The testis

The testes are the site of male gamete or spermatozoa production, and the primary site for androgenic hormone production in the stallion. As such, they play a central role in the reproductive function of the stallion (Figs 3.13 and 3.14).

Scrotum

The testes are held within the scrotal sacks, outside the abdominal cavity, and suspended from the abdomen by the spermatic cord area and the cremaster muscles. The scrotum in the horse is relatively short and non-pendulous in nature, when compared with that of other farm livestock such as the bull or the ram. As in other mammals, it is evident that the positioning of the testes outside the main body cavity is necessary to maintain a testes temperature of 35-36°C

Testis Outside
Fig. 3.13. The testes of the stallion after slaughter and dissection (see also Fig. 3.14).

(approximately 3°C below the main body temperature), which is a requirement for optimal production of spermatozoa (Cox et al., 1979). This temperature difference is maintained by means of the cremaster muscle and the pampiniform plexus. The cremaster muscle, as indicated earlier, allows the testes to be drawn up towards or let down away from the abdomen, in response to a reduction or increase, respectively, in environmental temperature. The pampiniform plexus is the term given to the intimate association between the fine network of capillaries branching from the central testicular vein, which returns blood from the testes to the body, and the similarly highly coiled arterial supply entering the testes. This arrangement acts as an arteriovenous counter-current heat exchange mechanism, allowing heat in the warmer arterial supply to be lost to the cooler venous return system. As a result the blood entering the testes is cooled, helping to maintain a temperature difference between the testes and the body core. In addition, the arrangement of the blood vessels in this area of the spermatic cord also acts to eliminate the pulse in the arterial blood supply entering the testes, without significantly reducing the blood pressure. The significance of this is unclear, but it may be important in the passage of diffusible elements between the incoming and outgoing blood supplies (Setchell, 1991).

Pampiniform Plexus Veins

pampiniform plexus

Fig. 3.14. A labelled diagram of Fig. 3.13 illustrating the major features of the stallion's testes after dissection from the body, post slaughter.

inguinal canal area through which runs the cremaster muscle, testicular artery and vein, and the vas deferens pampiniform plexus visceral tunica_^_ \ lj / A

vaginalis \/ ^^ —-_epididymis cauda epididymis

\ ] J/' —testis covered by the j/ tunica albuginea

Fig. 3.14. A labelled diagram of Fig. 3.13 illustrating the major features of the stallion's testes after dissection from the body, post slaughter.

The testes of the stallion descend through the body cavity and into the scrotum at birth (or soon after), passing down from a position ventral to the kidneys, through the inguinal ring and into the scrotum. Problems may be encountered with incomplete descent of the testes and inadequate closure of the inguinal ring. Such conditions are not uncommon and often result in cryptorchidism; animals with this condition are sometimes referred to as rigs (section 4.3.2). There is reputed to be a higher incidence of failure of the testis to descend on the left side than the right (Bergin et al., 1970).

The testes of the horse lie along the longitudinal axis; that is, horizontal to the body of the stallion - unlike the bull and ram, where the testes lie in a vertical axis. The positioning of the testes is such that the caput epididymis is directed cranially and the corpus epididymis lies horizontally over the dorsal aspect of the testes, with the cauda epididymis lying caudally. On retraction of the cremaster muscles the testes are drawn up towards the body, turning slightly away from the horizontal plane.

The size of a stallion's testes varies considerably with age during the first 5 years of life. At birth the testes weigh on average 5-10 g in a lightweight riding horse and do not change significantly during the first 10-12 months of life, increasing in size allometrically with general body growth (Thompson, 1992). With the advent of spermatogenesis at 18 months of age, testicular growth is accelerated, resulting in near final testes size at sexual maturation, around 5 years of age (Amann, 1993a,b). The final weight is 300-350 g per testis, with a longitudinal axis length of 6-12 cm, a depth of 4-7 cm and a width of 5 cm. In many stallions there is a tendency for the left testis to be slightly smaller than the right. The significance of such a size difference is unclear (Thompson et al., 1979).

