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310 Ap - Chapter 1 ANATOMY AND PHYSIOLOGY Introduction A...

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Unformatted text preview: Chapter 1 ANATOMY AND PHYSIOLOGY Introduction A management strategy that promotes maximum reproductive efficiency depends on an understanding of the structure and function of the reproductive organs. Often one or more of these organs malfunc- tion in the stallion. To properly evaluate a stallion sus- pected of having reproductive problems, anatomical terms and location, shape and relative size of each organ must be known. Development of a prognosis and an effective treatment or management plan also requires an understanding of major reproductive func- tions. These include: the process of spermatogenesis (formation of spermatozoa; Chapter 2); epididymal Lateral Proximal (toward the point of origin) Distal (away from the point of origin) 1 function and accessory gland secretion, which influ- ence seminal quality; and nature of the ejaculatory process. Presented in Figure 1-1 are some terms commonly used in Veterinary medicine to describe location as it applies to the position of any organ or body part in relation to another. The male reproductive organs consist of two testes, each suspended by a spermatic cord and external cremaster muscle; two epididymides; two deferent ducts each with an ampulla; paired vesicular glands; a prostate gland; paired bulbourethral glands and the Caudal———-> Dorsal Ventral Figure 1~1. Terms used to describe direction or position within an animal. Deferent duct Spine Ampulla Vesicular gland Rectum Prostate gland Genital fold Urinary bladder Testicular Bulbourethral artery and vein gland \ rs'r‘I'i-i’v'vruvrIsEm . I4 , \ , . We ,1 Just» a“: A N as Y t '-E':‘.{‘~’“§"5 t“ a f '” %k&5-$V\ ’/ Floor of pelvis muscle \. a , .\\ Internal inguinal ring ‘ \ l Spermatic cord Al“ ‘_ “ I Penile urethra within inguinal canal Bulbospongiosus muscle Corpus spongiosum Glans penis Corpus cavernosum penis Prepuce Scrotum Testis Epididymis Figure 1-2. Drawing of the reproductive tract of the stallion as seen in a left lateral dissection. penis (Figure 1-2)a. The vesicular glands, prostate gland tubules of the testes; b) altered and acquire fertilizing and bulbourethral glands often are termed the acces- capacity during maturation in the head and body of sory sex glands. The reproductive tract is supported the epididymis; c) maintained in a fertile state in the within the pelvic cavity by the hammock—like genital tail of the epididymis; and d) eiaculated by the coordi- fold and externally by the scrotum and prepuce. nated contractions of the deferent ducts, accessory sex The sequence of events central to normal reproduc- glands and muscles associated with the penis. These tive function in the stallion is illustrated in Figure 1-3. processes depend upon and are coordinated by the Spermatozoa are: a) produced in the seminiferous neuroendocrine system. a The general anatomical description is based on the authors' observations and standard texts (13,28,58,67,70l. References for newer anatomical observations and for specific physiological principles are supplied in the text. Pelvic urethra Retractor penis A STALLION IS SIMPLY FINISHING DELIVERY .. _.- - TESTIS HEAD 8 BODY TAIL OF PENIS OF EPIDIDYMIS EPIDIDYMIS COORDINATION SYSTEM THE NEUROENOOCRINE SYSTEM HYPOTHALAMUS a ANTERIOR PITUITARY Figure 1-3. An overview of the function of the reproductive organs in the stallion. The Scrotum The scrotum is an outpouching of the skin, com- posed of two scrotal sacs, one for each testis, separated by a septum. These sacs lie on either side of the penis (Figure 1-4). If one testis is larger than the other, the scrotum may appear asymmetrical or lop-sided. Corpus cavernosum penis Body of epididymis Deferent duct Central vein Parietal vaginal albuginea tunic . isceral va inal tunic Testlcular V g parenchyma Figure 1-4. Drawing of the penis, scrotum and testes of the stallion as seen in a vertical cross-section. The scrotum consists of four layers. The outermost layer is the skin, which contains an unusually large number of sweat glands. Underlying the skin and asso- ciated connective tissue is thetunica dartos. The tunica dartos is a layer of smooth muscle fibers intermingled with connective tissue, rather than a discrete muscle. This layer forms the outermost component of each scrotal sac and the smooth muscle fibers aid in control of testicular temperature. The third layer is loose connective tissue, or scrotal fascia, which allows the testis great mobility for vertical or horizontal movement within the scrotal sac. Nor- mally, the loose connective tissue prevents 180° rota- tion of the testis within the scrotal sac, although in certain stallions such rotation does occur (73,74). The innermost layer of the scrotum is the parietal vaginal tunic (some consider this covering as part of the testis). The parietal vaginal tunic (also called the common vaginal tunic) is a membranous sac that