PRENATAL DEVELOPMENT

Sex determination in mammals occurs in three stages: the establishment of chromosomal sex, the development of primary sex characters (the gonads), and the subsequent development of secondary sex characteristics under the influence of gonadal hormones.

During embryogenesis, the gonad first arises from the mesonephros as undifferentiated tissue, which has the capability to subsequently develop in either male or female form. The precursors to the male reproductive tract system (i.e., the wolffian duct system) and the female tract (the müllerian duct system) are both present. Subsequent sexual differentiation is decided, in mammals, by the presence or absence of the Y chromosome, with females being XX and males XY. The presence of a Y chromosome results in male development, regardless of the number of X chromosomes present. Thus, the Y chromosome must contain the dominant inducer of testis formation; the testis determining gene (TDF or SRY). SRY is activated early in embryogenesis to commit the undifferentiated genital ridge to the testicular pathway. The early testis produces both testosterone (T; from the Leydig cells) and müllerian inhibiting substance (MIS; from the Sertoli cells). The latter induces the müllerian ducts to regress. Subsequent hormonal production induces male sexual differentiation. In the bovine, the differentiating gonad may be identified as a testis by 41 days after conception, with testosterone production (from fetal Leydig cells) evident soon thereafter.² By 3 to 4 months following conception, the testes have generally passed through the inguinal canal and entered the scrotum, which is derived from the urogenital folds. Although the basic components of a functional male gonad are present at birth, the spermatogonia do not undergo meiosis until the onset of spermatogenesis at puberty. Thus, the basic structure of the testis (seminiferous cords and interstitial tissues) remains much the same from early fetal life until the onset of puberty.

REPRODUCTIVE ANATOMY

The reproductive organs of the mature bull include paired testes, each with a spermatic cord, an epididymis, and a deferent duct (ductus deferens), which culminates in an ampulla.³ In addition are paired vesicular glands, a prostate gland, paired bulbourethral (Cowper’s) glands, and a fibroelastic penis, which incorporates a sigmoid flexure (Fig. 30-1).⁴ The vesicular, bulbourethral, and prostate glands are often referred to as the accessory sex glands.

Fig. 30-1 Reproductive organs of the bull.

(From Amann RP: How a bull works. Proceedings of the 11th Technical Conference on A. I. and Reproduction (NAAB), 1986, p 7.)

The testes are suspended within the scrotum, a feature that is important for testicular thermoregulation, as discussed subsequently. Within the testis, most (70% to 90% by weight) of the parenchyma is composed of seminiferous tubules (Sertoli cells and layers of germ cells). The remainder consists of interstitial tissue (Leydig cells, blood and lymph vessels, and connective tissue). The mediastinum, an area of connective tissue extending lengthwise in mid-testis, contains blood vessels and tubules of the rete testis. The testicular parenchyma is encased in a thick, connective tissue capsule (tunica albuginea), which is, in turn, covered by a thin, serous membrane (tunica vaginalis propria).³

In general, the spermatogenic efficiency of healthy testicular parenchyma in bulls is remarkably consistent (approximately 10–12 × 10⁶ spermatozoa per gram daily). As testes weight is highly correlated with scrotal circumference (r = 0.91 to 0.98 in young beef bulls¹), this highly repeatable measure has gained wide acceptance as an estimate of sperm-producing capability, especially as testicular size is heritable in beef bulls (h² ∼ 0.5).¹ Although breed and environment may also influence testicular development, most variation in daily sperm production can be attributed to testicular size (represented by scrotal circumference), at least in younger (<4–5 years old) bulls.

Sertoli cells are amorphous, nucleated somatic cells that span the seminiferous epithelium and play a critical role in supporting and controlling germ cell development.⁴ Specialized junctions between adjacent Sertoli cells form the blood-testis barrier, dividing the seminiferous epithelium into basal and adluminal compartments.⁵ Spermatogonia lie between Sertoli cells and the tubule basement membrane, and other germ cells are located either in cytoplasmic crypts within Sertoli cells or sandwiched between adjacent Sertoli cells.

Spermatozoa are produced within the seminiferous tubules. Both ends of each tubule form tubuli recti (straight tubules), which join the rete testis (a complex of anastomosing spaces within the mediastinum testis). Spermatozoa pass from the rete testis to the head of the epididymis via the efferent ducts (n = 6–20). The epididymis, an elongated, torturous duct extending from the rete testis along the medioposterior border of the testis, comprises the head (caput), body (corpus), and tail (cauda) regions. Epididymal functions include sperm transport and maturation, as will be discussed.

At birth, the bull penis is short and slender and lacks a sigmoid flexure, and its apex is fused to the inner lining of the prepuce. With time (and under the influence of androgens), penile and preputial tissues separate, the penis elongates, and a sigmoid flexure develops. Tissue separation proceeds irregularly and in many bulls is completed only after the onset of erectile activity. Thereafter, incomplete separation is defined as a persistent penile frenulum (otherwise known as a persistent raphe or tied penis). This condition, most commonly present in Angus, Beef Shorthorn, Hereford, Polled Hereford, and Beefmaster bulls, probably has a genetic basis in many cases.⁶ Although this condition is usually correctable with minor surgery, the consequences of perpetuating a genetic defect should be considered when this is done.

