Chapter 3: The Male Reproductive System

The male reproductive system:Figure 3-1
Embryonic origin:
Testes: primary sex cords of the genital ridge
Male duct system: Wolffian ducts
Major functions od male reproductive organs: Table 3-1.

3-1 Testes


The testes are the primary organs of reproduction in males,just as ovaries are primary organs of reproduction in females.Testes are considered primary because they produce male gametes (spermatozoa) and male sex hormones (androgens).Testes differ from ovaries in that al potential gametes are not present at birth.Germ cells,located in the seminiferous tubules,undergo continual cell divisions,forming new spermatozoa throughout the normal reproductive life of the male.

Testes also differ from ovaries in that they do not remain in the body cavity.They decend from their site of origin,near the kidneys,down through the inguinal into the scrotum.Descent of the testes occurs because of an apparent shortening of the gubernaculum, a ligament extending from the inguinal region and attaching to the tail the epididymis. This apparent shortening occurs because the gubernaculum does not grow as rapidly as the body wall.The testes are drawn closer to the inguinal cannals and intra-abdominal pressure adis passage of the testes through the inguinal cannals into the scrotum.Both gonadotropic of hormones and androgens regulate descent of the testes.This descent is completed in the fetus by midpregnancy in cattle and just before birth in horses.In some cases one or both testes fail to descend due to a defect in development.If neither descends,the animal is termed a biaterral crytorchid.Bilateral cryptochids are sterile (section 3-2).if only one testis descends he is a unilateral cryptochid. The unilateral cyptochid is usually fertile due to the descended testis.The cyptochid condition can be corrected by surgery,but this is not recommended for farm animals (Chapter 23).The condition can be inherited;therefore,surgical correction would result in the perpetuation of an undesirable trait.



Figure 3-1 Diagram of the reproductive system of the (a) bull; (b) ram; (c) boar; and (d) stallion.(Redrawn from Sorenson. 1979. Animal Reproduction: Principles and Practices. McGraw-Hill.) 

3-1.1 Functional Morphology


The testis of the bull is 10 to 13 cm long,5 to 6.5 cm wide and weighs 300 to 400 gm.THe testis is of similar size in boars,but is smaller in rams, bucks (goats),and stallions.

In all species testes are covered with the tunica vaginalis,a serous tissue,which is an extension of the peritoneum.This serous coat is obtained as the testes descend into the scrotum and is attached along the line of the epididymis.The outer layer of the testes,the tunica albuginea testis, is a thin white memberane of elastic connective tissue.Numberous blood vesselsare visible just nuder its surface.Beneath the tunica albuginea testis is the parenchyma,the functional layer of the testes.The parenchyma has a yellowish color and is divided into segments by incomplete septa of connective tissue (Figure 3-2).Located within these segments of parenchyma tissue are the seminiferous tubules.Seminiferous tubules are formed from primary sex cords.They contains germcells(spermatogonia) and nursa cells (Sertolicells).Sertoli cells are larger and less numberous thanspermatogonia.With stimulation by FSH,Sertoli cells produce both androgen binding protein and inhibin (Chapter 4).Seminiferous tubules are the site of spermatozoa production.They are small,convoluted tubules approxilately 200¥ìin diameter.It has been estimated that the seminiferous tubules from a pair of bull testes,streched out and laid end,approach 5 km in length.They make up 80£¥ of the weight of the testes.seminiferous tubules join a network of tubules,the rete testis, which connects to 12 to 15 small ducts,the vasa efferential.which converge into the head of the epididymis.Prodution of spermatozoa will be discussed in Chapter6.


Figure 3-2 Sagittal section of testis illustratingsegments of parenchymal tissue which contain the seminiferous tubules, rete testis, vasa efferentis, epididymis, and scrotal portion of the vas deferen. 

Leydig (interstitial) cells are found in the parenchyma of the testes between the seminiferous tubules (Figure 3-3).LH timulates Leydig cells to produce testosterone and small quantities of dther androgens.


Figure 3-3 Cross section of parenchymal tissue showing relationship between the seminiferous tubules and interstitial tissue containing Leydig cells. 

Testosterone is needed for development of secondary sex characteristics and for normal mating behavior.In addition,it is necessary for the function of the accessary glands,production of spermatozoa and maintenance of the male duct system.Through its effects on the male,testosterone aids in maintenance of optimumcondtions for spermatogenesis,transprot of spermatozoa and deposition of spermatozoa into the female tract.Normal body temperature will not affect the function of the Leydig cells.For example,bilaterial crytochids develop secondary sex characteristics,have normal sexual vigor,and can do all things associated with reproduction exceptrpoduction of spromatozoa.