extends from the abdominal cavity through the ingui- nal canal (an opening in the abdominal wall through which the spermatic cord passes) to the bottom of the scrotum. Part of this membrane covers the testis and epididymis and also contributes to the spermatic cord. The space within the vaginal cavity, between the parie- tal vaginal tunic and visceral vaginal tunic, which is the outermost covering around the testis and epididymis (Figure 1-4), contains a watery, serous fluid, which serves as a lubricant and facilitates movement of the testis within the sac formed by the parietal vaginal tunic. In some older stallions, adhesions may develop between the parietal vaginal tunic and the visceral vaginal tunic, which covers the testis. Such adhesions impede mobility of the testis and may reduce effec- tiveness of temperature control mechanisms. Descent of the Testes A description of the embryology of the equine reproductive system is beyond the scope of this bul- letin. However, understanding the process by which the testes normally descend from the abdominal cavity into the scrotum is essential. In the normal colt, both testes should descend into the scrotum between 30 days before and 10 days after birth (17,35). Failure of normal testicular descent is common in horses and is termed cryptorchidism. By day 40 of gestation, the testis is suspended from the abdominal wall (Figure 1-5) and the mesonephric duct, which later gives rise to the epididymis and defe- rent duct, leads into the pelvic area (17). A narrow evagination, termed the vaginal process, starts to form about day 43 of gestation and progressively develops to form the inguinal canal. Around day 150, the devel- oping tail of the epididymis is drawn to or just within the internal inguinal ring (Figure 1-6), but the testis is very large and cannot enter the inguinal canal (17). Entrance of the testis into the inguinal canal typically begins between 270 and 300 days of gestation (17). This occurs only after the vaginal process and internal inguinal ring have been stretched sufficiently by the enlarging tail of the epididymis (Figure 1-7) to allow entrance of the testis that has diminished in size from 50 to 30 grams. Pressure from fluid in the abdominal cavity, and possibly from the intestines, forces the testis down through the inguinal canal. This places the vis— ceral vaginal tunic, the outer covering of the testis, in apposition with the parietal vaginal tunic (which is the former vaginal process), separated by the vaginal cavity (Figure 1—7A). Cephalic ligament Mesonephric of testis duct Dorsal body wall Mesorchium Mesonephric duct Peritoneal covering over testis Tunica albuginea ‘of testis ' ‘ Caudal gonadal ligament lnvaginated peritoneal lining Gubernacular Vaginal processes r M cord l ’ it“ Vaginal ' ‘ ‘ processes Gubernacular bulb it i Figure 1-5. Drawing of a horse fetus at 75 days of gestation. epididymis and deferent duct), is continuous with the 11 The testis is suspended within the abdominal cavity by a thin gubernacular cord that extends to the gubernacular bulb. ‘l ‘ double-layered band of fused tissue termed the mesor- The vaginal process (future parietal vaginal tunic) encloses l l! chium. The caudal gonadal ligament, extending from the about one-third of the gubernacular bulb. Modified from 1 testis to its point of fusion with the mesonephric duct (future (17). Epididymal Mesorchium . . t 1‘: duct Cephalic Irgamen of testis Deferent duct Dorsal body wall Mesorchium Epididymal duct Peritoneal covering of testis (future visceral layer of vaginal tunic) Tunica albuginea testis of testis lnvaginated peritoneal lining (inner= future visceral layer of vaginal tunic outer = future parietal layer of vaginal tunic) ‘Jvr‘f'ri‘v‘jir. , .lmr...-.-..y-,§«: ‘1 a _u Gubernacular cord Mesorchium Gubernacular bulb Vaginal processes Figure 1-6. Drawing of a horse fetus at 175 days of gestation, to the testis. The epididymis and deferent duct have formed The vaginal process extends almost to the scrotum and has from the mesonephric ductand the future tail ofthe epididy— virtually enclosed the gubernacular bulb. The mesorchium is mis will form where the duct is sharply reflected. Modified continuous from the gubernaculum and the vaginal process from (17). duct Mesorchium epididymidis ‘ Gubernacular i Figure 1-7. Drawing of a horse fetus near term. The testis has l passed through the inguinal canal, but is not fully within the scrotum. The gubernacular bulb, gubernacular cord, tail of the epididymis and caudal gonadal ligament all have regressed. The testis now is connected by the mesorchium to Bergin et al. (17) reported that the earliest complete descent of both testes was at 315 days of gestation, about 25 days before parturition. In 32 fetuses between 9 mo of gestation and birth, descent of the right testis was further advanced than the left in 78% of the fetuses, while the left testis was more advanced in only 3%. Of 12 fetuses icollected at term, 42% had com- pletely descended testes; 25% had both testes