Fixation

A thin fibrous capsule (capsula fibrosa) covers the surface of the kidney. The capsule follows the hilus to line the walls of the sinus and to form the adventitia of the renal pelvis. It also invests the renal vessels and nerves before they pass into the sinus. The fibrous capsule of normal kidneys is easily removable, except in the renal sinus, where it is adherent to blood vessels and to the renal pelvis (pelvis renalis). Fat of the adipose capsule (capsula adiposa), in which the kidney is partially embedded is external to the fibrous capsule and extends through the hilus into the sinus.

Position and Relations

The craniolateral surface of the left kidney is in contact with the dorsal end of the medial surface of the spleen, the greater omentum, and the greater curvature of the stomach. Cranially, it is in contact with the left lobe of the pancreas and left adrenal gland. Dorsally, the kidney, with its adipose capsule, is related to the quadratus lumborum, transversus abdominis, and psoas muscles, as well as to the deep layer of the thoracolumbar fascia underlying the retroperitoneal or pararenal fat. Caudally, the left kidney of the female is in contact with the descending colon and the mesovarium. The peritoneum on the ventral surface of the kidney blends with the peritoneum suspending the ovary. In the male, the renal peritoneum is reflected onto the dorsal body wall as parietal peritoneum. Medially, the left kidney of the male is related to the left adrenal gland descending colon, mesocolon, and ascending duodenum. The descending colon is also related to the ventral surface of the kidney. The medial edge of the left kidney is located approximately 1 cm from the middorsal line in an average-sized dog; the cranial pole lies approximately 5 cm caudal to the dorsal third of the last rib.

The right kidney has its cranial pole embedded in the fossa of the caudate process of the caudate lobe of the liver. This extremity is located at the level of the thirteenth rib. It may be a few centimeters craniad or caudad, depending on the degree of gastric or, in the female, uterine distention. It may be in contact with the diaphragm and retractor costae muscle. The right adrenal gland is also related to the cranial pole of the right kidney. Medially, the right kidney is in close proximity to the caudal vena cava, and, ventrally, it is in contact with the right lobe of the pancreas and the ascending colon.

The kidney has an indentation or cavity on its medial border referred to as the renal hilus (hilus renalis). The space defined by the walls of the hilus is the renal sinus (sinus renalis) (see Figs. 9-3 and 9-5). The sinus contains the renal pelvis, a variable amount of adipose tissue, and branches of the renal artery, vein, lymphatics, and nerves. After they pass through the sinus, the vessels and nerves enter the parenchyma of the kidney.

The renal pelvis (pelvis renalis) (see Fig. 9-5) is a funnel-shaped structure that receives urine from the papillary ducts of the kidney and passes it into the ureter. The pelvis of the kidney is elongated in a craniocaudal direction, and is curved to conform with the lateral border of the kidney. It extends into the renal parenchyma both dorsally and ventrally by means of curved diverticula, the recesses of the renal pelvis (recessus pelvis). There are generally five or six recesses curving peripherally from each border of the pelvis.

Structure

The parenchyma of the kidney is made up of an internal medulla (medulla renis) and an external cortex (cortex renis) (see Fig. 9-5). When cut transversely (see Fig. 9-5C) the peripheral portion of the renal parenchyma or cortex appears granular, owing to the presence of numerous renal corpuscles and convoluted tubules (nephrons). When the kidney is cut in a dorsal plane, numerous cut ends of arcuate arteries and veins are apparent at the corticomedullary junction. The thickness of the renal cortex is approximately the same as the transverse diameter of the renal medulla. The peripheral surface of the cortex is covered by the fibrous capsule.

A median plane longitudinal section of the kidney shows the medulla as a continuous striated structure with its free edge facing the renal pelvis. This is the renal crest (crista renalis). Similar dorsal plane longitudinal sections on either side of the renal crest show the medulla separated into cone-shaped renal papillae (papilla renalis) with interlobar vessels between them. These renal papillae are the apices of the renal pyramids (pyramides renales), the base of which is at the level of the renal cortex. These pyramids extend from the cortex on the dorsal and ventral surfaces of the kidney into the center where they fuse into the renal crest. A variable number papillary foraminae (foramina papillaria) open on the border of the renal crest that faces the renal pelvis. These are the openings of the papillary ducts (ductus papillares) that pass urine into the renal pelvis, which leads to the ureter. These foraminae compose the area cribrosa of the renal crest. The renal papillae of the pyramids are surrounded by extensions of the renal pelvis called pelvic recesses (recessus pelvis) but there are no papillary foraminae on these papillae.