Internal structure of the testis

The testes lie protected within the scrotum, the outer layer of which is the skin. Scrotal skin is of a similar structure to the skin covering the remainder of the horse's body, but with a higher concentration of sweat glands and nerve endings. Within the outer skin layer lies the tunica dartos, a muscle fibre and connective tissue layer, contraction of which again aids in drawing the testes up towards the body. Within the tunica dartos lies the connective tissue layer of the scrotal fascia, and within this is the tunica vaginalis (Figs 3.15 and 3.16).

Testis Sertoli Cell Tem
Fig. 3.15. The testes of the stallion, as shown in Fig. 3.13, after dissection.

The tunica vaginalis is divided into two parts: the outermost is the parietal vaginalis and the inner layer is the visceral vaginalis (Fig. 3.17). Adhesions between these two layers may occur, largely as a result of trauma, age and general senescence. Such adhesions restrict the movement of the testis within the scrotum and hence affect the efficiency of the testicular temperature-control mechanisms. Within the tunica vaginalis lies the tunica albuginea, which forms a relatively thick fibrous capsule. Within the tunica albuginea lies the functional area of the testes: the parenchyma. Extending parietal tunica vaginalis and connective tissue parietal tunica vaginalis and connective tissue

Sertoli Cells Diagram
seminiferous tubules

and testicular tissue

Fig. 3.16. A labelled diagram of Fig. 3.15 illustrating the internal structures of the stallion's testis.

Testis Tissue Diagram Sertoli Cells
Fig. 3.17. Vertical cross-section through the testes, epididymis and part of the penis of the stallion.

into the parenchyma are strands of connective tissue originating from the tunica albuginea, forming lobes within the testes. The colour of testicular parenchyma is indicative of stallion age. At puberty, as the testes become functional, they become lighter in colour. Dark-coloured testes in stallions under 5 years of age is indicative of small, inactive seminiferous tubules composed mainly of gonocytes and Sertoli cells, with no lumen. Active seminiferous tubules, with all types of germinal cells and Sertoli cells and with a lumen, appear lighter in colour (Clemmons et al., 1995). Post puberty, the testes parenchyma tends to darken gradually with age, due to greater pigmentation of the Leidig cells (Johnson and Neaves, 1981).

The testicular parenchyma may be divided into two main functional compartments: the seminiferous tubules and the interstitial tissue. The relative composition of the two is reported to be 58-72% seminferous tubules and 28-42% interstitial tissue (Swierstra et al., 1974; Christensen, 1975; Johnson and Neaves, 1981; Berndtson et al., 1983).

Seminiferous tubules

Seminiferous tubules are defined by the lamina propria, which consists of fibroblasts, myoid cells (specialized smooth muscle cells that give rhythmic contraction of the tubules) and laminin. The tubules are not penetrated by the blood, lymph or the nervous system. Within the seminiferous tubules is an epithelial cell lining, consisting of germinal cells at different stages of development (Kroning, 1986), plus somatic cells (Sertoli, support or nurse cells). The seminiferous tubules are arched in shape, with a straighter tapered portion at either end of a central convoluted area. It is this central convoluted portion that is primarily involved in spermatogenesis and contains up to 15-20% Sertoli cells, the remainder being germinal cells at various stages of development. The Sertoli cells may be found radiating out into the lumen of the tubules in association with their attached germinal cells. On release into the lumen, these germinal cells, now spermatids, progress, by rhythmic contractions of the tubules, towards either tapered end of the seminiferous tubule. These tapered ends connect to the rete testis situated in the cranial area of the testis. The rete testis penetrates through the tunica albuginea to form the extracellular rete testis. Each of the tubules within the rete testis fuses with the 13-15 efferent ducts that lead on to form the initial part of the caput epididymis (Amann et al., 1977).

Sertoli cells surround all germinal cells, except for spermatogonia, and as such closely control the environment of the developing spermatids. They fulfil three main functions. Firstly they provide a barrier between the spermatids and the stallion's system; this is particularly important in considering the protection of spermatids from rejection by the stallion's immune system. Secondly, they are a source of nutrients for the germinal cells, acting either as transport systems, or by producing the nutrients required themselves. Thirdly, they provide a means of movement for the developing spermatids which, as they develop, move progressively towards the lumen of the tubules in readiness for release at spermiation. In association with this, Sertoli cells also have a phagocytic role, removing degenerating germinal cells plus residual spermatozoan cytoplasmic bodies.