within the inguinal canals and equally distant betv'Veen the internal and external inguinal rings; 17% had one testis in the scrotum and one in the inguinal canal; and 17% had both testes within the abdominal cavity. Five of nine colts less than 1 wk old had complete bilateral descent of the testes into the scrotum. As reviewed by Bergin et al. (17), failure of the testes to descend has been attributed to abnormalities of the testis, devel0pment of adhesions between the testis and adjacent structures or an abnormal out-pouching of the vaginal process. Bergin et al. (17) discounted these factors as causes of cryptorchidism and sug- gested the most obvious reasons for the testis to remain in the abdominal cavity included: a) insufficient abdominal pressure to properly expand the vaginal process; b) stretching of the gubernacular cord; C) insufficient growth of the gubernaculum and tail of the epididymisso that they are unableto expandthe inguinal ring sufficiently to allow entrance of the testis; Deferent Body of epididymidis Tail of Testis Caudal gonadal bulb ligament Body of epididymidis Visceral layer of vaginal tunic Vaginal cavity Parietal layer of vaginal tunic Mesorchium Deferent duct SurrOunded by mesoductus deferens Vaginal cavity Blood vessels + B nerve surrounded by visceral layer of vaginal tunic Parietal layer of vaginal tunic the dorsal wall of the parietal vaginal tunic (formally the vaginal process). The deferent duct, blood vessels and lym- phatics are connected to the parietal vaginal tunic by folds of the mesorchium. Modified from (17). d) displacement of the testis to a position where intes- tinal pressure prevents gubernacular tension from pulling the testis into the vaginal process. The truly cryptorchid testis, one retained within the abdominal cavity, apparently occurs when the testis fails to enter the inguinal canal before closure of the internal inguinal ring during the first 2 wk after birth. The high incidence of failure of the left testis to des- cend (83) might result from the relatively slow rate of descent "of the left epididymis and testis. Obviously, this could result in a higher incidence of retention of the left testis compared to the right. Diagnosis of cryptorchidism should include careful external palpation of the scrotum and external ingui- nal ring, as well as rectal palpation of the internal inguinal ring and pelvic area. During the first several weeks after birth, the gubernaculum may be quite large; and should not be confused with a testis. At birth, the weight of each testis is 5 to 10 g, and testicular size increases very little through 10 mo of age. More rapid development of the testes generally starts around 12 to 18 mo of age, but age at which rapid growth of the testes commences varies considerably. Somatic Cells The testis (plural = testes) is the male gonad and is the site of production for both spermatozoa and the pre- dominant male-sex hormone, testosterone. The testes \i‘ ‘ are ovoid or walnut-shaped structures, slightly compressed from side to side with their long axis almost horizontal (Figure 1—2). When the testis is retracted, the long axis becomes almost vertical. Testes of post-pubertal stallions range considera- bly in size (Chapters 4 and 5). An average testis might measure 80 to 140 mm in length by 50 to 80 mm in width and weigh about 225 grams. How- ever, age and season greatly affect testis weight (Figure 2-10). If the testis is exposed, as during open castra- tion, a thick covering of connective tissue, termed the‘tunica albuginea, is seen (Figure 1-4). Fused to. the outer surface of this capsule is the thin visceral vaginal tunic. Supporting strands of con- nective tissue extend from the tunica albuginea to divide the testis into lobules. The parenchyma, or non-capsular part of the testis, is lightly pigmented in a young, post- pubertal stallion; moderately pigmented in a4-to 5-yr-old stallion; and darkly pigmented in an aged stallion (52). The testicular parenchyma consists of seminiferous tubules and interstitial tissue (Figure 1-8). The seminiferous tubules are lined by a seminif- erous epithelium that consists of different types of germ cells (see Chapter 2) and the Sertoli cells. Leydig cells are the major component of the interstitial tissue in adults (52). These cells are responsible for produc— tion of the male-sex hormone, testosterone. Table 1—1. Age-Related Differences in Testicular Compositiona Age (yr) 2 to 3 4 to 5 13 to 20 Testicular weight (g) Parenchyma 1osb was 164* Tunica albuginea 12b 15c 29d Parenchymal composition (96) Leydig cells 6b 12c 18d Other interstitial tissue 22b 16c 10d Seminiferous tubules 72 72 72 Seminiferous tubule Diameter (pm) 212b 230C 242c Length/testis (m) 2040b 2390136 met Daily sperm production (TIP/testis) 13') 27c 32c a Modified from (52). b,c,d Means in the same row with different superscripts differ (P < 0.05). As a stallion grows older, post-pubertal Leydig cells are gradually replaced by adult Leydig cells, which contain abundant pigmented lipids and are intercon- nected by numerous interdigitions (1), and they may produce more testosterone per cell than Leydig cells of younger stallions. The ratio of Leydig cells to semi- niferous tubules increases from 1:12 to 1:4 as a horse grows older (Table 1-1). Although certain of the changes presented in Table 1-1 are a consequence of testicular growth, composition of the testis on a per gram basis also changes with age. This is especially evident for the number of Sertoli cells per gram of testicular parenchyma (55), and the decline with Lymphatic vessel ' Fibroblast .q/l 7 l6 © Bl d 00 {mi vessel ”I. l l o \l 4 fl" .