3-2 scrotum and Spermatic Cord


The scrotum is two-lobed sac which enclosed the testes.It is located in the inguinal region between the rear legs of most species.The scrotum has the same embryonic origin as the labia majora in the female.It is composed of an outer layer of think skin with numerous large sweat and sebaceous glands.This outer is lined is with a layer of smooth muscle fibers,the tunica dartos,which is interspersed with connective tissue.The tunica dartos divides the scrotum into two pouches,and is attached to the tunica vagimals at the bottom of each pouch.

The spermatic cordconeects the testis to its life support mechanisms,the convoluted testicular arteries and surrounding venus plexus,and nerve trunks.In addition,the spermatic cord is composed of smooth muscle fibers,connective tissue,and a portion of the vas deferens.Both the spermatic cords and scrotum contribute to the support of the testes.Also,they have a joint function in regulating the temperature of the testes.


3-2.1 Temperature Control


Serveral examples can be given to illustrate the inportance of temperature control of the testes.If a ram^,s scrotum is insulated,or the testes are tied against the abdomen,sterility results.The higher temperature causes degeneration of the cells lining the wall of the seminiferous tubules.Fetility will be restored if the testes and scrotum are returned to their natural state before total degeneration occurs.However,a few weeks will be required before fertile semen is again produced.(Sometimes men with high fevers are sterile for short period after recovery.) The bilateral crytorchid is sterile,again illustrating that production of spermatozoa stops when the temperature inside the testes is as high normal body temperature (Section 23-5).

Low fertility semen produced by several species of farm animals during the summer has been attributes to the inability of the body^,s cooing mechanisms to keep the testes cool enough.In cattle,when ambient temperaturesrange from 5¡Éto 21¡É,the temperature inside the testes will be about 4¡É below body temperature(38.6¡É).As the ambient temperature increasea to approximately 38¡É,the temperature of both body and the testes will be elevated,and the difence between the two will be reproducdd by about onehalf(2¡É).The elevation in temperature inside the teste will be sufficient to stop spermatogenesis.Ther is no evidence that low ambient temperature will lower fertility.

The role of the scrotum and spermatic cord in temperature control of the testes involved drawing the testes closer swing further away from the body as amsient temperature rises.Two smooth muscles are involved.The tunica darttos,the smooth the spermatic cord,are sensitive to temperature.During cold weather,contraction of these muscles causes the scrotum to pucker and the spermatic cords to shorten,drawing the testes closer to the body.During hot weather,these muscles relax,peritting the scrotum to stetch and the spermatic cord to lengthen.Thus,the testes swing down away from the body These muscles do not rsepond to changes in temperature until near the age of puberty.They must be sensitized by testosterone to respond to changing ambient temperature.

Actual cooling of testes occurs by two mechanisms.The skin of the scrotum has both sweat abd sebaceous glands which are more active during hot weather.Evaporation of the secretions of these glands cools the scrotum and thus the testes.The external scrotum has been obsered to be 2¡É to 5¡É cooler than the temperature inside the testes.As the scrotum stretches during hot weather,more surface area is provided for cooling by evaporation.In addition to cooling occurs through heat exchange in the circulatory system(Figure 3-4).As arteries transproting blood at internal body temperature transcend the spermatic cord,their convoluted folds pass through a network of veins,the pampiniform venous plexus,transporting cooler blood back towards the heart.some cooling of arterial blood then occurs before it reaches the testes.The lengthening of the cord during hot weather provides more surface area for this heat exchange.


3-3 Epididymis


The Epididymis,the first external duct leading from the testis,is fused longitudinally to the surface of the testis and is encased in the tunica vaginalis with the testis and is encased in the tunica vaginalis with the testis.The single convoluted duct is coverd with an extensionof the tunica albuginea testis (Figure 3-5).The captut (head)of the epididymis is a flattened area at the apex of the testis,where 12 to 15 small ducts,the vasa efferentia,merge into a single duct.The corpus(body) extenging along the longitudinal axis of the testis is a single duct which becomes continuous with the cauda(tail).The total length of this convoluted duct is about 34 meters in the bull and longer in the ram,boar,and stallion.The lumen of the cauda is wider than the lumen of the corpus.The structure of the epididymis and other external ducts (vas deferens and urethra is similar to that of the tubular portion of the female tract.The tunica serosa (outer layer)is followed by a smooth muscle layer (middle)and an epithelial layer(innermost).



Figure 3-4 Cooling of the testis by heat exchange through the circulatory system.(Setchell. 1977. Reproduction in Domestic Animals.(3rd ed) ed. cole and Cupps. Academic Press 

3-3.1 Transport


As a duct leading from the testes,the epididymis serves to transport spermatozoa.In sexually active males the time involved in transprot is 9 to 1 days in boars,13 to 15 days in rams,and 9 to 11 days inbulls.Frequent ejaculation has been reported to speed transport by 10 to 20£¥.

