Anatomy of the male, female, and everything in between (Proceedings)

2009-11-01
Walter Threlfall, DVM, MS, PhD, ACT

The testicles in the stud dog are housed in the scrotum, a skin sac located between the thighs.

Anatomy of the stud dog

The testicles in the stud dog are housed in the scrotum, a skin sac located between the thighs. The dartos, a smooth muscle and fibrous tissue layer, acts as a cutaneous muscle and also forms the septum. The scrotum is important as it houses the testicles outside of the body, and enables the dog to regulate the temperature of the testicle through contraction of the dartos. Contraction of the dartos causes the skin of the scrotum to wrinkle.

The testicles themselves are ellipsoid and lie within the scrotal cavity with the long axis directed craniocaudally. They are tilted so that the cranial end is most ventral. Several layers of fascia, the most important being the tunica albuginea, surround the testicles. This is a thick fibrous capsule. The function of the testicle is to produce spermatozoa. This is accomplished by the seminiferous tubules. The tubules empty into a network of ductules, called rete testes and are eventually transported to the epididymis. In order to produce spermatozoa, the testicles must be at a temperature lower than the normal body temperature of the dog as mentioned before. The testicle is formed embryologically inside the body and migrates through an opening in the inguinal region of the abdominal wall. If this does not occur, the testicle does not produce spermatozoa and the dog is termed a cryptorchid.

Once the spermatozoa have been produced within the testicle, they move to the epididymis. The epididymis is basically a long tube that stores and transports the maturing spermatozoa. It is divided into a head, body, and a tail. The head of the epididymis is firmly attached at the cranial end of the testis. The body lies on the dorsolateral surface, while the tail is attached to the caudal end of the testis. It is important to know the orientation of the epididymis for the diagnosis of testicular torsion and other reproductive problems.

The tail of the epididymis gradually transforms into the ductus deferens. The ductus deferens ascends as a component of the spermatic cord. Other structures housed within the spermatic cord include the testicular artery, the sympathetic nerve plexus, and lymphatics. The visceral vaginal tunic surrounds these structures. Together the testicular artery and veins constitute a counter current exchange system for simultaneously conserving body heat and keeping the testicles cool. The spermatic cord enters the abdomen through the inguinal canal. The vaginal process, internal spermatic fascia, cremaster muscle, external pudendal vessels, and the genitofemoral nerve pass from inside to outside through this opening as well. The cremaster muscle is especially important as it provides yet another method of temperature regulation for the testicles, as well as a means of protection. The cremaster, when contracted, pulls the testes and other contents of the fibrous tunic closer to the body where they are less pendulous and vulnerable during physical activity. The inguinal canal itself consists of the deep inguinal ring, the inguinal ligament, and the superficial inguinal ring.

The dog has only one accessory sex gland, the prostate. A median septum divides the gland into right and left lobes, which are then further divided by septae into lobules. The prostatic capsule and septae contain smooth muscle fibers that contract to expel prostatic fluid during ejaculation. The prostate is located within the pelvic canal. It surrounds the proximal end of the urethra, caudal to the neck of the bladder and positioned between the rectum and symphysis pubis. Disseminated prostate gland is present surrounding the pelvic urethra. The pelvic urethra itself lacks smooth muscle, but is rich in elastic tissue.

The penis of the dog is composed of numerous venous sinuses enclosed within a fibro elastic capsule termed the tunica albuginea. Engorgement of the venous sinuses with blood increases internal pressure, which stretches the fibro elastic wall, rendering it and the penis turgid. The caudal portion of the glans penis is called the bulb glandis. This portion of the penis continues to swell following copulation, and its slow collapse after ejaculation, is responsible for the prolonged "tie" common between mating dogs. The penis contains a bone surrounding the penile urethra. This is termed the os penis. The paired retractor penis muscles run superficially along the caudal ventral surface of the penis and insert on the distal end of the body of the penis.

The penis is housed within the prepuce. The prepuce has an outer layer of skin and forms the preputial cavity. During erection, the penis protrudes through the preputial orifice. The internal lamina pulls away from the preputial wall and coats the caudal end of the free penis like skin. Fascicles of cutaneous trunci, called preputial muscle, leave the parent muscle and insert on the lateral and ventral wall of the prepuce to assist return of the penis into the prepuce.

