The Male ReproductiveOrgans

The male reproductive system consists of primary sex organ testis (testes in plural), excurrent tract, accessory sex glands, and ancillary organs. The excurrent tract begins from the rete testis, followed by efferent ducts (vasa efferentia), epididy­mis, vas deferens, and urethra.

Fig. 19.1 Reproductivetract of bull

Fig. 19.2 Reproductive tract of a male dog

19.1.1 Testes

Testes are the paired structure situated outside the abdomen (in most species) in a purse-like structure called the scrotum made of skin and fascia. The spermatic cord attached with the superior pole of the testes helps to suspend the testis within the scrotal sac, and the distal end of the testes is attached with the scrotum by a scrotal ligament. The right and left testicles are separated by a muscular septum formed by dartos muscles. In some animals like mice, bat, the testes lie inside the body during their non-breeding season. But, it comes outside when breeding approaches. The testes are situated in rats, rabbits, and camels’ perineal regions. Testes remain in the sub-anal region in the feline species like cat, tiger, lion, etc. The intra-abdominal testes are seen in monotremes, armadillos, sloths, elephants, rhinoceros, and birds. There is species variation in the morphological features of testes (Table 19.1). The testicular size varies throughout the year in seasonal breeders, like stallion, ram, and camel.

The testes are composed of testicular capsule, mediasti­num, and parenchyma.

19.1.1.1 TesticularCapsule

Four layers encapsulate the testes. The innermost serous covering of the testes is called the vaginal process or tunica vaginalis. It is a double layer peritoneal process that envelopes the whole testes except the region where the epi­didymis and spermatic cord are attached. The outer and inner layers of tunica vaginalis are called parietal and visceral vaginal tunics, respectively (Fig. 19.3). The watery fluid

Fig. 19.3 Structure of testis. [The schematic diagram showing the longitudinal cross section of the testis along with the excurrent tract in bull]

Table 19.1 Morphological features of the testes in different domestic animals

between these two layers acts as a lubricant and allows free movement of the testicles within the scrotal sac, and prevents the testes from injury. The visceral vaginal tunics is attached with a fibrous white capsule of dense collagenous connective tissue, called tunica albuginea. This layer acts as an insulator and helps maintain testes’ internal temperature.

19.1.1.2 Mediastinum Testes

It is a connective tissue sheet that runs through the centre of the testes from top to bottom (Fig. 19.3). The testicular mediastinum is divided into lobules by the trabeculae of the tunica albuginea. The lobules are filled with composed of seminiferous tubules. The mediastinum testes support the rete testis and allow the blood vessels and lymphatics.

19.1.1.3 Testicular Parenchyma

The testicular parenchyma comprises seminiferous tubules lined by different generations of maturing germ cells (spermatogonia, spermatocytes, spermatids spermatozoa) and Sertoli cells.

19.1.1.3.1 SeminiferousTubules

These tubules are highly convoluted structures that converge at the apex of each testicular lobule (Fig. 19.3). In bull, each seminiferous tubule is 80 cm in length and about 110-250 μm in diameter. The seminiferous tubule cross section reveals three layers: the outer capsule, basement membrane, and testicular cells. The testicular cells comprise the germinal epithelium, Sertoli cells, and Leydig cells. The germinal epithelium and Sertoli cells are the lining cells of seminifer­ous tubules, whereas the Leydig cells are present in the interstitium outside the seminiferous tubule.

The Outer Capsule The basal lamina is composed of fibroblasts and peritubular myoid (PTM) cells. The PTM cells are spindle-shaped smooth muscle cells surrounding the seminiferous tubules and separating them from interstitial space. The PTM cells give structural support to the seminif­erous tubules as it contains contractile proteins like actin, myosin, desmin, vimentin, and alpha-actinin.

The Basement Membrane Seminiferous tubule basement membrane (STBM) is composed of fibronectin, proteoglycans, and collagens (type I and IV) secreted from PTM cells. It gives structural support to the germinal epithe­lium and Sertoli cells.

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The left testis is generally larger in most mammals and birds, except in sharks, where the right one is larger. Intra-abdominal testes are found in birds, elephants, rhinoceros, sloths, whales, dolphins, and others in monotremes mammals. Testes remain in the sub-anal region in feline species, viz. cat, lion, and tiger. Testes

appear in the perianal region in camel, rat, and rabbit. The total length of the convoluted seminiferous tubules in mice is about 2 m, in domestic cat 50 m, crab-eating fox 80 m, fowl (Nigerian indigenous chicken) 600 m, human 300-1000 m, goat 3.7 km, and bull 5.2 km.

19.1.1.4 Testicular Cells

The lining cells of the seminiferous tubules are of two types: the stratified germinal epithelium and Sertoli cells (named after Enrico Sertoli, Italian Physiologist who discovered the cells) (Fig. 19.4). The Leydig cells (named after German zoologist and anatomist Franz Leydig) are present at the interstitial space (between the tubules) and play a pivotal role in controlling the activities of seminiferous tubules. In crossbred bulls, the population of Sertoli cells are less due to small-sized seminiferous tubules, which affects their fertility.

19.1.1.4.1 Germinal Epithelium

The seminiferous epithelium is capable of producing spermatozoa hence called the germinal epithelium.

19.1.1.4.2 SertoliCells

These cells are triangular or oval with a prominent nucleolus, fine chromatin, elongated mitochondria, small fibrils, and lipid droplet with glycogen (Fig. 19.4). The number of Sertoli cells varies with species. The number of Sertoli cells per gram of testes is about 40 x 10⁶, 30 x IO⁶ and 25 x 10⁶ in bull, cat, and human, respectively. The numbers of Sertoli cells gener­ally remain constant even after puberty. In seasonal breeders, the volume and functional activity increase during the breed­ing season.