The Nephron

The nephron (nephronum) (Fig. 9-6) is a continuous contorted tube that serves for urine production and for the regulation of the volume and composition of the extracellular fluid (Reese, 1991). There are approximately 500,000 nephrons in the dog kidney. Each nephron begins at the double-layered glomerular capsule (capsula glomeruli), which is invaginated by a spherical rete of blood capillaries, the glomerulus. Vimtrup (1982) commented on the number, shape, and structure of glomeruli in several mammals. The glomerulus and capsule together form the renal corpuscle (corpusculum renale) (see Fig. 9-6). Renal corpuscles are present in the renal cortex, but not in the medulla. The following components compose the tubular nephron in order from the glomerular capsule to the collecting tubules and papillary ducts in the renal medulla: proximal convoluted tubule, proximal straight tubule, attenuated (thin) tubule that forms a loop, distal straight tubule and distal convoluted tubule.

Eisenbrant and Phemister (1979) investigated normal postnatal development of the dog kidney in puppies between 2 and 200 days of age. They found a subcapsular nephrogenic zone was present until approximately 8 days of age. This zone produced new nephrons and interstitial tissues. Deep to the nephrogenic zone there were renal corpuscles of increasing maturity at successively deeper levels. They estimated the total number of nephrons to be 445,000 per kidney and this did not vary significantly during subsequent growth. The corpuscular volume per nephron increased 249% between day 14 and day 200, whereas the increase in the tubular volume per nephron was 303%.

Books on renal morphology and function include Smith (1951); The Kidney, in four volumes, by Rouiller and Muller (1969-1971); and The Kidney, in two volumes, by Brenner and Rector (1991).

Vessels and Nerves

The kidney is a highly vascular organ, as would be expected (Fuller & Heulke, 1973; Morison, 1926). Briefly, blood enters the renal artery from the aorta, goes through end arteries, interlobar vessels, arcuate vessels, interlobular arteries, and finally to glomeruli via afferent arterioles. Efferent arterioles leave the glomeruli and course directly into the outer layer of the medulla, giving rise to long capillary nets that extend to the apical end of the pyramid, or they branch directly into intertubular capillary networks. The intrarenal vascular system of the puppy kidney was described by Evan et al. (1979).

The renal artery (arteria renis) bifurcates into dorsal and ventral branches. The site of bifurcation is extremely variable (Christensen, 1952). Variations in the renal artery are common, ranging from a single vessel to one with numerous branches or to completely doubled renal arteries. The two primary branches of the renal artery, end branches, divide into two to four interlobar arteries (aa. interlobares renis). These branch into arcuate arteries at the corticomedullary junction. The arcuate arteries (aa. arcuatae) radiate toward the periphery of the cortex, where they redivide into numerous interlobular arteries (aa. interlobulares). Afferent arterioles (arteriola glomerularis afferens) leave these to supply the glomeruli and thence the efferent arterioles (arteriola glomerularis efferens). The mean glomerular diameter in a 35-pound dog is 170 µm. According to Rytand (1938), there are 408,100 glomeruli in one canine kidney, with a total glomerular volume of 1247 mm³. Finco and Duncan (1972) found a correlation between kidney and nephron size and the body size of the dog.