- 417 «go Leydig cells Seminiferous tubules Figure 1-8. A scanning electron micrograph (upper; X200) and drawing of a stallion testis (lower) showing the relation- ship among the blood vessels (V), lymphatic vessels and Leydig cells (L) of the interstitial tissue and seminiferous tubules (ST). Upper figure from (49). advancing age (Figure 1-9) may be partially responsible for the decline in daily sperm production (Figure 2-10), which often occurs in older stallions. Although per- centage of the testis occupied by seminiferous tubules does not change with age (Table 1-1), total volume of Leydig cells per gram oftesticular parenchyma increases more than three-fold with age (Figure 1-9). The total length of seminiferous tubules in the testis also increases by about one-third (52). .._._....._.._... .........__._ .2.» o 40 2° :9 g a) 5 E LrJ :50 30 or '— \ U) U) \ j 2 20 '00 Lu .1 O 5 <2 3 IO 50 E 0 Lu 10—: _i % o . ~ ll. 3 2-3 4-5 13-20 2-3 45 13-20 AGE (yr) Figure 1-9. Effect of age on number of Sertoli cells and volume of Leydig cells per gram of testicular parenchyma. Data for Sertoli cells are for testes collected in June and July and data for Leydig cells are for testes obtained between February and May. Based on data from (52,55). A seminiferous tubule is arch-shaped and consists of three zones. The major portion of a seminiferous tubule, the convoluted seminiferous tubule, is highly coiled and is the site of sperm production. A convo- luted seminiferous tubule is lined by a seminiferous epithelium, which contains Sertoli cells and several types of germ cells (13,19,21,27,29,48,49,52,55,84). Ser- toli cells and spermatogonia line the basement mem- brane of these tubules, and Sertoli cells extend toward the lumen in a radial pattern (Figure 2-2) to occupy 15 to 25% of the seminiferous epithelium in an adult stal- lion. Although it is conventionally accepted that Sertoli cells do not divide after puberty (27,29,78,79), from results of recent‘ research (48,53-55), it appears that Sertoli cells in the stallion proliferate as the breeding season approaches. The number of Sertoli cells in a testis is an important determinator of the number of spermatozoa that a testis can produce (20). Both ends of a convoluted seminiferous tubule con— tinue as tapered transitional zones leading to the straight portions of the seminiferous tubule. Straight tubules are the first component of the passageway through which spermatozoa pass from the seminifer- ous epithelium to the epididymis. Straight tubules converge in the cranial two-thirds of the testis (9) in an area termed the rete testis (Figures 1-10 and 1-11). Tubules of the intratesticular rete testis penetrate the tunic albuginea and continue as an extratesticular rete (9). Eventually, each rete tubule fuses with one of the 13 to 15 efferent ducts that lead to the epididymal duct (43). Horses are seasonal breeders and characteristics of the testes (Table 1-2) change markedly throughout the Year. However, unlike some seasonal breeders, stal- lions continue to produce spermatozoa throughout the year. Maximum testicular development and func- tion occurs in May, June and July (21—24, 54; Figures 2-10 and 2—11, and Chapter 3). From September through February, the testes are regressed; probably minimal size and function occur in November to January. At that time of year, the testes of typical stallions are 25% lighter, contain about 35% fewer Leydig cells and con- sequently less smooth endoplasmic reticulum (SER; the part of the Leydig cell involved in production of testosterone), contain 31% fewer Sertoli cells, and pro- duce 40 to 50% fewer spermatozoa (Table 1-2). Con- centrations in blood of hormones involved in c0ntrol of reproductive function also tend to be lower in the nonbreeding season (Chapter 3). Table 1-2. Effect of Season on Characteristics of Testes of Stallions 4 to 20 Yr Olda Breeding Nonbreeding Differenceb Parenchymal weight paired testes (g) 154 116 -25% . Leydig cells (105g testis) 22 20 ~ 9% (109/2 testes) 7.26 4.70 ' 65% Leydig cell SER (ml/2 testes) 34.0 22.2 35% Testosterone (pg/g testis) 700 580 47% (mg/2 testes) 224 154 -31% Sertoli cells . (106/g testis) 24.2 23.6 - 2%. (109/2 testes) 3.69 2.54 —31% Elongated spermatids per Sertoli Cell 9.36 7.54 49% Daily sperm production (106/g testis) 19.1 14.6 -23% (109/2 testes) 5.96 3.53 -41% S...
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