Intersexes and disorders of sexual development in the dog are common (Hare, 1976; Meyers-Wallen & Patterson, 1986). A dog may be a genetic female with a bicornuate uterus and have abdominal testes (Bodner, 1987 in a Doberman; Stewart et al., 1972 in a Pug). For a comprehensive review of intersexes and freemartins and a well-referenced general treatment of the reproductive system in vertebrates see van Tienhoven (1983) and Lamming (1990). A perceptive book by Willis (1962) provides a background for an understanding of pathologic problems resulting from the duality in development of the urogenital system, as does the embryology text by Noden and de Lahunta (1985) and McGeady et al (2009). Pathologic conditions of the reproductive system have been discussed by McEntee (1990), and congenital malformations by Szabo (1989). The kidney (ren), nephros in Greek (Figs. 9-1 to 9-5), is a reddish-brown, paired structure lying against the lumbar hypaxial muscles on either side of the vertebral column. Each kidney has a cranial and a caudal pole, a medial and a lateral border, and a dorsal and a ventral surface. The cranial and caudal extremities are joined by a convex lateral border. The medial border has an indentation, the hilus, that defines a space, the renal sinus. The sinus contains the ureter, renal artery and vein, lymph vessels, and nerves. Of these structures, the renal artery is the most dorsal, and the renal vein the most ventral. Commonly the renal vein is paired on one or both sides, and sometimes the renal artery may also be paired. The nerves and lymphatics lie in close relationship to the renal vein (Bulger et al 1979). The kidneys lie in an oblique position, tilted cranioventrally. The right kidney is more firmly attached to the dorsal wall than is the left and has a correspondingly larger retroperitoneal area. Both kidneys are invested with a fibrous capsule surrounded by adipose tissue and are held in position by transversalis fascia. They are not rigidly fixed and may move during respiration or may be displaced by a full stomach. In some lean animals it is possible to palpate the kidneys, especially the left kidney. The right kidney lies more cranially than the left (see Fig. 9-1) and is in contact with the liver. Grandage (1975) has considered some effects of posture on the radiographic appearance of the kidney. 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 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. 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. 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). 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. 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). 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. The ureters (Figs. 9-1, 9-2, 9-7, and 9-8) carry urine from the kidneys to the bladder. The diameter of a single ureter measures 0.6 to 0.9 cm when it is distended. The length of the ureter depends on the size of the animal, averaging between 12 and 16 cm in a 35-pound dog. The right ureter is slightly longer than the left because of the more cranial position of the right kidney. 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 The reproductive system in vertebrates is a most varied assemblage of primary and accessory organs and parts, which begin developmentally in a similar fashion but result in strikingly different forms in the adult. In recent years the study of reproduction in animals (theriogenology) has made great strides, and as a result of our new understanding we are now able to manipulate the system in many ways such as to facilitate artificial insemination, egg or embryo transfer, freezing and storage of eggs and embryos and cloning of various species. For explanations of developmental processes in domestic animals see Noden and de Lahunta (1985). For an overall view of the reproductive system from fish to humans there is nothing better than Marshall’s Physiology of Reproduction. The most recent revision of Marshall’s by Lamming (1990-1992) devotes one volume to a consideration of reproductive cycles and female anatomy, a second to reproductive structures and functions of the male, and a third to pregnancy and lactation. Other books on the reproductive system include Austin and Short (1982), Segal et al. (1973), and Cupps (1991). For information on the reproductive habits, cycles, and gestations of mammals of the world, including canids, reference should be made to Asdell’s Patterns of Mammalian Reproduction: A Compendium of Species-Specific Data by Hayssen and van Tienhoven (1993). The canine male genital organs (organa genitalia masculina) (Figs. 9-10 to 9-34) consist of the scrotum, the testes, the epididymides, the deferent ducts, the spermatic cord, the prostate gland, the penis, and the urethra. The scrotum (see Figs. 9-14 and 9-15) is a pouch of skin divided by a median septum into two components, each of which is occupied