The prepuce is a double invagination of skin, with its internal lining everting upon penile erection to constitute much of the penile surface.³ A fan-shaped protractor prepuce muscle raises and lowers the distal portion of the prepuce and also controls the size of the preputial opening. Retraction of the membrane lining the inner prepuce is under the control of the retractor prepuce muscle. Lack of development of this muscle (a condition genetically linked with the polled gene in bulls), predisposes to chronic eversion of this membrane with increased risk of traumatic injury.

The development and normal function of the accessory glands depend upon the effects of androgens. Castration results in marked depression of both development and secretory functions of these glands.

EPIDIDYMAL FUNCTIONS

The epididymis is far more than a passive organ, with functions including both sperm transport and maturation. Spermatozoa leaving the testis lack both the ability to survive in the female tract and to achieve unassisted fertilization. These capabilities are acquired in the epididymis. Other epididymal functions include the (1) energy-efficient storage of sperm while maintaining sperm fertility; (2) intermixing of recently formed and older spermatozoa to provide a temporal spectrum of optimal sperm function, and (3) changing sperm attributes and environment to permit survival and ensure fertilizing capability within the female tract.⁷

Immotile spermatozoa are carried into the lumen of the seminiferous tubule following separation of their connections to Sertoli cells. Most of the residual cytoplasm is retained by the Sertoli cell, although some remains attached to the spermatozoa (as cytoplasmic droplets). Initial transport into the rete testis and caput epididymidis seems to be dependent upon fluid secreted by Sertoli cells. Once into the efferent ducts, sperm movement is facilitated by ciliated epithelial cells (within the ducts) as well as by smooth muscle contractions. The efferent ducts and initial segment of the caput epididymidis resorb most fluid and protein emanating from the testis and secrete new compounds.

Sperm maturation occurs within the caput and corpus of the epididymis with this process requiring the coordinated secretion of specialized enzymes and proteins. During this process, changes occur in the sperm DNA-protein complex, plasma membrane, mitochondria, axonemal complex, plasma and acrosomal membranes, and sperm surface characteristics.⁷

As spermatozoa progress through the epididymis, they achieve progressive motility, the cytoplasmic droplet migrates from the proximal to the distal position on the sperm midpiece, and seminal fluids are resorbed and exchanged. Heat stress can adversely affect epididymal function. The epididymis, particularly the tail region, acts as a storage region for sperm, such that a mature Holstein bull may have epididymal reserves representing the equivalent of 6 or 7 days of daily sperm output. Most sperm that are not ejaculated are voided in urine, with a small proportion resorbed by the male tract. Some evidence exists for selective resorption of spermatozoa within the epididymis.

Sperm are transported from the cauda epididymis to the urethra in the ductus deferens (vas deferens) via muscle contractions that are strongest during precoital stimulation. The terminal portions of the ductus deferens expand to form the ampullae (each approximately 10 × 1.5 cm in adult bulls). These ampullae act as minor sperm storage areas and they also secrete fructose and citric acid into the seminal plasma. The ductus deferens open (via the ampullae) into the cranial portion of the pelvic urethra. The vesicular glands (or seminal vesicles) also open into the pelvic urethra. These glands provide much of the fluid component of the bull ejaculate, as well as sperm nutrients and semen buffers. The vesicular glands are lobulated organs, approximately 10 to 15 cm in length and 2 to 4 cm in diameter in mature bulls. The prostate gland, consisting of a relatively small body and larger disseminate region, produces 25% to 40% of seminal volume as well as semen odor. The bulbourethral glands of the bull each open into the pelvic urethra at the ischial arch. The urethra is an elongated tube extending from the bladder to the tip of the penis. It is surrounded by the urethralis muscle, which contracts strongly during ejaculation.

SCROTAL TESTICULAR THERMOREGULATION

Scrotal testicular thermoregulation has been recently reviewed.⁸ Testicular temperature in bulls must be 2 to 6° C cooler than core body temperature for effective production of fertile spermatozoa. Several mechanisms act to regulate the testicular temperature. Of major importance is the pampiniform plexus, a complex venous network that surrounds the highly coiled testicular artery within the neck of the scrotum. This entire structure (vein and artery) is properly called the testicular vascular cone.⁸ The cone functions as a countercurrent heat exchanger whereby heat is transferred from arterial to venous blood. Scrotal skin is thin and relatively hairless, with extensive blood vessels that can dilate to increase heat loss. The testes and scrotum have complementary temperature gradients that result in a nearly uniform intratesticular temperature.⁸ Other mechanisms that help to cool the testes include relaxation of scrotal muscles, scrotal sweating, and whole-body responses (e.g., panting and peripheral vasodilation). Spermatogenesis occurs in an environment that verges on hypoxia. When testicular temperature increases, spermatogenic metabolism increases faster than does blood flow, and hence the testes become hypoxic.⁸ Therefore, testicular function is very susceptible to temperature increases due to either endogenous or exogenous factors (e.g., fever or high ambient temperatures, respectively). Increases in testicular temperature cause increased production of defective spermatozoa. The proportion of defective spermatozoa and the time required for recovery depend upon the nature and duration of the thermal insult.⁸ Severe insult may cause irreversible spermatogenic damage.