Venous drainage of the kidney stems from the numerous stellate veins (venulae stellatae) in the fibrous capsule. These connect with veins of the adipose capsule and empty into interlobular (vv. interlobulares), arcuate (vv. arcuatae), and interlobar veins (vv. interlobares) before entering the main trunk of the renal vein (vena renis), which joins the caudal vena cava. Venous arcuate vessels, unlike their arterial counterparts, unite to form elaborate arches. Arcuate veins span the medulla to join the dorsal and ventral parts of the kidney. Evan et al. (1979) investigated the vascular system of the puppy kidney between 1 and 21 days after birth. They found the vascular system of the puppy kidney to be strikingly different from that of the adult. The most obvious difference was the lack of peritubular capillaries throughout the cortex. In their place were large sinusoidal vessels directly continuous with the venous system. The vascular arrangement of the efferent arterioles and the sinusoidal vessels appeared to function as a postglomerular shunt.

Bentley et al. (1988) examined the architecture and vasculature of the dog kidney using a dynamic spatial reconstructor. This device is a high-speed, volume-scanning, computed, radiotomographic imaging system, which, when used in conjunction with radiopaque methacrylate injections, allowed them to compare casts with the reconstructed images. Interlobar arteries and occasionally arcuate arteries could be clearly detected. Analysis of artery-to-vein transit times showed some to be as short as 3 seconds.

Capsular and parenchymal lymphatics are connected to interlobular plexuses that pass into trunks that leave the kidney at the hilus. They terminate in the lumbar lymph nodes. According to Peirce (1944) lymphatics in the kidney accompany the interlobular, arcuate, and interlobar vessels, surrounding them in an irregular network. The periarterial rete is thicker than the perivenous network. Cortical and perirenal lymphatics anastomose (O’Morchoe & Albertine, 1980).

A renal plexus (plexus renalis) surrounds the renal arteries where they enter the renal sinus. This plexus consists of postganglionic sympathetic axons with their cell bodies in aorticorenal ganglia (ganglia aorticorenalia). The plexus also contains preganglionic parasympathetic axons from the vagus nerve. Innervation is provided to the nephrons, blood vessels, and muscle in the renal pelvis.

Anomalies

Malformations of the kidney are fairly common. Congenital renal cysts and polycystic kidneys occur, although isolated renal cysts are more commonly found. Other congenital anomalies include hypoplasia and aplasia (Hofliger, 1971; Pearson & Gibbs, 1971). Fetal lobation of the kidney may persist in the adult dog.

Structure

The muscular wall of the ureter is divided into three thin layers: an outer longitudinal, a middle circular, and an inner longitudinal. Only longitudinal fibers are present at the junction of the ureter with the bladder. The ureteral mucosa (tunica mucosa) is made up of transitional epithelium.

Vessels and Nerves

The cranial ureteral artery to the ureter is derived from the renal artery, whereas the caudal ureteral artery comes from the prostatic or vaginal artery. Cranial and caudal ureteral arteries anastomose on the ureter. The ureteral arteries have venous counterparts. The autonomic nerves to the ureter come from the celiac and pelvic plexuses.

Anomalies

Congenital anomalies include duplication, ectopic openings into the vagina, ureteral atresia, or dilation of the renal pelvis. The latter condition may result from an obstructed ureter. Obstruction may be caused by calculi, tumors, scars, ligatures, or developmental anomalies.

Structure

There are three layers of muscle in the wall of the urinary bladder, similar to the arrangement of muscle fibers in the ureter: outer and inner longitudinal layers, and a relatively thick middle circular layer. The muscle fibers all take on an oblique appearance at the urethral-bladder junction. This bladder muscle is often referred to as the detrusor muscle. The tunica mucosa of the urinary bladder, like that of the ureter and renal pelvis, is made up of transitional epithelium. It is irregularly folded when the bladder is empty but the mucosal folds disappear during distention. A loose tela submucosa lies between the mucosa and the muscular layer. Internally, a triangular area near the neck of the bladder is termed the trigone of the bladder (trigonum vesicae). The apex of the trigone is at the urethral orifice, and the base is indicated by a line connecting the ureteral openings. This area is free from the characteristic mucosal folds, but poorly developed ridges, converging toward the urethral crest, denote the boundaries of the trigone.