by a testis, an epididymis, and the distal part of the spermatic cord. The scrotal septum (septum scroti) is a median partition that is made up of all the layers of the scrotum except the skin. In the dog, the scrotum is located approximately two thirds of the distance from the preputial opening to the anus. It lies between the thighs and has a spherical shape, indented in an oblique craniocaudal direction by an indistinct raphe scroti. The left testis is usually farther caudad than the right, allowing the surfaces of the testes to glide on each other more easily and with less pressure. Extending into each scrotal sac is an evaginated pouch of peritoneum, the vaginal tunic (tunica vaginalis) (see Figs. 9-12 and 9-14), covered by spermatic fascia of the abdominal wall. The vaginal tunic and fascia wrap the descended testis and spermatic cord in such a way as to result in a double-walled extension of abdominal peritoneum. This was a vaginal process before the descent of the testis with its duct system, vessels, and nerves (see Figs. 9-12 and 9-15) Zietzsehman (1928). The outer wall, or parietal layer of the vaginal tunic, is separated by a space, the vaginal canal (canalis vaginalis) or vaginal cavity (cavum vaginale), from the visceral layer of the vaginal tunic. The vaginal canal surrounds the spermatic cord and the vaginal cavity surrounds the testis. The vaginal canal is continuous with the peritoneal cavity at the vaginal ring. The scrotum, because of its thin, hairless skin, its lack of subcutaneous fat, and its ability to contract toward the body, functions as a temperature regulator for the tail of the epididymis. Evidence indicates that the epididymis, as the site of sperm storage, is the most heat-sensitive region of the male reproductive tract (Bedford, 1978). When the question is raised as to why a scrotum exists in some animals and not in others (there are approximately 1500 ascrotal species) we still do not have a satisfactory answer. Freeman (1990) has reviewed the question and came to the conclusion that the scrotum evolved to provide a cool environment for sperm storage, and testicular descent evolved because it improves sperm quality so that fewer are needed. He provides tables that show the proportional size of the testes in many species of animals. There are six mammalian orders that have species with internal testes as well as species with external testes. The surface of the testis is invested by the tunica albuginea, a dense, white fibrous capsule. Covering the testis most immediately is the visceral vaginal tunic, a serous membrane continuous with the peritoneum of the spermatic cord and the abdominal cavity. The tunica albuginea joins the centrally located mediastinum testis by means of interlobular connective tissue lamellae (septula testis), which converge centrally. The mediastinum testis is a cord of connective tissue running lengthwise through the middle of the testis. The lobuli testes (wedge-shaped portions of testicular parenchyma) are bounded by the septula. The lobuli contain the convoluted seminiferous tubules (tubuli seminiferi contorti), a large collection of twisted canals. Spermatozoa are formed within the epithelial lining of the tubules, which contains spermatogenic cells and sustentacular (Sertoli) cells. The organization, motility, and structure of sperm cells have been reviewed by André (1982). The longevity of spermatozoa in the reproductive tract of the bitch can be several days (Doak et al., 1967), and at least 6 days as shown by Concannon, Whaley, and Lein (1983). HE Straight seminiferous tubules (tubuli seminiferi recti) are formed by the union of the convoluted seminiferous tubules of a lobule. The mediastinum testis contains a network of confluent spaces and ducts called the rete testis. These connect the straight tubules with the efferent ductules (ductuli efferentes testis). Testicular blood vessels and lymphatics enter and leave through the mediastinum. The lobuli testis also contains interstitial cells (of Leydig) between tubular elements. Johnson et al. (1970) and Setchell (1978) consider the anatomy, physiology, biochemistry, and other parameters of the testis. 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. 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). 