glandular sweating [19]. Thus the scrotum not only plays an important role in housing and protecting the testes but also has a role in thermoregulation of the testes. The spermatic cord connects the testes to the body and provides access to and from the body cavity for the vascular, neural, and lymphatic systems that support the testes. In addition, the spermatic cord accommodates the cremaster muscle, the primary muscle supporting the testes, and the pampiniform plexus, a complex and specialized venous network that wraps around the convoluted testicular artery [21]. This vascular arrangement is very important in temperature regulation of the testicular environment. The plexus consists of a coil of testicular veins that provide a counter‐current temperature exchange system: this is an effective mechanism whereby warm arterial blood entering the testes from the abdomen is cooled by the venous blood leaving the testes. Testicular arteries originate from the abdominal aorta and elongate as the testis migrates into the scrotum [19]. In cattle and other large domestic ruminants these arteries are highly coiled, reducing several meters of vessel into as little as 10 cm of spermatic cord [19]. The arterial coils and venous plexus are complex structures that form during fetal life in cattle [19, 22]. Because of the pendulous nature of the bovine scrotum, testicular cooling is facilitated by the contraction and relaxation of the cremaster muscle, which draws the testes closer to the abdominal wall during cooler ambient temperatures and vice versa during warmer temperatures. Figure 2.1 shows bright‐field and thermal images of the bovine testes that demonstrate the change in temperature from the neck to the tip of the scrotum as the testes thermoregulate during elevated environmental temperatures. Scrotal and testicular thermoregulation is a complex process involving a number of local mechanisms that strive to maintain the testes at environmental and physiological conditions conducive for normal spermatogenesis. For additional reading on testicular thermoregulation in the bull the reader is referred to the review by Kastelic et al. [23] and Chapter 4 of this book.
Figure 2.1 Bright‐field and thermal images of two‐year‐old Hereford bull testes. The images were taken using an FLIR SC600 thermography unit (FLIR Systems Sweden, AB) on 25 June 2013 at midday with atmospheric temperature of 29 °C and humidity of 75%. (a) Bright‐field image of the testes. (b) Thermal image of the testes as seen in (a), identifying three regions of interest (RO1, RO2, and RO3) as indicated by the vertical bars along which temperature values were obtained by means of the software ThermaCAM Research Pro 2.7. The image is pseudo‐colorized with temperature scale bar to help visualize the change in temperature gradient from scrotal neck to scrotal tip. (c) Temperature values, minimum (Min), maximum (Max), the difference between Min and Max, average (Avg) temperature for each region of interest (RO), and standard deviation (St dev) which assess the variation of temperature in each region. Note the almost 4 °C change in average temperature from RO1 to RO3.
Interstitial Tissue (Leydig Cells)
Franz Leydig, a German zoologist, first described the interstitial cells of the testes in 1850 and these cells have since been known as Leydig cells. The Leydig cells reside in the interstitial tissue of the testis, a meshwork of loose connective tissue filling the spaces between the seminiferous tubules and blood vessels. In mammalian testes, the Leydig cells occur mainly as clusters in the angular interstices between the seminiferous tubules and are closely associated with the walls of small arterioles [8, 24]. The Leydig cell content of testes varies from species to species. The Leydig cells are thought to be the principal source of androgens in the testis. The development of the Leydig cells, via metamorphosis of mesenchymal precursor cells, has been observed to be continuous throughout life after the time of puberty in the bull [25]. Christensen [26] provides a very detailed and interesting review of the history of Leydig cell research dating from Leydig's description of the cells in the 1850s to the confirmation provided in the mid‐1960s that these cells were indeed primarily responsible for testicular androgen synthesis and secretion. There is extensive evidence to suggest that early fetal Leydig cells are steroidogenically active in some mammalian species including the pig [27] and sheep [28].
The Leydig cells of most mammalian species studied are basically similar, with some minor variations in appearance, size, and the relation of Leydig cell clusters to the lymph or blood vessels of the interstitial tissue. Some variation in the extent of cytoplasmic structures exists, but ultrastructurally Leydig cells show considerable overall similarity [8]. Fawcett et al. [29] described in detail the morphology of interstitial tissue of several mammalian species, and categorized three groups based on the abundance of Leydig cells and the relationship between volume of intertubular lymph structures and connective tissue. In the first group are the guinea‐pig and rodents (rat, mouse). In these species only 1–5% of the testicular volume is occupied by Leydig cells, for example 2.8% in the rat [30]. The bull, monkey, elephant, and human fall into the second group. In these species, the connective tissue of the interstitium is very loose and the Leydig cells are scattered throughout the interstitium and are closely associated with a well‐developed lymph system. The Leydig cells comprise only a small portion of the testicular volume (~15%) [8]. In the third group are the domestic boar and horse. In these animals there is abundant interstitial tissue packed with Leydig cells (20–60% of testicular volume) [31]. The reason for the high density of Leydig cells in these species is not known, but Parkes [10] and Fawcett et al. [29] have attributed this phenomenon to the vast amounts of estrogens produced by the boar and stallion and the large quantities of musk‐smelling 16‐androstenes secreted by the boar testes [32]. For more detailed discussions on the cytology of Leydig cells the reader is referred to an excellent chapter by de Kretser and Kerr [33].
Endocrine Function of the Testis
Hypothalamic–Pituitary Hormone Regulation of the Testis
Hormone action depends on the release of hormones from the appropriate endocrine gland and transportation via the vascular circulatory system to the target tissue where the hormone binds to cellular receptors, thus inducing a physiological response. In some cases these receptors are very hormone‐specific. The response at the target tissue may depend on the level of receptor expression and concentration of hormone. Some hormones regulate their own receptors, others may require synergism between two hormones, and others still may have their receptors regulated by other hormones [34]. The general characteristics of neuroendocrine regulation of the mammalian testis by the hypothalamic–pituitary axis are well established [35], but in some species this may be seasonally regulated. Domestic cattle are considered to be continuous or non‐seasonal breeding species [36]. However, there is information in the literature to indicate that domestic beef and dairy bulls have a functional hypothalamic–pituitary–gonadal axis that is seasonally regulated and thus may influence levels of gonadal steroidal and germ cell production. In a study using composite breeds of mature bulls, Stumpf et al. [37] observed that season of the year influenced the profile of gonadotropins in the circulation of both gonadectomized and intact males, and that the greatest secretion of gonadotropins occurred at the spring equinox. In addition, these authors observed that season of the year also influenced secretion of testosterone in intact males, but that more testosterone was released in response to LH during the time of the summer solstice. Others have reported similar effects, where location affected reproductive traits of bulls including semen quality and blood concentrations of LH and testosterone [38, 39]. In addition, sensitivity in responsiveness to exogenous gonadotropin administration and testosterone secretion in bulls was observed to be seasonally influenced [40].
The regulation of testicular function by hormonal mechanisms depends on the integrated actions of gonadotropins, such as LH, FSH, and prolactin, and steroids (androgens and estrogens) on the Leydig cell [41]. Gonadotropin‐releasing hormone is the primary hypothalamic hormone governing regulation of the synthesis and release of the gonadotropins LH and FSH by the anterior portion of the pituitary gland. LH is primarily responsible for testosterone production by the Leydig cells, while FSH facilitates