Fixation

The reflection of the peritoneum from the lateral and ventral surfaces of the urinary bladder to the lateral walls of the pelvis and to the ventral abdominal wall are known as ligaments of the bladder. These are made up of double layers of peritoneum separated by intercalated blood vessels, nerves, lymphatics, and adipose tissue, as well as by the ureters, deferent ducts, and vestiges of embryonic structures. The largest peritoneal fold, the median ligament of the bladder (lig. vesicae medianum) is reflected from the ventral surface of the bladder to the symphysis pelvis and the linea alba of the abdominal wall as far cranially as the umbilicus. It is median in position and triangular in shape. In the fetus the median ligament contains the urachus (stalk of the embryonic allantois) and the umbilical arteries. These normally disappear shortly after birth, leaving only the peritoneal fold. A vestigial fibrous urachus may sometimes be found in the free edge of the ligament. In an average-sized dog this median ligament has its greatest height caudally (6 cm) and narrows cranially to form an acute angle with the abdominal wall at the umbilicus. Caudally, the ligament ends approximately at the level of the vaginovestibular junction in the female, and at the level of a transverse plane through the middle of the prostate gland in the male.

The lateral ligaments of the bladder (lig. vesicae laterales) (Fig. 9-9) connect the lateral surfaces of the bladder to the lateral pelvic walls. They are also triangular in shape. The lateral ligaments of the bladder contain the round ligament of the bladder and the ureter. In the fetus each lateral ligament contains a large umbilical artery that extends cranially along the rudimentary bladder and courses in the median ligament of the bladder with the urachus to the umbilicus. The urachus is the stalk of the allantois, which connects from the apex of the rudimentary bladder to the allantoic portion of the fetal membranes. Before birth, the bilateral umbilical arteries (branches of the internal iliac arteries) carry blood from the fetus to the placenta and are components of the umbilical cord. When the umbilical cord is severed at birth, the arteries retract and become fibrous cords between the bladder and the umbilicus that disappear in the young dog and are rarely visible in adult dogs. The narrowed lumen of each umbilical artery remains patent between the internal iliac artery and the bladder, where the relatively minute cranial vesical artery leaves the umbilical artery to vascularize the apex and body of the bladder. The remnants of these arteries in the lateral ligaments of the bladder are referred to as the round ligaments of the bladder (lig. teres vesicae). The ureter and round ligament of the bladder cross at nearly right angles to each other at the junction of the broad and lateral ligaments. The ureter is the more mesial of the two structures. The lateral ligaments of the bladder, in the female, blend laterally with the broad ligament of the uterus (mesometrium) as well as with the lateral pelvic wall. In the male, the ureter and ductus deferens cross each other a few centimeters from the entrance of the ureters into the bladder (see Fig. 9-7). The ductus deferens is suspended by the mesoductus deferens, a fold of peritoneum, which at the vaginal ring separates from the mesorchium, which is the peritoneal fold containing the testicular vessels and nerves. The ductus deferens and its mesoductus deferens course dorsocaudally, dorsal to the ureters in the lateral ligaments of the bladder to terminate in the prostatic urethra. Dorsal to the bladder a short fold of peritoneum connects between each ductus deferens. This is the genital fold (plica genitalis). In the female this genital fold connects between the two uterine horns. The peritoneal pocket between the rectum and the genital fold and the two ductus deferens or the initial part of the two uterine horns, the uterus and cranial vagina is the rectogenital pouch (excavatio rectogenitalis). A small pubovesical pouch (excavatio pubovesicalis) is present between the bladder and its lateral ligaments and the pubis

Vessels and Nerves

The urinary bladder receives its major blood supply through the caudal vesical arteries. These are branches of the vaginal or prostatic arteries that are branches of the internal pudendals from the internal iliac arteries. The small cranial vesical artery from the umbilical supplies the bladder apex. The venous plexus on the urinary bladder drains primarily into the internal pudendal veins. The lymphatics of the bladder drain into the hypogastric and lumbar lymph nodes (Baum, 1918).