Functions of Sertoli Cell Sertoli cells are involved in regulating spermatogenesis, formation of blood-testes bar­rier, and many other activities (Figs. 19.5 and 19.9). The proliferation and development of germ cells during foetal growth are regulated by Sertoli cells. The proliferation and differentiation of Sertoli cells cease after puberty. The cells then act as ‘nurse cells’ to feed the germ cells and aid the spermatogenesis by secreting stem cell factors (SCF). The maturation of spermatids into spermatozoa, i.e. spermiation occurs within Sertoli cells. The Sertoli cells provide nutrients to spermatozoa as the blood nutrients, such as glucose, are limited to spermatozoa due to the blood-testes barrier. Sertoli cells convert glucose into lactate, which the growing sperm cells utilize. They also help develop germ cells’ iron transport by producing transferrin and ceruloplasmin. The androgen binding proteins (ABP) secreted from Sertoli cells bind with testosterone and transport it to the epididymis through semi­niferous tubule fluid and aids epididymal transit, maturation of spermatozoa, and health of accessory sex glands. Sertoli cells also potentiate the action of testosterone after converting it to its active form 5α-dihydrotestosterone (DHT) by secret­ing the 5α-reductase enzyme.

Fig. 19.4 Cross section of seminiferous tubules of rat. [The photomicrograph at 20X magnification showing the internal structure of the seminiferous tubules, contains a = interstitial space between the seminiferous tubules, b = lumen of the seminiferous tubule, c = blood-testis barrier (BBT), d = Sertoli cells, e = Leydig cells at the interstitial space, and f = peritubular myeloid cells surrounded the basement of the tubules]

Fig. 19.5 Functions of the Sertoli cells. [The flow diagram showing the various function of the Sertoli cell with the involvement of different hormones, enzymes and growth factors. FSH follicle-stimulating hor­mone, mainly involved in hormone and various protein synthesis; T testosterone; IP6K1 inositol hexakisphosphate kinase-1, an enzyme have role in cell metabolism and male fertility; E oestrogen; T₃∕T₄ thyroid hormones; ATRA all trans retinoic acid, form of vitamin A;

OCLNoccluding; CLDNclaudins; JAMsjunctional adhesion molecules; ZO zonula occludens; 17β-HSD 17 β-hydroxysteroid dehydrogenase; 5-AR 5α-Reductase; ABP androgen binding protein; Mct monocarboxylate, lactate transporter; GATA4 transcription factor GATA-4, a protein required for lactate synthesis; DHT 5- α-dihydrotestosterone; SCP Sertoli cell proteins; AMH anti-mullerian hormone; IGF-1 insulin-like growth factor-1]

immune-related proteins (cytokines, chemokines, comple­ment inhibitors, adhesion molecules) and provide immune protection to germ cells. However, Sertoli cells phagocytose, defective germ cells failed to complete spermatogenesis.

The Sertoli cells also produce activin, inhibin, anti- mullerian hormone (AMH), and insulin-like growth factor-1 (IGF-1). IGF-1 helps in spermatogenesis and testosterone production from Leydig cells. The liver borne IGF-1 is unable to pass the blood-testes barrier. Thus, the IGF-1 secreted from Sertoli cells is of great importance. Sertoli cells give structural support to the seminiferous tubule by adding the ground substances of the matrix. The proliferation and maturation of Sertoli cells are controlled primarily by follicle-stimulating hormone (FSH). But, the exogenous administration of FSH has a limited role in stimulating sper­matogenesis as the numbers of Sertoli cells become fixed after puberty. In addition to this, several endocrine and para­crine factors are also involved in regulating Sertoli cell functions (discussed later in Sect. 19.2.7).

19.1.1.4.3 LeydigCells

These are the large polygonal cells situated at the interstitial space of seminiferous tubules in extra-tubular connective tissue (Fig. 19.4). It is also known as an interstitial cell, and it is involved in the secretion of androgen (testosterone). Depending on their degree of maturation, LCs are of two types, foetal Leydig cells (FLCs) are present in male foetuses and matured LCs are called adult Leydig cells (ALCs). The numbers of ALCs decline with age, and testosterone produc­tion is subsequently decreased.

Foetal Leydig Cells (FLCs) The FLCs perform various functions, including determination of male sex and musculation, testicular descent and brain development together with the expression of the male behavioural pattern (Fig. 19.6). The androgens produced from FLC are responsi­ble for developing the male genital tract during foetal life and testicular descent. The insulin-like peptide 3 (INSL3) secreted from FLC also potentiates testicular descent. The FLC derived testosterone helps in metabolic and neuroendo­crine functions of the foetus, including brain development in conjugation with various growth and transcription factors. The action of FLC is gradually ceased after birth as its population diminished around puberty to support spermato­genesis but sufficient to develop male secondary sexual characteristics.

Adult Leydig Cells (ALCs) After puberty, the population of FLCs are gradually replaced by ALCs characterized by

Fig. 19.6 Functional activity of foetal Leydig cells (FLC). [The diagram showing the functions of the FLC with the involvement of different hormones and growth factors. T testosterone, required for major activities of FLC; INSL3 insulin-like-3, a peptide hormone responsible for descending of the testis; PDGF-A and PDGF-B are the platelet-derived growth factors; GATA-4 is a transcription factor and IGF-I is insulin-like growth factor 1]

cellular hypertrophy (probably due to an increase in the nuclear volume, e.g. 5.6 pm in 4th week vs. about 7.4 μm in 75th week, in a bull) and hyperplasia. ALCs express LH/CG receptor (LHCGR), a G protein coupled receptor under the relaxin family peptide receptor 2 (RXFP₂). Luteinizing hormone or interstitial cell stimulating hormone (LH or ICSH) causes higher expression of the key steroido­genic enzymes like Hsd3β6 and Hsd17β3 to catalyse the conversion of androstenedione into testosterone (Fig. 19.7) with higher amplitude than FLCs. The testosterone produced from ALCs is either diffused into the seminiferous tubules and Sertoli cells by the paracrine process or enters into the systemic circulation. Sertoli cells further convert testosterone into its potent form and aid the spermatogenic process. The endocrine regulation of Leydig cell function is discussed in the next chapter.

19.1.1.4.3 The Cross-Talk Between Testicular Cells

All the cells are interrelated in their function (Fig. 19.8). The normal functioning of the testes depends upon the interaction between different testicular cells through endocrine and para­crine factors. The LH stimulates Leydig cell steroidogenesis and initiates a cascade of cellular cross-talk to promote sper­matogenesis. The action of FSH on Sertoli cells facilitates cellular communications for germ cell development, differ­entiation of peritubular myoid cells, and Leydig cell function. The growth and differentiation of different testicular cells are regulated through several paracrine factors like IGF, TGF-α and β, and NGF. Some of these factors promote cell prolifer­ation, and others regulate differentiation. The coordinated actions between these factors promote the rapid proliferation of germ cells and slower growth of peritubular and Leydig cells in adult testis.

