CHAPTER 94 Reproductive Physiology and Endocrinology of Boars
Modern swine operation viability depends on herd productivity and profitability. Herd productivity generally is assessed by reviewing herd reproductive performance and, in particular, performance of females in the breeding herd. Although gilts and sows constitute well over 95% of the breeding herd, the fact that overall reproductive performance is shared equally with the boars should not be overlooked. For this reason it is important to understand the underlying determinants of reproductive performance as they relate to boars used for natural service and those used for artificial insemination (AI).
The prenatal testis develops in the fetal abdomen, near the embryonic kidney, and migrates caudally through the inguinal rings as it passes into the scrotum. In the pig, both testes should be fully descended at birth. Failure of one or both testicles to enter the scrotum results in cryptorchidism, a condition in which the retained gonad is capable of full endocrine function, although spermatogenesis is impaired owing to the thermal environment. Sperm production appears to be closely correlated with the number of Sertoli cells present in the testis, with a fixed ratio of spermatids to Sertoli cells being reported.1 Sertoli cell proliferation in pigs is marked by two distinct phases, the first encompassing the period between birth and 1 month of age, and the second between 3 and 4 months of age.2 Depending on the animal’s genetics, steroid production from the Leydig cells of the testis and its subsequent effect on behavior (e.g., mounting pen mate, aggressiveness) can be apparent as early as 1 month of age onward.
Puberty can be thought of a complex process leading up to full sexual maturity and is accompanied by distinct changes in the physiology and anatomy of the reproductive system.3 The penis of the prepubertal boar lacks a sigmoid flexure and is attached to the lining of the prepuce by the penile frenulum. Under androgen influence, penis growth occurs and the boar’s physical mounting activity increases; these events lead to the development of the sigmoid flexure and frenulum breakdown, respectively. Boars as young as 4.5 to 5 months of age can have spermatozoa present in the epididymis, but adequate libido for mating and ejaculation generally is not displayed until after 5 months of age.4 Age at attainment of puberty can vary greatly between breeds and genetic lines.
The boar has a fibroelastic penis with a sigmoid flexure along its shaft and a characteristic corkscrew-shaped glans penis (Fig. 94-1). The boar’s penis is retracted into the prepuce while at rest. The boar’s prepuce runs medially along the distal third of the abdomen, with its wider cranial portion tightly attached to the ventral abdomen suspending the narrower caudal portion of the prepuce. It is normal for boars to maintain the penis in a retracted state and to display a jerking motion of the penis or prepuce during urination. In the dorsal aspect of the cranial portion of the preputial cavity, the boar has a unique structure known as the preputial diverticulum (see Fig. 94-1). This cavity collects urine, semen, and other fluids and harbors a large bacterial population. Contraction of the cranial preputial muscles in concert with mounting activity appears to empty the contents of the preputial diverticulum onto the shaft of the penis, providing lubrication for intromission. The glabrous, nonpendulous scrotum of the boar is located immediately ventral to the anus and contains a prominent medial raphe. Variation between boars in scrotal attachment may predispose some boars to be more susceptible than others to disruptions of spermatogenesis during the hot season of the year, especially those boars with testes held close to the body wall.
The testes are the site of spermatogenesis and hormone production. Boar testis weight has been shown to increase most dramatically between 4 and 6 months of age, reaching mature size by 8 months.4 A recent study using B-mode ultrasound examination to measure testis diameter in vivo confirmed that in crossbred boars a plateau in testis growth is achieved by 8 months of age, with only a 2-cm difference in testicular diameter for boars 8 months to 2 years old.5 The testes should by symmetrical and relatively uniform in size and have a distinct firmness to digital palpation. The epididymis functions as a site of sperm transport, maturation, and storage. The caudae epididymidis are palpable as distinct nodules located on the dorsal aspect of the testis within the scrotum. Proper location and identification of the caudae epididymidis are necessary in order to successfully perform epididectomy surgery in young pigs, a technique commonly used to create sterile teaser boars for estrus detection programs. The ductus deferens is the conduit used for passage of sperm from the cauda epididymidis to the pelvic urethra at the time of ejaculation. During vasectomy, sterility can be induced via ligation and segmental excision of a portion of the ductus deferens. The spermatic cord contains the ductus deferens and also serves as a vascular, lymphatic, and neural connection from the testis to the rest of the body. The scrotum, pampiniform plexus, and the cremaster muscle all aid in autonomic control of core testis temperature.
The boar reproductive tract contains accessory sex glands that contribute the seminal plasma portion of the semen (see Fig. 94-1). The vesicular glands are large, diffuse, lobulated structures, which contribute the bulk of the boar’s ejaculate fluid volume. The prostate gland is much smaller relative to the other accessory glands and is located between the pelvic urethra and the ventrocaudal portion of the vesicular glands. The bulbourethral glands are large paired structures in the boar that straddle the internal pelvic inlet and produce the large gel fraction characteristic of boar semen.
The reproductive endocrinology of the boar is discussed fully elsewhere6; a brief review is presented here. The pattern of gonadotropin-releasing hormone (GnRH) release from the hypothalamus stimulates the release of follicle-stimulating hormone (FSH) or luteinizing hormone (LH), or both, from the anterior pituitary gland. Sertoli cell function is regulated primarily by FSH. Sertoli cells are the only somatic cells found in the seminiferous epithelium and form the specialized blood-testis barrier. Sertoli cells are intimately involved in spermatogenesis, in addition to producing the hormones inhibin and estrogen. Inhibin provides the necessary feedback to the anterior pituitary to regulate FSH secretion. Estrogens influence sexual behavior and appear to control critical events during sperm maturation in the epididymis.7 Leydig cells are the primary interstitial cells of the testis, and produce androgens under the influence of LH. Testosterone, the primary androgen produced, is necessary for normal sperm production and maintenance of the accessory sex glands and is involved with libido. Sexually excited boars chomp their jaws, eliciting the production of copious quantities of foamy saliva containing pheromones that stimulate sexual receptivity in estral gilts and sows. Active components in the saliva include the androgen metabolites 3α-androstenol and 5α-androstenone.
Spermatogenesis is the process by which highly specialized spermatozoa are formed from primitive male germ cells. Spermatocytogenesis is the initial phase of spermatogenesis and consists of a series of mitotic divisions of the spermatogonia, ending in the production of primary spermatocytes. Primary spermatocytes undergo meiosis to produce haploid spermatids known as secondary spermatocytes. During spermiogenesis, haploid spermatids undergo dramatic morphologic transformations to become fully differentiated spermatozoa. After completion of spermatogenesis, spermiation ensues, which is the release of spermatozoa into the lumen of the seminiferous tubule. Sperm cells then undergo a final process of maturation in the epididymis in order to acquire motility and potential fertility. The cauda epididymidis also functions as the sperm storage reservoir until the time of ejaculation. Spermatogenesis takes approximately 35 days in the boar, with an additional 9 to 12 days needed for epididymal passage and maturation before becoming the fertile gametes emitted during ejaculation.8–10 Therefore, it is not unrealistic to observe a 6- to 7-week lag time before a normal spermiogram can again be expected following a disruption in spermatogenesis.