I. EMBRYOLOGY AND ANATOMY OF THE MALE REPRODUCTIVE TRACT

Determination of gender is dependent on the type of sex chromosomes present in the embryo. All dogs have 78 chromosomes, two of which are sex chromosomes; in females both are X chromosomes, and in males there is one X and one Y chromosome. Two tubular tracts are present in the developing embryo: the müllerian (paramesonephric) duct system, which goes on to form the female reproductive tract, and the wolffian (mesonephric) duct system, which goes on to form the tubular portions of the male reproductive tract.

In the presence of a Y chromosome, the indifferent gonad is stimulated to form a testis. A gene on the Y chromosome, called the SrY gene, encodes for production of a protein, sometimes called testis-determining factor, which stimulates formation as a testis. The testis secretes testosterone and müllerian-inhibiting factor. Testosterone is metabolized to a similar compound, dihydrotestosterone. Both these compounds must be present for normal development of the male reproductive tract to occur. Müllerian-inhibiting factor prevents development of the female ductal system. The mesonephric ducts form the epididymis and vas deferens. The genital tubercle forms the penis. The genital swellings close to form the scrotum.

The reproductive anatomy of the male dog consists of the paired testes-epididymes within the scrotum, the vasa deferentia, the prostate, the penis, and the prepuce (Figure 22-1). The testes consist of multiple seminiferous tubules in which spermatogenesis takes place. The seminiferous tubules empty into a fibrous center space, the rete testis. The rete testis connects to the head of the epididymis. The testicular tissue is surrounded by a tight capsule, such that the testes have the consistency of a peeled hard-boiled egg when palpated. An epididymis is tightly adhered to each testis. Fluid and spermatozoa from the testis enter the head of the epididymis; move through the body of the epididymis, which lies laterally on each testis; and are stored in the tail of the epididymis.

Figure 22-1 Anatomy of the male reproductive tract. A, Penis; B, urinary bladder; C, prostate; D, testis; E, epididymis.

The tail of the epididymis becomes the vas deferens, the tubule running from the epididymis to the urethra in the spermatic cord. Other tissues within the spermatic cord include the testicular arteries and veins. The vas deferens on each side empties into the urethra at the level of the prostate through multiple small openings collectively called the seminis colliculus. The prostate encircles the urethra at the neck of the urinary bladder.

The urethra runs through the center of the penis. It is surrounded throughout its length by cavernous tissue. Cavernous tissue is an empty honeycomb space within the penis that fills with blood during erection. The urethra also is surrounded by part of its length within the penis by a bone, the os penis (Figure 22-2). The os penis allows the male to introduce the penis before erection is complete; this allows the extreme increase in size of the penis to occur within the vagina, permitting formation of the copulatory lock, or tie.

Figure 22-2 Anatomy of the canine penis. A, Os penis; B, urethra; C, cavernous tissue.

The proximal portion of the penis, which always is larger than the rest of the shaft of the penis, is the bulbus glandis. This tissue will treble in size when the penis is erect and is the portion of the penis that is caught within the vulvar lips during the tie that usually occurs during breeding (Figure 22-3). The tip of the penis contains the external urethral orifice, visible as a small opening or slit. When the penis is erect, the tip takes on the appearance of a thick leaf; it is thought that this shape helps promote ejaculation of spermatozoa toward the cervix of the female during natural service. The prepuce is a piece of haired skin that encloses the flaccid penis at rest.

Figure 22-3 Normal erect canine penis. Note engorged proximal portion, the bulbus glandis.