19.1.1.5 Blood-TestisBarrier

A specialized multicellular barrier present between the vas­cular endothelium of the capillary blood vessels and the epithelium of the seminiferous tubules is called the blood­testis barrier (BTB) (Figs. 19.4 and 19.9). The tight junctions between the adjacent Sertoli cells form a barrier to dividing the seminiferous tubule into basal and adluminal compartments. The primary function of BTB is to isolate the germ cells from circulatory and lymphatic systems and local immune suppression to provide a biochemically and immunologically distinct microenvironment to support germ cell maturation. BTB restricts the transport of water, steroids, antibody-producing cells (B lymphocytes), electrolytes, para­crine factors, hormones, and toxic substances across the epithelium of Sertoli cells and allows FSH and testosterone to enter. The BTB also protects the developing germ cells from the exposure of germ cell-specific antigens, which oth­erwise leads to the production of anti-sperm antibodies to cause male infertility. The locally produced immune- suppressive substances (interleukins, interferons, and prostaglandins) from the Sertoli cells also provide an immune-suppressive microenvironment for germ cell matu­ration. The functional activity of BTB is mediated through several junctional proteins, namely occludin, claudins, junc­tional adhesion molecules (JAMs), and zonula occludens (ZO). The androgens, oestrogens, thyroid hormones, retinoic acid, and opioids influence the development of BTB.

Fig. 19.7 Molecular mechanism of biosynthesis of testosterone in adult Leydig cell (ALC) by the influence of Iuteinising hormone (LH). [LHCGR G protein-coupled receptor for LH, activates AC; AC adenylate cyclase, the enzyme which converts ATP to cAMP, it activates PKA; PKA protein kinase, it de-esterifies the lipid droplet to cholesterol (free cholesterol); cAMP and PKA activate nucleus to pro­duce steroidogenic proteins and enzymes like, PAP7, AKAP121, StAR along with voltage-dependent anion channel to import cholesterol into mitochondria; PAP7 is A-kinase anchoring protein; AKAP121 is

A-kinase anchor protein 121; StAR is steroidogenic acute regulatory protein (transport protein); CYP11A1 is an enzyme of the cytochrome P450 cholesterol side-chain cleavage (P450_scc) superfamily member (family 11, subfamily A, polypeptide 1) converts cholesterol to preg­nenolone within mitochondria; pregnenolone then migrate to smooth endoplasmic reticulum and converted to testosterone by involving enzymes 3β-HSD, CYP17, 17β-HSD; 3β-HSD 3β-Hydroxysteroid dehydrogenase; CYP17 cytochrome P450 17α-hydroxylase; 17β-HSD 17β-Hydroxysteroid dehydrogenase]

19.1.1.6 Spermatic Cord

It is a tubular structure that suspends the testes into the scrotum. It originates from the inguinal ring, descends into the scrotum, and ends at the posterior margin of the testicle. The spermatic cord consists of the spermatic artery, spermatic vein, spermatic nerve, internal cremaster muscle, lymphatic vessels, vas deferens, and tunica vaginalis propria (Fig. 19.10). The spermatic cord provides blood and nerve supply to the testes.

19.1.1.7 DescentofTestes

The migration of testes from their intra-abdominal location to the scrotal sac is called testicular descent (Fig. 19.11). It starts around the 8th week of gestation in humans and is completed around the 35th week. Testicular descent is completed around the mid-gestation period in bull, ram, and buck. In boar, it occurs at the last quarter of pregnancy. The testicular descent in the stallion is completed just before birth or immediately after birth. Testicular descent in camels generally happens within a couple of days of postnatal life. The process needs

2- 6 months after birth to complete the testicular descent in dogs. It may take about 2-3 years to complete descent. The descent of testes is a three-stage process. In the first stage of nephric displacement, the detachment of gonads from meso­nephros occurs. In the second stage of transabdominal descent, the testes migrate towards the inguinal ring. The last stage is called inguinal descent when testicular migration is completed from abdominal cavity to scrotum. The descent of testes is guided by myxofibrous structure extending from testis to scrotum called gubernaculum testes. The testicular descent is mediated through the outgrowth and regression of gubernaculum testes. During the gubernacular outgrowth, rapid swelling of the gubernaculum dilates the inguinal canal and makes the way of testicular migration through the inguinal ring. During the regression of the gubernaculum, cellular remodelling occurs and it becomes a fibrous tissue rich in collagen and elastic fibre. The mechanical factor-like intra-abdominal pressure transmitting to the gubernaculum initiates the testicular descent, and protrusion of processus

Fig. 19.8 Interactions among the testicular cells. [The involvement of different hormones and growth factors in testicular cells cross-talk is presented. Ttestosterone; IGF-1 insulin growth factor 1; β-EP beta endorphin; E2 oestradiol; Inh inhibin; Act activin; PModS peritubular modifying substance; TGF transforming growth factor; OT oxytocin; ABP androgen binding protein; Trans transferrin; IL interleukin; bFGF basic fibroblast growth factor; SGP sulphated glycoprotein; NGF nerve growth factor; CRBP cellular retinol-binding protein]

Fig. 19.9 Blood-testis barrier. [The cross section of seminiferous tubule depicting blood-testis barrier (BTB) in dotted line divides the testicular tissues in basal and adluminal compartments]

vaginal through the inguinal ring completes the descent process.

Many factors are involved in testicular descent. Androgen-independent insulin-like factor 3 (INSF-3) pro­duced from Leydig cells plays a pivotal role in testicular descent. The mutation of this gene or antibody against INSF-3 leads to the failure of testicular descent. It causes the swelling of the gubernaculum and initiates testicular descent. The androgens have very little effect on the gubernaculum as gubernaculum doesn’t possess androgen

Fig. 19.10 Spermatic cord. [Diagram showing the spermatic cord in a dog. The cross section view of the spermatic cord depicting vas deferens, various blood vessels, nerves, and cremaster muscle is illustrated at the left top corner]

Fig. 19.11 Testicular descent. [Stages (i, ii, iii, and iv) of testicular descent from the foetal abdomen to scrotum through gubernaculum]

receptors; rather, androgen causes masculinization of genitofemoral nerves and stimulates to secrete calcitonin gene-related peptide (CGRP), causing rhythmic contraction of the gubernaculum testes. The other factors involved in testicular descent are oestrogen and epidermal growth factor (EGF). Oestrogen prevents the swelling of the gubernaculums, and EGF stimulates human chorionic gonad­otropin (hCG) production from the placenta, which in turn stimulates androgen production from Leydig cells.