Spermatogenesis

At the level of the seminiferous tubule, spermatogonia (the cells that give rise to mature sperm) are distributed around a basement membrane and surrounded by Sertoli cells. The Sertoli cells provide nutrition and an appropriate environment for developing sperm. The stem cells can undergo many divisions to produce more stem cells as well as to produce differentiating germ cells, which are destined to become mature sperm. The initial differentiated cells are called primary spermatocytes, which undergo the first meiotic division to become secondary spermatocytes. During this process, meiosis, cell divisions occur without duplication of genetic material, thus resulting in cells with one half the chromosomal material of normal somatic cells. The purpose of this process is to produce germ cells that can produce an embryo with the normal amount of DNA following fertilization. With the completion of the second meiotic division, the spermatids have the haploid set of chromatids.

Once the spermatid is formed, many morphologic changes occur. These changes include development of a flagellum (tail) and acrosome, elimination of excess cytoplasm, and chromatin condensation. These unique changes to the cell ultimately make it capable of independent streamlined movement, penetration of the egg membrane and surrounding structures, and delivery of the genetic material for completion of fertilization. The fully differentiated cell is the spermatozoan. Spermatogenesis is completed at spermiation or the release of the spermatozoa into the lumen of the seminiferous tubule. Sperm maturation, however, is not completed until passage through the epididymis occurs.

Within the testicle, spermatogenesis is regulated such that spermatozoa are continually produced as opposed to being produced in "batches." This is accomplished by staggering, in time and space, the initiation of the cells into the differentiated pool. At a given location within a seminiferous tubule, there are several germ cells that are in specific maturational association. There are several stages that occur throughout the length of the seminiferous tubule and over time at a particular location. The spermatogenic wave is the phenomenon as it occurs down the length of the tubule. In the dog, it takes 13.8 days for a location in the seminiferous tubules to contain the same stage again; i.e. the cycle is 13.8 days. It takes approximately 48-50 days for spermatogenesis, from start to finish, and thus takes approximately 3.5 cycles to produce a spermatozoan. The take home point is that sperm production is continual and not sporadic.

Although spermatogenesis is completed in the testicle, sperm maturation is not achieved until the spermatozoa pass through the epididymis. In the canine, this takes approximately 12-14 days. During this time, the sperm have the ability to be motile, suppressed, then returned, and the cytoplasmic droplets are eliminated. All in all, form start to ejaculation of mature sperm; the process takes about 62 days in the dog.

Anatomy of the bitch

The vulva is positioned caudoventral to the ischial arch. It consists of a left and right labium which is composed of fat connective tissue / smooth muscle / striated constrictor vulvae muscle. The dorsal and ventral labial commissures are the fusion lines of the labia. The rima pudendi is the opening common to both urinary and genital systems.

The clitoris is the homologue of the male penis). It is located ventral and cranial to the labia in the vestibule. The glans is erectile tissue, the body is fat within a fibrous capsule, and the paired crura consist of fibrous tissue surrounding a small core of erectile tissue. Each crus attaches to the ischial arch and is covered ventrally by a small ischiocavernosus muscle. The fossa clitoridis is the caudal extent of the clitoris, located at the ventral labial commissure and visible when the vulva cleft is parted.

The vestibule is the common urogenital chamber and extends from the vulva to the vagina. The urethral opening is located cranially in the vestibule. The wall of vestibule consists of the striated constrictor vestibule muscle, minor vestibular glands, and bilateral vestibular bulbs. The vestibular bulbs are masses of erectile tissue that are homologous to the bulbs of the penis.

The constrictor vestibuli and constrictor vulvae are two muscles of considerable importance in the bitch in that they assist with the "tie". These striate muscles that encircle the vestibule and vulva comprise the bulbospongious muscle. Constriction of the penile veins by the constrictors vulvae and vestibuli enhance erection in the dog.

The vagina is located between the vestibule and the uterus. The longitudinal and transverse folds (RUGAE) in the vagina allows for vaginal expansion. The fornix of the vagina is the cranial extent of vaginal lumen. Located at the cranial vaginal wall is the cervix. The orifice is normally small. The cervix is short, greatly thickened by accumulation of circular smooth muscle.

The uterus consists of a body and two horns. The body of the uterus is short and bounded caudally by the cervix and cranially by the two horns. The uterine horn courses along the lateral abdominal wall. The intercornual ligament connects the two cornua near this junction with the uterine body. The horns are bordered medially by the descending colon on the left side and the jejunum on the right side.

