Both sexes have a pair of gonads (ovaries or testes). However, in birds the right ovary does not develop; only the left ovary and oviduct are present in adult females. The main functions of the gonads are gametogenesis and steroidogenesis. Both functions are regulated primarily by gonadotropins, follicle-stimulating hormone (FSH), and luteinizing hormone (LH), released by the anterior pituitary gland under the influence of the hypothalamus. Hypothalamic control of the pituitary is mediated by a peptide, gonadotropin-releasing hormone (GnRH); the secretion and release of GnRH are governed by CNS stimuli and, through a feedback mechanism, by hormones produced by other endocrine organs such as the gonads, pituitary, thyroid, and adrenal glands. (Also see Endocrine System Introduction, et seq.)
The size and location of the ovaries vary with the species. The ovaries can be directly examined by ultrasonography or by palpation per rectum only in larger animals (eg, cows, mares, and camelids). In small ruminants and companion animals, ovarian function can be monitored by transabdominal ultrasonography, endoscopy, or hormone assay. Once puberty is reached (see Table: Features of the Reproductive Cycle), the size and form of the ovaries are altered by cyclic functional structures, namely the corpus luteum (CL) and ovarian follicles.
FSH is responsible for development of ovarian follicles and synthesis of estrogens. Once a critical estrogen level is attained, LH is released from the anterior pituitary gland in spontaneously ovulating species. For induced ovulators like the alpaca, cat, and rabbit, the LH surge and subsequent ovulation are triggered by physical or chemical factors associated with copulation. In either case, LH interacts with follicular receptors to initiate ovulation, which is followed by development of a new CL. The increase of luteal cells parallels an increase in progesterone output from the CL. In nonpregnant polyestrous and seasonally polyestrous females, the functional and morphologic life of the CL is terminated by endogenous prostaglandin F2alpha from the uterus. As the CL regresses, a new ovulatory follicle(s) develops, which completes the estrous cycle.
The hormonal changes during the estrous cycle can be monitored by laboratory assay of hormones in blood, milk, or feces. Estrual cycling is continuous after puberty unless interrupted by pregnancy and, in some species, by season or by lactation in the immediate postpartum period. Cycling may also be blocked by pathologic conditions of the ovaries (eg, nutritional atrophy or ovarian cysts) as well as by uterine disease (eg, pyometra), which may result in persistent luteal function. Estrogens and progesterone act locally, affecting target organs such as the tubular genital tract, and distally, regulating gonadotropin release by a feedback mechanism on both the hypothalamus and anterior pituitary. In addition, they play a prominent role in sexual behaviors, lactation, and the development of secondary sex characteristics.
The testes function in both spermatogenesis and secretion of steroid hormones. Spermatogenesis is stimulated by FSH and augmented by androgens, primarily testosterone. Interstitial cells of the testes, under the influence of LH, produce testosterone. Testosterone and its metabolites are required for development and function of accessory glands, copulatory organs, male sex characteristics, and behavior. The testes of birds are permanently retained in the abdominal cavity. For optimal spermatogenesis, mammalian testes must descend into the scrotal cavity; however, steroidogenesis occurs in testes that remain within the abdomen, and the libido of cryptorchid males is usually not impaired. Photoperiod affects both sperm cell formation and steroidogenesis in males of species that have a seasonal reproductive pattern. Semen quality, libido, and mating ability are reduced during the seasonally anestrous period of females. Function of the testes can be assessed by evaluation of representative semen samples and hormone assays. Examination and measurement of the testes help predict potential sperm output and may reveal pathologic conditions.
Except for the vestibule, which develops from the urogenital sinus, the female genital tract is derived from the embryonic paramesonephric ducts. Each of the segments is adapted to fulfill its function. The oviduct acquires the egg(s) and moves the resulting zygote(s) into the uterus, while its secretion provides a proper environment for survival of gametes, fertilization, and the first few critical days of embryonic life. Interference with motility or secretion leads to infertility. Species variation of the bicornual, Y-shaped uterus involves the size of the body and length of horns. These adaptations accommodate the species-specific number and form of fetuses and placentas. The cervix provides a protective barrier that is relatively effective against ascending infections. Morphologic and functional integrity of the uterus and cervix are required to establish and maintain pregnancy and for parturition.
Infections contracted at mating or during parturition and the puerperium are common causes of female infertility. Applicability of diagnostic methods for detection of uterine and cervical abnormalities depends on species, size of the animal, and anatomy of the cervix. Clinical diagnosis is by transrectal and abdominal palpation, vaginoscopy, hysteroscopy, radiography, and ultrasonography. In small animals, laparotomy or laparoscopy may be necessary to assist with diagnosis. Laboratory diagnostic aids include microbiologic and cytologic examination of exudate or secretion, histologic examination of biopsies, endometrial cytology, and hormone assays.
The posterior tract, consisting of the vagina, vestibule, and vulva, serves as the copulatory organ and as the last segment of the birth canal. It also provides a pathway for ascending infections, particularly when effectiveness of the vestibulovaginal sphincter or the vulvar labia is lost or reduced because of trauma, relaxation, or defective conformation of the perineal region. Puerperal infections commonly involve the entire tubular tract. In addition, vestibulovaginal infection perpetuated by urovagina and pneumovagina may sustain chronic infection or inflammation of the uterus. However, the vestibule and vagina can be inflamed even when the uterus is normal or pregnant. Conversely, in closed-cervix pyometra in cows and female dogs, the vagina and vestibule may essentially be normal.
In males, the tubular tract provides a pathway for sperm cells and seminal fluid. It begins in the testes as the rete testes and efferent ductules, which exit the testes and merge to become the head, body, and tail of the epididymis; it then continues as the ductus deferens. In mammals, the ductus deferens ascends into the abdominal cavity via the inguinal ring and passes over the dorsal aspect of the bladder to enter the pelvic urethra. The pelvic and penile urethras are shared as an outlet for semen and urine.
Along this pathway, certain segments of the tract have evolved morphologically and functionally to perform additional specific functions. The epididymides are involved in sperm cell maturation and storage and in selective absorption of abnormal spermatozoa. Ampullae and accessory sex glands (ie, seminal vesicles, prostate, bulbourethral glands) contribute to the formation of seminal plasma. The size, form, and function of the accessory sex glands vary among species. The seminal vesicles and the bulbourethral glands are absent in dogs. In bulls, the epididymides and seminal vesicles are common sites of infection. Epididymitis is also common in rams. Prostatic infections, hypertrophy, and malignancy are found primarily in dogs. Pathologic conditions of the epididymides can be diagnosed by scrotal palpation or ultrasonography in most animals. Other diseases or functional disturbances may require evaluation of one or several semen samples.
Common semen collection techniques used in veterinary medicine include artificial vagina, electroejaculation, and post-mating recovery from an estrual female. The preferred technique varies with the species and operator preference. The pelvic accessory sex organs can be assessed by transrectal palpation and ultrasonography in large animals and by digital palpation or transabdominal ultrasonography in small animals.