The major function of Sertoli cells - that of immunological protector - is largely conferred by their action as a blood-testis barrier consisting of junctional complexes between neighbouring Sertoli cells. This protection is required to prevent the rejection of germinal cells, which in essence (due to their haploid nature) are 'foreign bodies' within the stallion's system. The blood-testis barrier also acts to control the passage of molecules from the blood plasma to the seminiferous tubules and seems to actively exclude large water-soluble molecules, though allowing the passage of lipid-soluble molecules and water (Setchell, 1991). As a component of this protective blood-testis barrier, the Sertoli cells must also have a communication role allowing communication between themselves, the germinal cells, the lamina propria and the Leidig cells (Amann, 1993a). Damage to this blood-testis barrier, though rare, can occur and results in a significant reduction in reproductive ability, which may be irreversible.

Sertoli cells also produce and secrete lactate, a source of energy to the spermatids, along with inhibin and activin (regulatory hormones) and several proteins. Some of these proteins act as carriers providing a means of transport for iron, copper and vitamin A to the spermatids; some may act to regulate spermatozoa production (Amann, 1993a) and others (transferrin and cerulo-plasmin) regulate germinal cell movement (Amann, 1993b). This intimate association and the reliance of germinal cells upon Sertoli cells does have the major disadvantage that any effect on Sertoli cell number or function will have a direct effect on germinal cell development. Indeed it is likely that many of the effectors of spermatozoan production do act via the Sertoli cells, rather than directly upon the developing spermatids (Hochereau-de Reviers et al., 1990; Amann, 1993a,b). The proportion of Sertoli cells to germinal cells is reported to be highly heritable (Hochereau-de Reviers et al., 1987). This, plus the normally observed reduction in Sertoli cell numbers with age, may account for the apparent reduction in spermatogenesis over time and the considerable variation in spermatozoan production capabilities between stallions (Johnson and Thompson, 1983; Blanchard and Johnson, 1997).

Interstitial tissue

The interstitial tissue of the testes is made up largely of Leidig cells (21-57%, depending upon the stallion's age: Table 3.2), blood vessels, lymph vessels, connective tissue and nerves (Fig. 3.18). It is evident that the Leidig cells may make up the majority of the interstitial tissue, and in the horse they appear to be relatively large in size, compared with those of the rat, hamster, ram and bull (Johnson and Neaves, 1981).

Leidig cells are primarily involved in steroid hormone production, in particular the production of testosterone from cholesterol. Two types of Leidig cell have been identified and are associated with stallion age and cellular function. Post-puberty Leidig cells are gradually replaced, with age, by adult Leidig

Table 3.2. Changes in testicular composition evident with age in the stallion (Johnson and Neaves, 1981; Pickett et al., 1989).

Age (years)

13-20

Testicular weight (g) Parenchyma Tunica albuginea

Parenchyma composition (%) Leydig cells (%)

(a l g_1 testis) Other interstitial tissue Seminiferous tubules

Seminiferous tubules Diameter (m) Length/testis (m) Sertoli cell no. (x 106 g^1 testis)

Daily spermatozoal production per testes (x 109)

105a 12a

6a 50 22a 72

212a 2040a 39

1.3a

146b 15b

12b 115 16b 72

230b

2390ab

29 2.7b

184c 29c

18c 175 10c

242b 2790b 21

3.2b a,b,c Means in the same row with different superscripts differ (P < 0.05)

Cross Section Testis Labelled Diagram
Fig. 3.18. Cross-section through the testicular tissue, illustrating interstitial tissue and an enlarged area of seminiferous tubule with developing spermatids.

cells, which contain a higher concentration of pigmented lipids and have a greater number of interdigitations. Their production of testosterone is also greater (Almahbobi et al., 1988). In addition to changes in the function of Leidig cells, there is also a difference in the ratio of Leidig cells to seminiferous tubules. The older the stallion, the relatively greater is the number of Leidig cells present, with up to a threefold increase being reported to occur between 2-3-year-old stallions and ones of 13-20 years of age (Table 3.2).

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