The oviducts are small in diameter and divided into three segments. They serve to convey ova released by the ovary to the uterus. Also, fertilization takes place within the oviduct. The three portions of the oviduct are: the infundibulum, the ampulla, and the isthmus. The infundibulum is funnel-shaped and opens into the ovarian bursa near the slit between bursa and peritoneal cavity. The fimbria are extremely small finger-like processes on the free edge of the infundibulum that partially protrude from the bursa into the peritoneal cavity. The ampulla is where fertilization occurs. The isthmus is the portion which joins to the uterus.

The ovary is located just cranial to its corresponding uterine horn and close to the caudal end of its respective kidney. It is encased in an ovarian bursa. The ovaries are ellipsoid in shape and the surface appearance varies with the reproductive status of the animal. The proper ligament of the ovary connects the ovary to the uterine horn. The suspensory ligament of the ovary extends to the dorsolateral abdominal wall at the level of the last rib.

The broad ligament is the connection between the reproductive tract and the body wall. The mesometrium extends from the uterus and cranial vagina to the dorsolateral abdominal wall and lateral pelvic wall. The mesosalpinx is a lateral fold of mesovarium and extends ventral and medial to the ovary. The mesovarium connects to the ovary and at the cranial border is the suspensory ligament.

Select canine anatomic abnormalities

Cryptorchidism

If a testicle is housed anywhere except within the scrotum, the animal is considered a unilateral cryptorchid, The testis is incapable of producing sperm due to the increased temperature. It is believed to be inherited in the canine due to the proven genetic links existing in other species. If both testes are retained with in the body there may be some difficulty in the confirmation of the condition if the historical information regarding castration is lacking unless diagnostic testing is performed. Observation of the scrotum for surgical scars can be done but may be incorrect.

The best method to diagnose the presence of one or both testes remaining in the inguinal canal or abdomen: is to use the hCG stimulation test. The retained testicle is incapable of the production of sperm but retains its capability to produce testosterone. A blood sample obtained prior to the injection of hCG and a second sample 4 to 24 hours later.is diagnostic for the presence of a retained testicle if there is a two or more-fold increase in testosterone. The treatment for cryptorchidism is surgical castration.

This is the debate as to whether an inherited defect should be corrected hormonally or surgically. However, since the treatment of this condition hormonally is usually unsuccessful, it greatly reduces the concern regarding treatment. Administration of 50 ug of GnRH or 500 to 1000 international units of hCG intramuscularly every three days for two to four weeks has been recommended (does not work).

If the testicle is in the inguinal canal or located subcutaneously dorsal to the scrotum, it may descend on its own. However, testicles located within the abdominal cavity after the first few weeks of life are probably destined to remain these due to closure of the internal inguinal ring. Surgical repair is an unethical procedure and should not performed. Surgical placement of prosthetic material into the scrotum to replace a retained testicle should also not be performed.

Hermaphroditism

Hermaphroditism is a developmental abnormality of the reproductive tract. A true hermaphrodite has both testicular and ovarian tissue. Pseudohermaphrodites can be male or female. The males have testes but female external genitalia. The XX sex reversal syndrome has a genetic sex, which is opposite to the gonadal sex. They have testes and masculinized external genitalia. This one is most common. The XY testicular feminization syndrome has testes but normal female external genitalia. The testosterone is not converted to dihydrotestosterone, which is necessary for masculinization of external genitalia. The behavior is usually male. The female hermaphrodites have ovaries but male external genitalia.

Estrous in spayed bitches

Spayed bitches may have a heat cycle for possibly three endogenous reasons. First, it could be induced by estrogens and progesterone produced by the adrenal or the presence of a third ovary. The adrenal is capable of inducing estrus in some species but has not been reported in the canine. Nor has the presence of a third ovary. However, one should always look just in case. The second reason is referred to as the ovarian remiment syndrome. This is due to the failure of all ovarian tissue to migrate to the site of the ovary and some tissue remains in the mesovarium and is capable of producing follicles and estrogens and progesterone. The third reason is due to the failure of the entire ovary to be removed during the ovariohysterectomy procedure. One should always consider exogenous estrogens as a possibility of induced estrus before going forward with diagnostic procedures or surgery.

The useful diagnostic tests include vaginal cytology, estrogen (estradiol) concentration, hCG (diagnostic test), progesterone concentration. and exploratory surgery.