19.1.1.7.1 Disorders OfTesticular Descent

Cryptorchidism and inguinal hernia are the two major disorders occurred due to disturbance in testicular descent.

Cryptorchidism The failure of testicular descent in the mature animal leads to a pathological condition called crypt­orchidism. It may be unilateral (involving a single testicle, commonly right testicle) or bilateral (both testicles). The highest prevalence of cryptorchidism is seen in horses, followed by pigs. The companion animals also have a higher cryptorchidism incidence than cattle and sheep. The aetiology of cryptorchidism is multifactorial—the genetic, anatomical, and endocrine factors are involved in cryptorchi­dism. The hereditary predispositions of cryptorchidism are documented in pigs and horses. The candidate gene of crypt­orchidism in pigs is SSC8, identified in Large White and Landrace. Thoroughbreds have less prevalence of cryptorchi­dism among the breeds of horses than American Quarter, Percherons, and American Saddlebreds. In sheep, cryptorchi­dism is associated with autosomal recessive mode inheri­tance, including the dominant gene with the incomplete penetrance. The endocrine factors involved in testicular descent are abnormal testosterone production or the absence of Mullerian inhibiting hormone. The foetus exposed to increased maternal oestrogen concentration may also develop cryptorchidism. The anatomical factors like torsion of the spermatic cord, scrotal hernia, and premature birth may inter­fere with testicular descent. The other miscellaneous causes of cryptorchidism include prolonged breech presentation, navel infections at the time of descent, exposure to anti- androgenic chemicals, and deficiency of maternal vitamin A. The cryptorchid animals can produce a subnormal amount of testosterone, and secondary sex characters are developed but fail to produce sperm due to elevated testicular tempera­ture in the abdominal cavity.

Inguinal HerniaZScrotal Hernia It results when the portion of the intestine drops into the scrotum with the testes due to enlargement of the inguinal canal. The abdominal viscera entered inside the scrotum may be located inside the vaginal process (indirect hernia) or within the vaginal process (direct hernia). Congenital inguinal hernia results from enlarged inguinal canal occurred due to abdominal compression dur­ing parturition. Acquired indirect hernias are common in stallions.

19.1.1.8 Thermoregulation of Testes

The efficient spermatogenesis requires an environment 2-6 ° C cooler than the core body temperature. The scrotum, there­fore, has to provide the necessary thermal environment to support spermatogenesis. Five main anatomical features con­tribute to regulating the testicular temperature: tunica dartos smooth muscle, cremaster muscle, a countercurrent heat exchange system, absence of subcutaneous fat layer, and abundance of sweat gland. The tunica dartos surrounding the scrotum relaxes in response to higher environmental temperature holding the testis away from the body core. The reverse occurs during lower environmental temperatures. The relaxation of tunica dartos also increases the surface area of the scrotum for better heat exchange. The cremaster mus­cle also functions similarly to the dartos muscle. Still, the basic deference is that the contraction and relaxation of cremaster muscle lead to elevation and distension of testes, whereas dartos muscle controls scrotal skin. The testes receive comparatively cooler blood than other body organs due to a countercurrent exchange system. This testis system is facilitated by a network of small veins around the spermatic cord. This venous plexus is called the pampiniform plexus. The arterial blood carried through the testicular artery loses heat in exchange with the venous plexus, and a lower testic­ular temperature is achieved. In ram, the arteries and veins supplying the testes are superficial to the scrotum. It gives additional benefits to this species in terms of thermoregula­tion through evaporative heat loss. But, during hot-humid conditions, these evaporative heat exchange mechanisms are lost and may lead to ‘summer sterility'.

The absence of insulating covers in the subcutaneous fat layer in the scrotum facilitates conductive heat loss. A higher abundance of sweat glands in the scrotal skin promotes

Fig. 19.12 Countercurrent heat exchange in testis. [The testicular artery (bold line) carries warmer blood towards the testes becomes cooler through the exchange of heat with the cooler blood of the testicular vein (dotted line)]

evaporative heat loss to maintain lower testicular temperature than the body core.

The testicular thermoregulation is less efficient in the boar due to the anatomical position of testes (less pendulous), and less sweat gland in scrotal skin resulted in more sperm abnormalities under extreme climatic conditions. The animals with intra-abdominal testes support spermatogenesis due to heat resistant spermatozoa or cooler core body tem­perature. The testicular temperature of sea mammals is gen­erally lower than the abdominal temperature due to cooling employing the dorsal fin and associated vessels by counter­current heat exchange mechanism (Fig. 19.12).

19.1.1.9 Testes of Birds

The avian testes are elliptical in shape and light yellow in colour, situated near the top of the kidneys. The testicular size shows seasonal variations and increases around the breeding season. The left testis is generally larger than the right. Birds have higher body temperatures than mammals. The testes lie adjacent to the air sac for an efficient heat exchange mecha­nism to maintain the desired testicular temperature for sper­matogenesis. Spermatogenesis also occurs at night when the body temperature is comparatively lower. The avian spermatozoa are also heat resistant.

Testes can be considered a mixed gland in which the exocrine part, comprised of seminiferous tubules, is involved in spermatogenesis. The endocrine portion made of the

Table 19.2 ThepH of fluid of various parts of the excurrent tract

Leydig cells produces male sex hormone through steroido­genesis. These functions are discussed in detail under subsequent sections/chapters.

19.1.2 Excurrent Tract

The excurrent tract of the male reproductive system extends from the rete testes up to the urethra and consists of rete testes, efferent tract, epididymis, vas deferens, and urethra (Figs. 19.1 and 19.2). Major functions of the tract are the maturation of spermatozoa, reabsorption of the excess fluid secreted from the seminiferous tubules, and providing pas­sage for the expulsion of spermatozoa. The functional activ­ity of the tract is controlled by the hormones in the paracrine and autocrine fashion with variable pH (Table 19.2).

