flow. Overall, the number of sperm passing through the ductus deferens is not increased by sexual activity. Muscular contraction of the wall of the duct is controlled by sympathetic autonomic nerves of the pelvic plexus derived from the hypogastric nerves. In normal stallions, α-receptor stimulation and β-receptor blockade increase the sperm concentration in the ejaculate (26). During ejaculation the bulbospongiosus muscle compresses the penile bulb and so pumps blood from the penile bulb into the remainder of the corpus spongiosum penis. Unlike the corpus cavernosum penis, this cavernous body is normally drained by distal veins; peak pressures recorded during ejaculation are much lower than those in the corpus cavernosum penis (27). The waves of pressure passing down the penile urethra may help to transport the ejaculate. Pressure changes in the corpus spongiosum penis during ejaculation are transmitted to the corpus spongiosum glandis; the glans penis enlarges in the ram, goat, and stallion but not in the bull. ## Laboratory Animals Species differences in the male reproductive organs are shown in Figure 1-1. These organs can move from a wholly scrotal to a wholly abdominal position. Differences in relative size of the accessory glands are reflected in the semen characteristics (Table 1-3).