Several factor contribute to movement of spermatozoa thorugh the epididymis.One factor is pressure from the production of new spermatozoa.As spermatozoa are produced in the seminiferous tubules,they are forced out through the rete testis and vasa efferentia into the epididymis.In a sexually inactive male,they are eventually forced through the epididymis.such movement of spermatozoa is aided by external perssure created by the massaging effect on the testes and epididymis thet occurs during normal exercise.The lining of the epididymis contains some ciliated epithelial cells,but the role of these cilia in facilitating movement of spermatozoa is aided by ejaculation.During ejaculation,peristaltic contractions involving the smooth muscle layer of the epididymis and a slight negative pressure (sucking action) created by peristaltic contractions of the vas deferens and urethra actively move spermatozoa from the epididymis into the vas deferens and urethra.


3-3.2 Concentration


A second function of the epididumis is concentration of spermatozoa.spermatozoa entering the epidiaymis from the testis of the bull,ram,and the epididymis,they concentrate to about 4x10⁹(4 billion)spermatozoa per ml.Concentration occurs as the fluids,which suspend spermatozoa in the testes,are absorbed by the epithelial cells of the epididymis.Absorption of these fluids principally in the caput and proximal end of the corpus.


3-3.3 Storage


A third functin of the epididymis is storage of spermatozoa.Most are stored in the cauda of the epididymis where concentrated spermatozoa are packed into the wide lumen.The epididymis of a mature bull may contain 50 to 74 billion sperm.Capacities of other species are not reported.conditions are optimum in the cauda for preserving the viability of spermatozoa for an extended period.The low pH,high viscosity,high carbon dioside concentration,high potassium-to-sodium ratio,the influence of testosterone,and probably other factors combine to conteibute to a lower metabolic rate and extended life.These conditions have not been duplicated outside the epididymis.If the epididymis is ligated to prevent entry of new spermatozoa and removal of old,spermatozoa have remained have alive and fertile for about 60 days.On the other hand,after a long period of sexual rest,the first fewejaculates may contain a high percentage of nonfertile spermatozoa.


3-3.4 Maturation


A fourth function of the epididymis is that of maturation of spermatozoa.When recently formed spermatozoa enter the caput from the vasa efferentia they the ability for neither motility nor fertility.As they passthrough the epididymis they sain the ability to be both motile and fertile.If the cauda is ligated at each end,those spermatozoa closet to the corpusincreased in fertility for up to 25 days.During the same period,those closest to the vasa deferens exhibited reduced fetilizing ability.Therefore,it appears that spermatozoa gain ability to be fertile in the cauda and then start to age and deteriorate if they are not removed.

While in the epididymis,spermatozoa lose the cytoplasmic droplet which forms on the neck of each spermatozoa during spermatogenesis (Chapter 6).The physiological significance of the cytoplasmic droplet is not known,but it has been used as an indicator of sufficient maturation of spermatozoa in the epididymis.If a high percentage of spermatozoa in fershly ejaculated semen has cytoplasmic droplets,they are considered immature and have low fertilizing capacity.



3-4 Vas Deferens and Urethra


The vas deferens are a pair of ducts with one leading from the distal end of the cauda of each epidymis.Initially supported by folds of the peritoneum,it passes along the spermatic cord,through the inguinal cannal to the pelvic region,where it merges with the urethra at its origin near the opening to the bladder.The enlarged end of the vas deferens near the urethra is the ampulla.The vas deferens has a thick layer of smooth muscles in its walls and appears to have single function of traspor of spermatozoa.Some have suggested that ampulllae serve as a short0term storage depot for semen.However,spermatozoa age quickly in the ampullae.It seems more likely that spermatozoa may pool in the ampullae during ejacualtion before being expelled into the urethra.

The urethra is a single duct which extends from the junction of the ampullae tothe end of the penis.It serves as an excretory duct for both urine and semen.During ejacualtion in bull and ram there is a complete mixing of spermatozoa concentrate from the vas deferens and epididymis with fluids from the accessory and boars,mixing is not as complete,with the ejaculate containing sperm-free and sperm-rich segments(Chapter 11).

3-5 Accessory Glands


The accessory glands(Figure 3-6)are located along the pelvic portion of the urethra,with ducts which empty their secretions into the urethra.They include the vesicular glands,the prostate gland and the bulbourethral glands.They contribute greatly to the fluid volume of semen.In addition,their secretions are solution of bufers,nutrients,and other substances needed to assure optium motility and fertility of semen.



Figure 3-6 Accessory glands of the bull, boar, ram, and stallion showing their relationship to the ampulla and urethra. (Redrawn from Ashdown and Hancock. 1974. Reproduction in Farm Animals.(3rd ed). ed. Hafez. Lea and Febiger.) 