19.1.2.1 ReteTestes

The highly convoluted seminiferous tubules are merged in straight tubules within testes are called rete testes (Fig. 19.3). In rats and mice, the rete testes are situated at the cranial pole of testes, but it is present at the centre of the testes in other species. The rete testis is lined by simple cuboidal or colum­nar epithelium but lacks germinal epithelium hence unable to produce spermatozoa. The columnar epithelium is either ciliated or non-ciliated. The ciliated cells are secretory and primarily involved in the movement of spermatozoa along with the fluid towards efferent ducts. The non-ciliated cells are mostly involved in selective reabsorption of the tubular fluid. The rete testes serve as collecting reservoirs of sperm. It acts as the first site for reabsorption of seminiferous tubular fluid and reabsorbs inhibin into the circulation. The junction between seminiferous tubule and rete testes is tubuli recti lined by Sertoli cells.

19.1.2.2 Efferent Ducts

Efferent ducts connect the rete testes with the epididymis (Fig. 19.3). These ducts are converged at the junction with epididymis. The lining cells are similar to the rete testes. The basement membrane of the efferent ducts is layered by smooth muscle and connective tissues. The smooth muscle cells are under sympathetic control and support the ciliary movement of the ducts. The lumen of the ducts is wider at the rete testes end and narrower at the epididymis end. In rats, mice, and guinea pigs, the ducts are merged and formed single duct before joining the epididymis. In other mammals, they join with the epididymis as parallel tubes. Before join­ing, the efferent ducts become tortuous and convoluted. Efferent ducts are mainly involved in the reabsorption of the testicular fluid and the movement of spermatozoa towards epididymis. Other functions of the ducts are ion transport, protein reabsorption, and steroid metabolism. The presence of Na⁺/K⁺ ATPase pump enables efferent ducts to absorb 70-95% of the testicular fluid, and around 50% of proteins released from the testes are reabsorbed from the efferent ducts. As a result, more viscous fluid and the spermatozoa reach the epididymis, which facilitates sperm maturation. The epithelium of the efferent duct also contains enzymes like acid phosphatase (endocytosis), carbonic anhydrase (bicar­bonate absorption), glutathione S-transferase (GST) (cellular detoxification), and sulphated glycoprotein-1 (SGP1) (endo- cytosis), as well as the receptors for testosterone, oestrogen, vitamin D₃, oxytocin, inhibin, opioid, and proenkephalin to control the activity of the duct.

19.1.2.3 Epididymis

It is a highly convoluted single tube that remains in close contact with testes (Fig. 19.3) and descends into the scrotum through the spermatic cord. Length of epididymis varies with species. It is 40-50 m in bull, nearly 50 m in boar and ram, 70-80 m in the stallion, 3 m in rat, 2 m in cat, and 6-7 m in human. Morphologically it is divided into three parts, head (caput), a body (corpus), and a tail (cauda). The epididymis is lined by ciliated and non-ciliated columnar epithelium cells with long microvilli, which increase absorptive surface. There are four different cell types in the epididymal epithe­lium: principal, basal (narrow cells), clear, and halo cells. The most abundant cells are principal cells involved in secre­tion and absorption. The tight junctions between two adjacent principal cells form a blood-epididymis barrier that continues with BTB and provides immune protection of post-pubertal germ cells. The basal cells are flat and elongated act like macrophages involved in detoxification due to glutathione

S-transferases (GST) and lysosomal enzymes. The clear cells are involved in the disposal of cytoplasmic droplets of the spermatozoa. Halo cells are believed to be lymphocytes and monocytes involved in the immune protection of the male reproductive tract. The cauda epididymis is connected to a highly muscular duct called vas deferens. The spermatozoa undergo final maturation during their transit through the epididymis, acquiring their motility and fertilizing capabilities (the role of epididymal factors in sperm maturation has been discussed in detail in the spermatogene­sis chapter). The cauda epididymis is the principal storage site of mature spermatozoa. The epididymis is also involved in the reabsorption of epididymal and testicular fluid to the tune of 95% and 5-30%, respectively, to increase the osmo­lality of testicular fluid with the help of Na⁺/K⁺-ATPase pump (10-20 mosM of the seminiferous tubules in contrast to 200 mosM at the epididymis, in rat). The concentration of the spermatozoa is also more in the cauda than rete testis (10⁹/mL from 104/mL, in rat). The epididymal secretions help acidify testicular fluid during its transit from caput (7.11-7.26) to cauda (pH 6.0-6.5) favoured by zinc and various proteins to promote sperm maturation. However, environmental pollutants, viz. heavy metals, oppose acidifi­cation. Epididymis also protects the spermatozoa from xenobiotics and oxygen radicals. The functions of the epidid­ymis are under the control of testosterone. Enzymes like steroid 5α-reductase and neurotrophins (nerve growth factor, NT-3) are also reported to control epididymal functions. The epididymal functions are impaired by certain immunosup­pressive drugs (cyclophosphamide), fungicides (benzimid­azole), industrial gas (methyl chloride), sulfonates (ethyl dimethyl sulfonate) that may lead to infertility.

19.1.2.4 Vas Deferens

The vas deferens or ductus deferens is a tubular structure that originates from the caudal epididymis at the posterior border of each testis (Fig. 19.3) and enters the pelvic cavity through the inguinal canal to join the ejaculatory duct (union of the vas deferens and the duct of seminal vesicle) (Figs. 19.1 and 19.2). The distal portion of the vas deferens is dilated to form a sac-like structure called the ampulla. The length of vas deferens is about 30 cm in bull, 15 cm in buck, 29 cm in ram, and 35 cm in camel. The vas deferens is lined by pseudostratified columnar epithelium made of columnar cells and basal cells. The columnar cells are lined with cilia at their luminal surface. Beneath the epithelial layer, a tissue stroma is made of elastic fibres. The boarders of vas deferens are lined by circular and longitudinal smooth muscles with sympathetic innervations. The contraction of these muscles during ejaculation causes slow peristaltic waves that facilitate sperm transport epididymis to the urethra. Vas deferens act as temporary storage of spermatozoa before ejaculation. The sperm released from the cauda epididymis into the vas deferens will never return to the epididymis again if failed to expel out from the body. In a vasectomy, the passage of vas deferens is blocked through surgical interventions. It prevents the movement of spermatozoa from the epididymis to the urethra and causes permanent sterilization of males. The vasectomy does not assure to inhibit testosterone pro­duction and spermatogenesis. The spermatogenesis in vasectomized animals continued and stored at the cauda epididymis, resulting in the rupture of the epididymis, leading to sterility. Reversible contraception can also be possible by inserting an obstructive device into the vas deferens to block spermatozoa’s flow temporarily.

