Both sexes have a pair of gonads (ovaries in females and testes in males). However, all female birds (except the brown kiwi, which has two functional ovaries) have only the left ovary and oviduct; the right ovary does not develop.
The main functions of the gonads are gametogenesis and steroidogenesis. Both functions are regulated primarily by follicle-stimulating hormone (FSH) and luteinizing hormone (LH), gonadotropins that are released by the anterior pituitary gland under the influence of the hypothalamus. Hypothalamic control of the pituitary gland is mediated by the peptide GnRH; 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 and the pituitary, thyroid, and adrenal glands. Kisspeptin production by hypothalamic neurons has been identified as a critical control mechanism for GnRH release (1). (See also Endocrine System Introduction.)
Ovaries in Animals
The size and location of the ovaries vary by species. In large animals (eg, cows, mares, and camelids), the ovaries can be directly examined by ultrasonography or rectal palpation. In small ruminants and companion animals, ovarian function is monitored by transabdominal ultrasonography, endoscopy, or hormone assay. Once an animal reaches puberty (see the table Features of the Reproductive Cycle), the corpus luteum and ovarian follicles, both cyclical functional structures, alter ovary size and form.
FSH is responsible for the development of ovarian follicles and synthesis of estrogens. In spontaneously ovulating species, once a critical estrogen level is attained, LH is released from the anterior pituitary gland. For induced ovulators, such as alpacas, cats, and rabbits, the LH surge and subsequent ovulation are usually triggered by physical or chemical factors associated with copulation. In either case, LH interacts with follicular receptors to initiate ovulation, which is followed by the development of a new corpus luteum. The increase in luteal cells parallels an increase in progesterone output from the corpus luteum. In nonpregnant polyestrous and seasonally polyestrous females, the functional and morphological life of the corpus luteum is terminated by endogenous prostaglandin F2alpha from the uterus. As the corpus luteum regresses, a new ovulatory follicle(s) develops, completing the estrous cycle.
Hormonal changes during the estrous cycle can be monitored by hormone assays of blood, milk, urine, or feces. Estrous cycling is continuous after puberty unless interrupted by pregnancy and, in some species, by season or lactation in the immediate postpartum period. Cycling may also be blocked by pathological 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 to both the hypothalamus and anterior pituitary gland. In addition, they play a prominent role in sexual behaviors, lactation, and the development of secondary sex characteristics.
Testes in Animals
The testes play a role 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 the development and function of accessory glands, copulatory organs, male sex characteristics, and behavior.
Avian testes are permanently retained in the abdominal cavity. For optimal spermatogenesis, mammalian testes must descend into the scrotum; 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 species that have a seasonal reproductive pattern. Semen quality, libido, and breeding ability are decreased during the seasonally anestrous period of females.
Testicular function can be assessed by evaluating representative semen samples and hormone assays. Testicular examination and measurement help predict potential sperm output and may reveal pathological conditions.
Female Tubular Genital Tract in Animals
Except for the vestibule, which develops from the urogenital sinus, the female genital tract is derived from the embryonic paramesonephric ducts. Each of the tract's segments is adapted to fulfill its function.
The oviduct acquires egg(s) and moves the resulting zygote(s) into the uterus, and oviductal secretions provide 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 bicornuate, Y-shaped uterus is related to body size and horn length. These adaptations accommodate the species-specific number and form of fetuses and placentas. The cervix provides a relatively effective protective barrier against ascending infections. Morphological and functional integrity of the uterus and cervix are required to establish and maintain pregnancy and prepare for parturition.
Infections contracted during breeding, parturition, and the puerperium are common causes of female infertility. Applicability of diagnostic methods for detecting uterine and cervical abnormalities depends on the patient's species, size, and cervical anatomy. 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 microbiological and cytological examination of exudates or secretions, histological examination of biopsies, endometrial cytology, and hormone assays.
The distal 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 decreased because of trauma, relaxation, or defective perineal conformation. Puerperal infections commonly involve the entire tubular tract. In addition, vestibulovaginal infection perpetuated by urovagina and pneumovagina may sustain chronic uterine infection or inflammation. 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 be essentially normal.
Male Tubular Genital Tract in Animals
In males, the tubular tract provides a pathway for sperm cells and seminal fluid. It begins in the testis as the rete testis and efferent ductules, which exit the testis and merge to become the head, body, and tail of the epididymis; the tract 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 urethra is a shared outlet for semen and urine.
Along this pathway, certain segments of the tract have evolved morphologically and functionally to perform additional specific functions. The epididymis is involved in sperm cell maturation and storage and in selective absorption of abnormal spermatozoa. Ampullae and accessory sex glands (ie, seminal vesicles, bulbourethral glands, and the prostate) contribute to the formation of seminal plasma. The size, form, and function of the accessory sex glands vary among species. Seminal vesicles and 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 occur primarily in dogs. Pathological 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 more semen samples.
Common semen collection techniques include artificial vagina, electroejaculation, and postbreeding recovery from a female in estrus. The preferred technique varies with the species and collector's 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.
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References
Xie Q, Kang Y, Zhang C, et al. The role of kisspeptin in the control of the hypothalamic-pituitary-gonadal axis and reproduction. Front Endocrinol (Lausanne). 2022;13:925206. doi:10.3389/fendo.2022.925206