Bladder innervation is complex from an anatomic as well as a physiologic understanding. The bladder receives autonomic innervation from both the sympathetic and parasympathetic general visceral efferent neurons. The sympathetic innervation primarily functions in urine storage and the parasympathetic innervation primarily functions in evacuation of urine. Sympathetic preganglionic cell bodies are located in the lateral horn of the first four lumbar spinal cord segments. Preganglionic axons enter the abdomen through the lumbar sympathetic trunk and splanchnic nerves to synapse on cell bodies of postganglionic axons in the caudal mesenteric ganglion or in the bladder wall. For the former, postganglionic axons leave the caudal mesenteric ganglion in the hypogastric nerves and course to the pelvic plexus, which is associated with the vaginal or prostatic artery. These axons follow the branches of the artery to the bladder to innervate the detrusor muscle with inhibitory synapses and the bladder neck sphincter muscle with excitatory synapses. The sympathetic preganglionic axons that did not synapse in the caudal mesenteric ganglion course to the bladder wall through the hypogastric nerves, the pelvic plexus and its branches that follow the arteries to the bladder. Synapse occurs on cell bodies of short postganglionic axons within the wall of the bladder. Parasympathetic cell bodies of preganglionic axons are located in the lateral horn of the sacral spinal cord segments. The preganglionic axons course into the pelvic nerve through the ventral branches of the sacral spinal nerves. The pelvic nerve courses to the pelvic plexus and synapses on cell bodies of postganglionic axons in pelvic plexus ganglia or on cell bodies in the bladder wall. These axons of postganglionic cell bodies are excitatory to the detrusor muscle causing contraction and bladder evacuation. The striated urethralis muscle which functions to store urine is innervated by general somatic efferent neurons with their cell bodies in the ventral horn of sacral spinal cord segments. Their axons are distributed to this muscle through the sacral plexus and the branches of the pudendal nerve. The axons of visceral afferent neurons that innervate the bladder and urethra reverse the pathway of the efferent neurons. Their cell bodies are in cranial lumbar and sacral spinal ganglia. This complex reflex pathway of innervation is under voluntary control by cranially projecting sensory spinal cord pathways, centers in the caudal brainstem and caudally projecting upper motor neuronal pathways.

Petras and Cummings (1978) describe the location of the cells of origin for the sympathetic and parasympathetic innervation of the urinary bladder and urethra in the dog. Their study, using horseradish peroxidase injected into the bladder and urethra and counterstained with cresyl violet, demonstrates the presence of both sympathetic and parasympathetic intramural ganglia and axons. Thus there is a direct preganglionic sympathetic pathway to the urinary bladder and urethra in addition to the postganglionic sympathetic innervation.

Anomalies

Anomalies of the urinary bladder include diverticula, strictures of the neck of the bladder, urachal cysts, and patent urachus. An enlarged prostate may be the indirect cause of dilation of the bladder.

Vessels and Nerves

The principal blood vessel to the scrotum is the ventral scrotal branch of the external pudendal artery. The cremasteric artery arises from the deep femoral artery. The scrotal arteries run along the cranioventral surface of the testis, superficial to the parietal layer of the vaginal tunic. The perineal branches of the internal pudendal artery supply dorsal scrotal arteries. The draining veins follow the same course in reverse.

The genital rami, branches of the genitofemoral nerve from the ventral branches of the third and fourth lumbar nerves, innervate the skin of the prepuce. The superficial perineal nerve, a branch of the pudendal from sacral nerves 1, 2, and 3, supplies all of the scrotum, according to Spurgeon and Kitchell (1982b). Postganglionic sympathetic axons supplying the tunica dartos enter via the sacral plexus and the pudendal and superficial perineal nerves.

Attachments

The short proper ligament of the testis (lig. testis proprium) attaches the testis to the tail of the epididymis. This is continued by the short ligament of the tail of the epididymis (lig. caudae epididymidis) that attaches the tail of the epididymis to the reflections of the layers of the vaginal tunic at its most distal extent as well as to the spermatic fascia. These two ligaments are remnants of the two divisions of the embryonic gubernaculums testis. The scrotal ligament (lig. scroti) is a thin layer of connective tissue between the tunica dartos and the tail of the epididymis. These latter two ligaments are difficult to distinguish from each other. These ligaments as well as the spermatic cord provide stability to the testis.