19.1.2.5 Urethra

The urethra is a fibromuscular tube of the urogenital system as it carries urine from the bladder and semen from the excurrent tract to the exterior of the body (Figs. 19.1 and 19.2). It is broadly divided into two segments, namely the pelvic urethra and penile urethra. The pelvic urethra extends from the internal opening of the bladder up to the prostate. The pelvic urethra can be subdivided into the pre-prostatic and prostatic urethra. The pre-prostatic urethra is extended up to the prostate, and the prostatic urethra continues through the prostate joins with the penile urethra. Two ejaculatory ducts, each from vas deferens and seminal vesicle, drains sperm and ejaculatory fluids to the prostatic urethra. The prostatic ducts also open in this portion through the prostatic sinus to con­tribute prostatic secretions into the ejaculates. These openings are called colliculus seminalis. The pelvic urethra is lined with transitional epithelium. The penile urethra is composed of membranous urethra located in the deep peri­neal pouch extending through the external urethral sphincter. It has a bend at the ischial arch. It is the narrowest portion of the penile urethra. The lower and longest part of the penile urethra remains within the penis up to the urinary meatus at glance penis. It is covered with spongy elastic fibres hence called the spongy urethra. The bulbourethral glands open in the proximal portion of the spongy urethra.

Like pelvic urethra, the inner lining of the penile urethra is made of transitional epithelium, but in some species, it may change to stratified squamous epithelium before termination. The penile urethra is covered with erectile tissue called the cavernous body and cavernous spongiosum to control fluid flow. The penile urethra in ruminants is ‘S’-shaped along with the sigmoid flexure of the penis (Fig. 19.1). Hypospa­dias is an anomaly of urethral opening at the body or head of the penis instead of the tip, making difficulty in urination and ejaculation. The aetiology may be congenital or endocrinal. There are two urethral sphincters, the internal urethral sphinc­ter situated at the junction between the urinary bladder and urethra and the external urethral sphincter surrounds the membranous urethra. The internal urethral sphincter is made of smooth muscle and under involuntary (autonomic) control, and the external urethral sphincter is formed by skeletal muscles (urethralis muscle) and under voluntary control. The internal sphincter regulates the involuntary flow of urine from the bladder to the urethra, and the external sphincter controls the voluntary urine flow from the bladder to the urethra. The internal sphincter blocks the retrograde flow of semen into the urinary bladder at the time of ejacula­tion by relaxing and contracting the internal urethral sphinc­ter during urination and ejaculation, respectively.

Fig. 19.13 Reproductive system of bird. [Long vas deferens and char­acteristic copulatory organ (penis) along with various components of the male reproductive system of bird are illustrated]

Fig. 19.14 Characteristic feature of the copulatory organ of male and female bird. [The enlarged penis of male (left) and genital eminence of female (right) are illustrated, which can be used to identify their sex at day-old birds]

19.1.3 Excurrent Tract of the Bird

There are two major parts of the excurrent tract of the birds (Fig. 19.13). These are (1) vas deferens and (2) copulatory organs.

19.1.3.1 Vas Deferens

The vas deferens originates from each testis leading to the cloaca. At the opening (from the testis), it has a small and flattened area and resembles mammals’ epididymis. The vas deferens is relatively narrow at the proximal part and gradu­ally widens towards the cloaca. It has several bends and twists in its passage. The spermatozoa become mature, transported, and stored into it. The vas deferens are terminated at the swollen seminal vesicle (glomus) in the cloaca wall. Unlike mammals, the seminal vesicles of birds store the spermatozoa for a limited period. The sperm of the bird can be collected for artificial insemination by pressing these vesicles.

19.1.3.2 Copulatory Organs

Most birds do not have a penis, like mammals, and they have a small quantity of erectile tissue, known as papilla. The papilla is a small bump-shaped structure situated on the posterior wall of the cloaca. It is the rudimentary copulatory organ, generally not used in mating, but mostly the sexing of birds can be done by identifying it as early as day-old (Fig. 19.14). During mating, the birds of two opposite sex positioned their cloaca openings opposite each other to trans­fer the spermatozoa from male to female. The spermatozoa are expelled from the body through the cloaca. The cloaca is a common body opening used for the expulsion of urinary and digestive waste also. The developed copulatory organ, like a penis, is present in male swans, ducks, geese, and ostriches.

In birds, the secretion of the epithelium cells of the excur­rent tract is the major source of seminal plasma as they lack accessory sex glands. In addition, some lymph-like secretions are found during ejaculation originate from paired paracloacal vascular bodies.

19.1.4 AccessorySexGlands

The male accessory sex glands include the ampulla, seminal vesicles, prostate, and bulbourethral (Cowper’s) glands (Figs. 19.1 and 19.2). The secretions from these glands are the main contributor to seminal plasma. All the glands drain their secretion into the urethra, except the bulbourethral gland, which opens into the penile urethra at the base of the penis. There are huge species variability regarding the func­tional morphology and nature of secretions of the accessory sex glands (Table 19.3). Animals that possess both seminal vesicles and the bulbourethral gland have seminal alkaline fluid, as secretion of both the glands is alkaline. The seminal alkaline pH favours the neutralization of the acidic secretions of the female genital tract. The seminal plasma acts as a vehicle to carry the spermatozoa to the site of fertilization. Moreover, the secretory products of the accessory glands promote the metabolic activity and potentiality of the spermatozoa by providing nutrients, buffering constituents, and bioactive substances. Androgens control the functions of the accessory sex glands. Most accessory sex glands contain the 5α-reductase enzyme, which converts testosterone to dihydrotestosterone.