3-5.1 Vesicular Glands

The vesicular glands (sometimes called seminal vasicles) are a pair of lobular glands that are easily that identiied because of their knobby appearance.They have been described as having the appearance of a "cluster of grapes." They are of similar length in the bull,boar,and stallion (35 to 15 cm),but the width and thickness of the vesicular glands of the bull is approxomately half that of the boar and stallion.They vesicular glands of the ram and buck are much smaller,being about 4 cm in length.The excretory ducts of the vesicular gkands open near the bifurcation where the ampullae merge with the urethra.In bulls,they contribute well over half of the total fluid volume of semen,and appear to make a substantial contribution in other species.Several organic compounds found in secretion of the vesicular glands are unique in that they are not found in substantial quantities elsewhere in the body.Two of these compounds,fruetose and sorbitol,are major sources of energy for bull and ram spermatozoa but are found in lower concentration in boar and stallion.Both phosphate and carbonate buffers are found in these secretions and are important in that they protect against shifts in the pH of semen.Such shifts in pH would be detrimental to spermatozoa.


3-5.2 Prostate Gland


The prostate is a single gland located around and along the urethra just posterior to the excretory ducts of the vesicular glands.A prostate body is visible in excised tracts and can be palpated in bulls and stallions.In rams,all of the prostate is embedded in urethra muscles as is part of this glandular tissue in bulls and boars.It makes a small contribution to the fluid volume of semen in mosr species stuided.however,some repory that the contribution of the prostate gland is at least as substatial as thet of the vesicular glands in boars.The prostate of the boar is larger than that of the bull.The secretions of the prostate are high in inorganic ions with sodium,chlorine,calcium,and magnesium all in solution.


3-5.3 Bulbourethral Glands

The bulbourethral (Cowpers)glands are a pair of glands located along the urethra near the point where it exits from the pelvis.They are about the size and shape of wanuts in bulls,but are much larger in boars.In bulls,they are embeded in the bulbospongiosum muscle.They contribute very little to the fluid volume of semen.In bulls,their secretions flush urineresidue from the urethra before ejaculation.These secretions are seen as dribblings from the prepuce just before copulation.In boars,their secretions account for that portion of boar semesn which coagulates.This strained from boar semen before it is used for artificial insemination.During natural service,the white lumps formed by coagulation may prevent semen from flowing bak through the cervix into the vagina of sows.


3-6 penis

The penixl is the organ of copulation in males(Figure 3-1).It forms dorsally around the urethra from the point where the urethra leaves the pelvis,with the external urethral orifice at the free end of the penis.Bulls,boars,and rams have a sigmoid flexure,an S-shaped bend in the penis which permits it to be retracted completely into the body.These three species and the stallion have retractor penis muscles,a pair of smooth muscles which will relax to permit extension of the penis and contract to draw the penis back into the body.These retractor penis muscles aries from the vertebrae in the coccygeal region and are fused to the ventral penis just anterior to the sigmoid flexure.The glans penis(Figure 3-7),which is the free end of the penis,is wwell supplied with sensory nerves and is homologous to the clitorisin the female(Chapter 2) In most species the penis is fibroelastic, containing small amounts of erectile tissue.The penis of stallions contains more erectile tissue is found in bulls,boars,bucks,rams.


Figure 3-7 Comparative diagram showing the shape of the glans penis of the bull, boar, ram. and stallion. Note the twisted groove containing the external urethral orifice in the bull, the urethral proxess (filiform appendage)extending beyond the glands penis ing the ram, the corkscrew spiral in the boar, and the flattened glans penis in the stallion with the small urethral process extending beyond.(Redrawn from Ashdown and Hancock. 1974. Reproduction in Farm Animals.(3rd ed.). ed. Hafez.Lea and Febiger.)

Figure 3-8 Cross section of penis showing corpus cavernosum penis and corpus spongiosum penis.(Redrawn from Sprensen. 1979. Animal Reproduction: Principles and Practices. McGraw-Hill) 

Erectile tissue is cavernous (spongy)tissue located in two regions of the penis(figure 3-8).The corpus spongiosum penis is the cavernous tissue around the urethra.It enlarges into the penile bulb,which is cavered with bulbospongiosum muscle at the base of the penis.The corpus cavernosumpenis is a larger area of cavernous rods from the ischiocavernosus muscole ,eventually fusing to from one cavernous area as it proceeds toward excitement,causing extension of the penis (erecrion)and facilitating the final ejection of semen during ejaculation (Chaper 11).Both the bulbospongiosum muscle and ischiocavernous muscle are striated,skeletal muscles,rather than the smooth muscle associated with most of the male and female tracts.


3-7 Prepuce


The prepuce (sheath)is an invagination of skin which completely enclosed the free end of the penis.It has the same embryonic origin as the labia minora in the female.It can be divided into a prepenile portion,which is the outer fold,and the penile portion,or inner folds.The orifice of the prepuce is surrounded by long and tough preputial hairs.