Vessels and Nerves

The testicular artery and the artery of the ductus deferens supply the testis and epididymis. The testicular artery (homologue of the ovarian artery of the female) arises from the ventral surface of the aorta at the level of a transverse plane through the fourth lumbar vertebra. The right artery originates cranial to the left, corresponding to the embryonic positions of the testes. The artery of the ductus deferens, a branch of the prostatic artery from the internal pudendal, follows the ductus deferens into the spermatic cord to the level of the epididymis. It sends branches to the epididymis and anastomoses with the testicular artery. The testicular vein follows the arterial pattern but forms an extensive pampiniform plexus (plexus pampiniformis) in the spermatic cord, surrounding the testicular artery lymphatics, and nerves. The right testicular vein empties into the caudal vena cava at the level of the origin of its arterial counterpart. The left drains into the left renal vein. Harrison (1949) made a detailed comparative study of the vascularization of the mammalian testis.

The testicular and epididymal lymphatics anastomose into a variable number of trunks that drain into the lumbar lymph nodes (see Chapter 13).

The nerve supply to the testis is derived from the sympathetic division of the general visceral efferent component of the autonomic nervous system. The nerves of the testicular plexus accompany the testicular arteries distally and enter the testis with either the blood vessels or the efferent ducts. Indirectly, they are derived from the fourth, fifth, and sixth lumbar sympathetic trunk ganglia. The testicular plexus is derived from the abdominal aortic plexus at the level of the origin of the testicular arteries. These testicular vessels, lymphatics and nerves are suspended in the spermatic cord by the mesorchium, which is a fold of the visceral layer of the vaginal tunic. This is continued in the abdomen as a fold of the parietal layer of the peritoneum. The blood vessels and smooth muscle fibers in the testis receive a sympathetic nerve supply, but the seminal epithelium and the interstitial secretory tissue do not. Elimination of the sympathetic nerve supply to the testis is followed by degeneration of the seminal epithelium and hypertrophy of the interstitial secretory tissue (Kuntz, 1919b). The degenerative changes are considered to be the result of paralysis of the blood vessels in the spermatic cord and testis.

Anomalies

Cryptorchidism, or failure of the testis to descend, is the most important congenital anomaly of the testis. This condition is comparatively frequent and is believed to be hereditary in some instances. Cox et al. (1978) investigated 12 cases of cryptorchidism in Miniature Schnauzers. Five were unilateral and seven were bilateral. All of the unilateral cases had retained testes on the right side. When retained testes were bilateral the right testis was always smaller. Their observations suggested a multigene defect. In a cryptorchid animal one or both testes are retained either in the abdominal cavity (in the region of the inguinal canal) or between the superficial inguinal ring and the scrotum. Sterile, cryptorchid dogs usually possess normal sexual desire. For a discussion of cryptorchidism see Wensing and van Straten (1980). Hayes et al. (1985) studied 1.8 million documented medical records and identified 2912 dogs (in 104 different breeds) that had cryptorchid testes. There were 14 breeds with significantly high risk.

According to Runnells (1954) testicular tumors of dogs have been reported to cause anatomic alterations, such as atrophy of the opposite testis and enlargement of the prepuce and prostate gland. Hayes et al. (1985) reported that testicular tumors were found in 5.7% of the 2912 cryptorchid dogs whose records they reviewed. Half had Sertoli cell tumors, and one-third had seminomas.

Male pseudohermaphroditism and true hermaphroditism have been reported in the dog by Lee and Allam (1952), Brodey et al. (1954), and Bodner (1987). Female pseudohermaphroditism, considered rare (Meyers-Wallen & Patterson, 1986), was reported by Olson et al. (1989) in three sibling Greyhounds.

Descent of the Testes

In the majority of mammalian species the testes migrate from their developmental position within the abdomen near the kidneys to a location outside of the body wall, usually in a scrotum (see Fig. 9-15). For these species, including the dog, if neither testis descends (bilateral cryptorchid) spermatogenesis is eliminated and the animal is infertile. If only one testis descends (monorchid or unilateral cryptorchid) fertility is lessened. (Many rodents have testes that descend periodically coincident with breeding. In such species the testes can be gently squeezed back into the body cavity at any time because of the large inguinal canal.)