19.1.4.1 Ampulla

Ampulla (Ampullae in plural) is the glandular swelling of the vas deferens above the urinary bladder. The ampulla is lined

Table 19.3 Presence of accessory sex glands in different male animals and pH of its secretion

+present, ++large size, absent, ^rudrudimentary

by simple columnar epithelium without excretory ducts. The ampulla is present in ruminants, horses, camels, rodents (rat, squirrel, etc.), and bats. It is well developed in camels and some species of bats. The primary function of the ampulla is to store sperm like reservoirs. The ampullary secretion is serous in nature and yellowish-white in colour. But, the secretion of ampulla contributes little in seminal plasma except for bull (0.5-2.0 mL). But, in the stallion, ampulla secretes ergothioneine, an antioxidant and cytoprotective substance of the seminal plasma. The ampullary contraction depends on its surrounding smooth muscle, which is stimulated along with vas deferens.

19.1.4.2 Seminal Vesicles

The seminal vesicles or the vesicular glands are paired, coiled, and extended fibromuscular glands present at the dorso-cranial aspect of the pelvic urethra lateral to the ampulla. It opens directly into the prostatic urethra, except in bull, where it opens at vas deferens. It is absent in almost all carnivores, camel, and domesticated rabbits but well developed in bull, boar, ram, stallion, rat, and guinea pig. The seminal vesicles are lobulated in bull, ram, and buck; and sac-like in stallion and boar. The glands and their ducts are lined by pseudostratified columnar epithelium, and the wall is composed of smooth muscles innervated with sympathetic and parasympathetic nerve fibre. The parasympathetic nerves control the secretion of glandular tissues, whereas the smooth muscles are under the control of both sympathetic and para­sympathetic nerves. Several neuropeptides are involved in seminal vesicle secretion, like neuropeptide Y (rats, guineapig, and humans); substance P (guineapig), vasoactive intestinal polypeptide (VIP) (mice), and gastrin-releasing peptide (rabbit).

The seminal vesicles contribute more than 50% of the ejaculate. The vesicular fluid is yellowish in colour, viscid, and alkaline. The predominant component of the vesicular secretion is the citric acid controlling the pH of the seminal plasma for its affinity towards divalent cations (viz. calcium, magnesium, and zinc). The seminal vesicle can synthesize fructose from either blood glucose or sorbitol, which acts as the chief energy source in seminal plasma. The concentration of fructose is inversely related to sperm motility as the spermatozoa utilize fructose as their major energy source.

Therefore, the concentration of fructose in semen indicates the functional index of seminal vesicles. Other than citric acid and fructose, various biologically active substances are secreted by seminal vesicles (Table 19.4). MHS-5 protein (antigen) is the marker to identify seminal vesicular secretions in chimpanzee, gorilla, orangutan, and human semen. It is also used to distinguish the ejaculatory sperm (semen) from the epididymal sperm. The epithelium of the seminal vesicle is infiltrated with macrophages and T lymphocytes (CD4 and CD8) as the gland is prone to infection.

19.1.4.3 Prostate

The prostate is a tubule-alveolar gland that surrounds the urethra at the base of the urinary bladder and opens into the urethra. It is present in almost all domestic species and the only accessory sex gland in carnivores and cetaceans (whales and dolphins). The prostate is a heterogeneous gland divided into two parts, the pars propria (body of the prostate) and pars disseminata (disseminated portion). In bulls, both parts are distinguishable. In rams, only pars disseminata is present. In stallions, the prostate has two lateral lobes connected through the isthmus. Pars disseminata is well developed in boars. Large-sized prostates exist in dogs and cats. The prostate of a cat has four lobes. The secretion of the prostatic is apocrine. It is thin, milky, alkaline, with low protein contents. The prostatic fluid is serous in dogs and mucous in bulls. The prostate contributes 30% of seminal plasma in human, but secretory volume is comparatively low in bulls and dogs. A proteolytic substance called prostate-specific antigen (PSA) of prostatic secretion helps to liquefy the vaginal plug for free passage of spermatozoa. PSA is used as a marker of prostate cancer. In dogs, Kallikrein-2 (KLK-2), a serine protease enzyme, induces PSA production. The prostate gland contains large quantities of chloride ions, calcium, zinc, citrate, and polyamines. Prostatin or prostatic binding protein has been identified in rats. Prostatic acid phosphatase (PAP) and prostate-specific protein (PSP) are identified in human prostatic secretions. The prostate is capable of secreting oxy­tocin that exerts a paracrine effect on the prostate to stimulate the growth of the prostate and its contraction during ejacula­tion. In some species, like a rat, mouse, guinea pig, hamster, rodents, and bats, the secretion of this gland helps to

Table 19.4 Major contribution of seminal vesicle

coagulate the semen immediately after intromission of the penis to form a vaginal plug; hence, it is called coagulating gland. The prostatic secretion in dogs is excreted through urine, other than ejaculate, and used to mark their territory. The growth and functions of the prostate are dependent on androgens, and glandular dysfunction is associated with endocrine deregulations. Benign prostatic hyperplasia (BPH) is one such pathological condition occurred in old age dogs and humans due to over production of dihydrotes­tosterone (DHT). Prolactin is also reported to induce BPH. BPH is associated with sperm abnormality and reduced motility. Bacterial prostatitis is also common in sexually mature males. The swollen prostate reduced the passage of the urethra, resulting in difficulty in urination. Anti-androgen drugs (to restrict 5α-reductase), progesterone therapy, or castration are recommended to control the BPH.

Know More...

The human prostate can secrete unique, organic sub­stance spermine synthesized from putrescine (precursor is arginine) under the influence of testosterone. The spermine formed spermine phosphate crystals together with phosphate, and it has bacteriostatic properties. This is used as a marker and used to identify the existence of human semen in veterolegal cases.

19.1.4.4 Bulbourethral (Cowper's) Glands

The gland was named after English surgeon; William Cowper discovered the gland in the seventeenth century. It is analogous to Bartholin’s glands in females. The two Cowper’s glands are situated beneath the prostate gland in the urogenital diaphragm covered by skeletal (bulbospongiosus, bulbocavernosus, and urethral) and smooth muscles. It is well developed in boar, camels, cats, rodents, and elephants, whereas small in bull, ram, stallion, and human. It is absent in dogs and most carnivores. Ducts of the gland Cowper ’s glands open into the penile urethra and are lined by pseudostratified epithelium. Cowper’s gland’s transparent, alkaline, and viscous secretion is released upon sexual arousal before coitus. It has five major roles:

1. The secretion helps to flush out the residual drops of urine and unwanted foreign substances from the urethra and penis for clean passage of semen.