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). The deferent duct (ductus deferens) (see Figs. 9-16 to 9-18) is the continuation of the duct of the epididymis. Beginning at the tail of the epididymis, it passes cranially along the dorsomedial border of the testis, continues dorsally in the spermatic cord, and enters the abdominal cavity through the inguinal canal. Running in a fold of peritoneum, the mesoductus deferens, it crosses ventral to the ureter at the lateral ligament of the bladder and penetrates the prostate to open into the pelvic urethra, lateral to the colliculus seminalis. Each spermatic cord (funiculus spermaticus) (see Figs. 9-12 and 9-16) is composed of the ductus deferens and its vessels, and the testicular vessels and nerves, along with their serous membrane coverings, the mesoductus deferens and the mesorchium. These structures pass through the inguinal canal during the descent of the testis. The spermatic cord begins at the vaginal ring, the point at which its component parts converge to leave the abdominal cavity via the inguinal canal. The ductus deferens, which arises from the tail of the epididymis, leaves the vaginal ring, runs caudomedially in the deferential fold of peritoneum, and enters the prostate gland before opening into the prostatic part of the pelvic urethra. The ductus deferens is accompanied by the small artery of the ductus deferens, which arises from the prostatic artery and the vein of the ductus deferens, which drains into the internal iliac vein. The testicular artery originates from the ventral surface of the aorta. The testicular arteries arise cranial to the origin of the caudal mesenteric artery. The testicular artery runs laterally and caudally, crossing the ventral surface of the ureter, at which point it is joined by the testicular vein and nerve. The left testicular vein empties into the left renal vein and the right into the caudal vena cava. The peritoneal fold, the proximal mesorchium, enclosing the testicular vessels is attached to the abdominal wall in a line slightly lateral to the junction of the transversus abdominis and psoas muscles. The plexus of the testicular nerves arises from the area of the sympathetic trunk between the third and the sixth lumbar sympathetic trunk ganglia. The testicular lymph vessels pass to the lumbar lymph nodes. The inguinal canal (see Fig. 9-12) is a fissure through the abdominal muscles that connects the deep and the superficial inguinal ring (Ashdown, 1963; McCarthy, 1976). It is located approximately 1 cm craniomedial to the femoral ring. The femoral ring affords passage for the femoral vessels. The inguinal canal is bounded medially by the rectus abdominis muscle, cranially by the internal oblique muscle, and both laterally and caudally by the aponeurosis of the external abdominal oblique muscle. The superficial ring, located 2 to 4 cm lateral to the linea alba, is merely a slit in the aponeurosis of the external abdominal oblique muscle. It represents where the abdominal wall formed around the gubernaculum in the fetus. The cranial wall of the inguinal canal is made up of the transversus abdominis and internal abdominal oblique muscles, as well as the aponeurosis of the external abdominal oblique muscle. Only the latter forms the caudal wall of the canal. The prostate gland (prostata) (see Figs. 9-7, 9-16, 9-18 and 9-20) completely envelops the proximal portion of the male pelvic urethra at the neck of the bladder. It is the only accessory sex gland present in the male dog. The prostate develops from a series of symmetric buds of the pelvic urethra that appear at approximately the sixth week of gestation (Price, 1963). The size and weight of the prostate varies, depending on the age, breed, and body weight of the dog (Berg, 1958a; O’Shea, 1962). In most dogs, progressive enlargement occurs with age (Schlotthauer & Bollman, 1936). The latter authors found that in all dogs in which there was more than 0.7 g of prostate per kg of body weight, the prostate was abnormal on histologic examination. They suggested 0.7 g prostate per kg of body weight as the upper limit of normality for the dog. O’Shea (1962) divided prostatic growth into three phases: normal growth in the young adult, hyperplasia during the middle of adult life, and senile involution. The prostate is bounded dorsally by the rectum and ventrally by the symphysis pubis and ventral abdominal wall. Its craniocaudal position is age-dependent, as discussed by Gordon (1961). The prostate lies entirely within the abdominal cavity until the urachal remnant breaks down at approximately 2 months of age. From that time until sexual maturity the gland is confined to the pelvic cavity. With sexual maturity it increases in size and extends cranially. By 4 years of age more than half of the gland is abdominal, and by 10 years of age the entire gland is in the abdomen. The degree of bladder distention was not found to alter these relationships to any significant degree. The prostate is androgen-dependent, and castration at any age results in a marked reduction in size (Hansel & McEntee, 1977). 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 Urogenital System
Urinary Organs
Kidneys
Position and Relations
Structure
The Nephron
Vessels and Nerves
Anomalies
Ureters
Urinary Bladder
Fixation
Reproductive Organs
Male Genital Organs
Scrotum
Testes
Anomalies
Descent of the Testes
Epididymis
Structure
Ductus Deferens
Spermatic Cord
Prostate Gland
Vessels and Nerves
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