Much important research on testicular descent in domestic animals has been conducted at the Institute of Veterinary Anatomy of the State University at Utrecht, the Netherlands, by Wensing and co-workers: Wensing (1968, 1973a, 1973b, 1980); Baumans et al. (1981, 1982, 1983); Baumans (1982); Wensing and Colenbrander (1986).

Whereas in most mammals testicular descent occurs in fetal life, in the dog it occurs at approximately the time of birth and this makes the dog a good subject for the study of the mechanics and hormonal control of descent. A descriptive and illustrated experimental study of normal development and the factors responsible for the descent of the testes in the dog was published as a thesis in the Netherlands by Baumans (1982). (Included in the thesis are six papers with co-workers, each with a bibliography.) Baumans found that the major factors essential for the descent were first an outgrowth and then a regression of the gubernaculum testis. The gubernaculum is a mesenchymal mass enclosed in a fold of peritoneum that extends from the testis across the mesonephros (kidney ridge) to the inguinal area. It is attached to the caudal pole of the testis and epididymal part of the mesonephric duct. Its distal end continues through the abdominal wall where the inguinal canal forms around it. Here, the gubernaculum is invaded by an outgrowth of parietal peritoneum, the vaginal process. During fetal growth the testis moves caudally to the level of the inguinal canal as the trunk elongates. There are two phases of the migration of the testis through the inguinal canal and into the scrotum. In the first phase the extra-abdominal part of the gubernaculum increases enormously in length and volume expanding beyond the inguinal canal and dilating it. When this expansion exceeds the size of the testis and passive resistance is reduced, the testis passes through the canal adjacent to the vaginal process. Here it rests in a mass of swollen gubernaculum, which is still covered by vaginal process peritoneum. An important feature of this testicular migration is that the testis brings with it the visceral peritoneum that formed around it at its site of development in the dorsal abdominal wall. Completion of this first phase of testicular descent initiates the second phase in which the gubernaculum is transformed by tissue degeneration from an expanded mucoid mass into a small fibrous structure in the scrotal sac, thus making room for the testis. This converts the gubernaculum into the proper ligament of the testis, which attaches the tail of the epididymis to the testis, and the ligament of the tail of the epididymis, which attaches the tail to the distal portion of the vaginal tunic and spermatic fascia. The latter is derived from the remnants of the gubernaculum that surrounds the vaginal tunic. The degeneration of the gubernaculum results in the testis being located at the distal extent of the vaginal tunic.

Baumans found that on the day of birth the testis was located halfway between the kidney and the deep inguinal ring. The gubernaculum had reached its maximum development and was beginning to show signs of regression histologically and histochemically. By day 3 or 4 after birth the testis passes through the inguinal canal and the gubernaculum regresses. By 35 to 40 days after birth the testis reaches its definitive position in the scrotum.

To test the factors responsible for gubernacular growth and regression, Baumans removed the testes in fetal dogs and found that gubernacular outgrowth ceased and there was no epididymal descent. Removal of the testis at birth resulted in retarded gubernacular regression and a delayed epididymal descent. After further investigation it was concluded that the testis induces gubernacular outgrowth and regression and thereby regulates its own descent. It was found that testicular hormones, particularly testosterone, are synthesized in the testis. In the first phase of descent an unidentified testicular factor stimulated gubernacular proliferation and swelling. Sustentacular cells are thought to be the source of this factor. In the second phase of descent testosterone induced gubernacular regression. In regard to the mechanical role of the gubernaculum, it was found that a connection between the testis and the gubernaculum (see Fig. 9-15) was essential for normal testicular descent. By the end of the second phase of descent the testis plays a mechanical role in distending the scrotum.

Wensing and Colenbrander (1986), in discussing normal and abnormal descent of the testis, make a distinction between the morphologic characteristics of testicular descent in mammals with a striplike cremaster muscle (ungulates, dog, and humans) and those with a saclike cremaster muscle (rodent, lagomorph). They believe that the bilaminar saclike cremaster muscle of rodents and lagomorphs continues to grow and increase in size even after the regression of the gubernaculum and thus plays a role in descent of the testis. Regardless of the morphologic characteristics of the cremaster muscle, they stress the importance of a normal gubernacular outgrowth and regression for affecting testicular descent. Their experimental results showed that the first phase in the descent process (gubernacular outgrowth) does not depend on the presence of active interstitial cells, testosterone, testosterone receptors, or gonadotropins, although the presence of the testis is essential.