2. The secretion contains many mucin or gelatinous substances that lubricate the passage of the penis and vagina for smooth ejaculation and transportation of spermatozoa.

3. The alkaline secretion buffers the acidic environment of the female genital tract and protects the sperm from the harsh environment.

4. The mucous facilitates the retention of the semen for a long time in the female genital tract. The secretion of Cowper ’s gland of boar is more viscous, having high sialomucin (sialomucoprotein) concentration, and holding the semen for nearly 4 days in the female genital tract.

5. The secretory products also provide energy to the spermatozoa.

The gland is highly responsive to oestrogens and estro­genic chemicals. Hence, the animals offered with a rich source of phytoestrogens (like clover pastures) may have metaplastic or cystic lesions in the bulbourethral gland.

19.1.5 AncillaryOrgans

19.1.5.1 Penis

Morphology The penis is a fibroelastic muscular urogenital structure that assists in urination and ejaculation. It is made up of erectile tissue (contains several elastic fibres, sinuses, and large free space), dense connective tissues, lymphatics, blood vessels, smooth and skeletal muscle innervated with the autonomic and central nervous system. The penis contains two types of tissue, a pair of corpora cavernosa and a single corpus spongiosum. The corpora cavernosa (cavernous body) is a kind of smooth muscle with a resting tone. The corpus cavernosum consists of cavernous tissue that form is paired columns and surrounded by connective tissue. The corpus spongiosum surrounds the urethra and is made up of vascular tissues. It appears as a spongy tissue enlargement of the pelvic urethra. The corpus spongiosum extends over the corpus cavernosum and forms a glans penis at the penile extremity. A considerable variation in the mor­phology of the glance penis corresponds to the morphology of the female genital tract. The penis has three segments, the root, body, and glans penis. The root is connected with the ischial arch and attached with corpora cavernosa and ischiocavernosus muscle. The body of the penis is surrounded by corpora cavernosa and corpus spongiosum with a thick connective tissue covering called tunica albuginea. The body of the penis is made up of erectile tissues, which promote the rigidity of the penis. The rigidity of corpora cavernosa is more in comparison to the corpus spongiosum. The corpus spongiosum thus facilitates the reduction of the pressure on the urethra for the passage of semen during ejaculation. The last part of the penis is the glans, also called the head of the penis. It contains touch and pressure receptors (Pacinian corpuscles) with several sensi­tive non-myelinated nerve endings.

Morjphological Variations In dogs, the distal portion of the corpus cavernosum transforms into bone called os penis. The penis of bull, boar, ram, and deer is fibroelastic, containing large connective tissue, elastic fibre, and small erectile tissues. In contrast, the musculovascular penis of stallions, dogs, cats, and humans have more erectile tissue and less connective tissue. The penis of ruminants and boar is ‘S’ shaped supported by a fibroelastic sigmoid flexure attached with retractor penis muscle (Fig. 19.1). During an erection, the retractor penis muscle is relaxed and facilitates the pro­trusion of the penis to the female genital tract. The rigidity of the penis is achieved after profuse blood supply in the venous sinuses through helicine arteries. The penis of dogs, bats, bears, seals, rodents, and certain primates have os penis (Fig. 19.2).

Fig. 19.15 Spiral deviation in penis of different domestic male

Functional Variations The bulbus glandis is present at the glans penis of the dog enlarging after intromission and helps to form copulatory tie during mating, which subsequently relaxes after ejaculation, and the penis comes out. The glans enlarges in ram, buck, and stallion. Os penis of the cat has cornified spines, and it stimulates the ovulatory response in the queen. In some species, the free end of the penis is spiraled (Fig. 19.15) after intromission. It depends upon the arrangement of adjacent supported lamellae or collagen fibres of the penis. After completion of erection, the penis returns to its normal shape by the dorsal epical ligament. If the penis’s spiralling fails to return to its normal state, the penis can appear as a ‘corkscrew penis’.

Penile Erection Penile erection is a coordinated process involving circulatory, nervous, and muscular systems. Increased blood flows into cavernosal sinuses through helicine arteries facilitate the erection process—increased arterial pressure and venous occlusion help trap the blood, resulting in penile engorgement and rigidity. The somatic nerves attached with the ischiocavernosus, ischio-urethralis, and bulbocavernosus muscles are contracted and support the rigid penis. The parasympathetic nerves, originating from the sacral region, are connected with these muscles and erectile tissues. In ruminant and boar, the retractor penis muscle relaxes, facilitating the penis to become elongated. After ejaculation, the sacral-originated sympathetic nerves return to their normal tone, causing contraction of helicine arteries and retractor penis muscle. Thus the pressure around the veins is reduced along with increased outflow and restoration of normal blood flow. The penis also returns to its normal position under the prepuce. The role of penis in ejaculation is discussed in detail in spermatogenesis chapter. The disorder of penile erection is called erectile dysfunction or incompe­tence that occurs due to structural or morphological defects without psychological influences.

Know More...

Most male marsupials comprise an ‘S’-shaped bifurcated penis used only during copulation. Its penis bifurcates into two columns corresponding with two lateral vaginas of the females.

19.1.5.2 Prepuce

The invagination of the abdominal skin covering the penis is called the prepuce. At birth, the epithelial lining of the penis and sheath of the prepuce are fused to form balanopreputial fold that obstructs the penile erection through prepuce. The fold disrupts at puberty under the influence of testosterone and allows free passage of the penis through prepuce during erection. In boar, this fold splits and enables the penis to move freely but remains as a ridge-like structure that holds urine and other wastes. In bull and dog, the fold may persist as a fibrous band causing incomplete protrusion. In a dog, the band is called the frenulum. The dog’s prepuce is very loose, which assists in holding the glans penis when the bulbus glandis bulge. The prepuce is absent in the cat, and its penis moves backwards and downward from the ischial arch.

19.2