Structure

The epididymis, especially the caudal portion, has a lower temperature than the rest of the body because of its position in the scrotum and is a favorable storage place for spermatozoa before their passage into the ductus deferens (Bedford, 1978). Circular smooth muscle fibers aid seminiferous tubule secretion in moving the germ cells. The length of the epididymis and the slowness of spermatozoa movement are important in allowing the spermatozoa to complete their maturation process, called capacitation (Mason & Shaver, 1952).

Structure

The layers of the ductus deferens are the tunica adventitia, t. muscularis, and t. mucosa. Three layers of smooth muscle are generally recognized: an outer and an inner longitudinal, and a middle circular. The mucosa is made up of simple or pseudostratified columnar epithelium. The terminal portion of the ductus deferens and the adjacent area of the urethra contain branched tubular glands. A distinct ampulla of the deferent duct is not obvious in the dog.

Vessels and Nerves

The artery of the ductus deferens is a branch of the prostatic artery, which, in turn, arises from the internal pudendal. It accompanies the ductus deferens to the epididymis, which it also supplies with blood. The artery of the ductus deferens anastomoses with the testicular artery in the spermatic cord. The vein of the ductus deferens runs in the spermatic cord with the deferent duct, and empties into the internal iliac vein. The lymphatics drain into the hypogastric and medial iliac lymph nodes. Nerves to the ductus deferens are autonomic, arising from the pelvic plexus. The hypogastric nerves (sympathetic), via the pelvic plexuses, supply the pelvic part of the ductus deferens. Parasympathetic axons are thought to be distributed via the pelvic plexus only to the epididymis and musculature of the ductus deferens.

Vessels and Nerves

The blood and nerve supply of the prostate have been studied by Gordon (1960) and Hodson (1968). The prostatic artery (a. prostatica) arises from the internal pudendal at the level of the second or third sacral vertebrae, although it may arise from the umbilical (Hodson, 1968) near its origin. The prostatic artery gives rise to the artery of the ductus deferens (a. ductus deferentis), which is homologous with the uterine artery in the female. The artery of the ductus deferens gives rise to the caudal vesicle artery (a. vesicalis caudalis). The caudal vesicle artery gives branches to the ureter (ramus uretericus) and urethra (ramus urethralis) and then ramifies on the surface of the bladder, anastomosing with the contralateral caudal vesicle and cranial vesicle arteries. When the cranial vesicle artery is not present, the caudal vesicle supplies the entire bladder.

The prostatic artery continues caudoventrally and gives rise to the small middle rectal artery (a. rectalis media) before ramifying on the surface of the prostate. These branches penetrate the capsule on the dorsolateral surface of the gland to become subcapsular arteries (Hodson, 1968). Radial tributaries pass along the capsular septae toward the urethra to supply the glandular tissue. Cavernous tissue, continuous with the corpus spongiosum, surrounds the pelvic urethra. It is supplied by the artery of the bulb of the penis.

Anastomoses occur between the prostatic vessels and the urethral artery (internal pudendal), the cranial rectal artery (caudal mesenteric) and the caudal rectal artery (internal pudendal). These anastomoses complicate prostatectomy. The venous network of the gland drains by way of the prostatic and urethral veins into the internal iliac vein. The prostatic lymph vessels empty into the iliac lymph nodes. The nerve supply to the prostate is closely allied to the vasculature. The hypogastric nerve, which supplies sympathetic innervation to the prostate, follows the prostatic artery from the pelvic plexus. The pelvic nerve, which may be single or double (Gordon, 1960), accompanies the prostatic artery as far as the lateral surface of the rectum. Here it forms the pelvic plexus, together with branches of the hypogastric nerve. The middle portion of the pelvic plexus forms the prostatic plexus, which innervates the gland. Parasympathetic stimulation increases the rate of glandular secretion.