Animal Reproductive Systems and Physiology

Fundamentals of Animal Reproduction and Puberty

• Reproduction in complex organisms occurs when an egg from the mother joins with a sperm from the father, involving the transfer of genetic information from parent to offspring.

• Puberty is defined as the age or specific time at which reproductive organs become functional and reproduction becomes possible.

• Reproductive capacity continues to develop even after the initial onset of puberty.

• In females, the hallmark of puberty is the commencement of ovulation, which is the release of an egg cell from a follicle.

• The onset of puberty is influenced by several factors:

  • Diet and Nutrition: Poor nutrition can significantly delay puberty, particularly in cattle and horses.

  • Environment and Disease: Presence of disease or exposure to environmental stressors and extreme temperatures often leads to a delay.

  • Genetic Factors: Variation across species and breeds.

• Survival Mechanism: The delay of puberty under stress is considered a biological survival technique to allow the female to conserve resources for her own health and ensure offspring are born into favorable conditions.

• Breeding Maturity: Puberty often occurs before the dam (mother) is mature enough for normal gestation (fertilization to birth) and parturition (birth). Consequently, breeding should be delayed until physical development is adequate.

Comparative Onset of Puberty in Domestic Animals

• Horses: $10\text{--}24 \, \text{months}$

• Cattle: $6\text{--}18 \, \text{months}$

• Sheep: $6\text{--}12 \, \text{months}$

• Swine: $5\text{--}8 \, \text{months}$

• Dogs: $6\text{--}12 \, \text{months}$

• Cats: $6\text{--}15 \, \text{months}$

Anatomy of the Male Reproductive System

• Scrotum: A sac of skin located outside the body between the thighs, containing the testes and epididymis. Its outer surface is generally free of hair.

• Testes (Testicles): The primary organs where sperm are produced. Each male possesses two testes.

• Epididymis: Attached to each testicle; it serves as the site for sperm maturation and storage until ejaculation.

• Ductus Deferens: A tube extending from the epididymis to the urethra that transports sperm during ejaculation.

• Accessory Sex Glands: These glands produce the fluid portion of the ejaculate and provide lubrication for coitus (mating). The resulting mixture of sperm and fluid is called semen.

  • Ampullae: Serve as temporary storage for sperm.

  • Seminal Vesicles: Secrete fluid containing mucus, amino acids (for coagulation), fructose (energy source for sperm), and prostaglandins (stimulate uterine contractions to aid sperm movement).

  • Prostate Gland: Secretes a thin alkaline fluid into the urethra to neutralize the acidity of residual urine and the vaginal environment. Note: This is the only accessory gland present in male dogs.

  • Bulbourethral Glands: Secrete a viscous fluid prior to sperm emission. These are very small in male cats.

• Penis: The male copulatory organ.

  • Prepuce: A double covering of skin surrounding the glans.

  • Glans: Often referred to as the head of the penis.

Male Reproductive Hormones and Spermatogenesis

• Hormonal Control: Reproduction is regulated by a complex system of checks and balances involving the nervous and endocrine systems. Triggers include light, sounds, odors, nutrition, stress, and temperature.

• Anterior Pituitary Hormones:

  • FSH (Follicle-Stimulating Hormone): Stimulates the production of sperm (spermatogenesis).

  • LH (Luteinizing Hormone): Also known as Interstitial Cell Stimulating Hormone in males; stimulates testes to produce testosterone and supports spermatogenesis.

• Testosterone: Produced in the testes; essential for sperm formation and the development of secondary sex characteristics, such as aggression.

• Spermatogenesis Process:

  • Sperm production is continuous in most domestic animals, allowing for year-round fertility.

  • Seasonal Breeders: Animals like the ram and stallion show decreased sperm production during nonbreeding seasons.

  • The process involves an orderly system within the testes where different sections contain sperm at various development stages before they move to the epididymis to mature.

Physiology of Male Reproduction: Erection and Ejaculation

• Erection: Blood fills large caverns within the penis, causing it to become firm. Muscles help maintain this state by holding blood in the penis.

  • Musculocavernous Type: Found in stallions and dogs; slow to become erect.

  • Fibroelastic Type: Found in bulls, rams, and boars; capable of rapid erection.

• Ejaculation: The forceful expulsion of sperm and glandular fluids through the penis.

  • Factors: Temperature, friction, and pressure are critical for producing ejaculation, varying by species.

  • Duration/Volume: Dogs, stallions, and boars have high volumes and long coitus; bulls and rams have low volumes and short coitus.

• Three Phases of Ejaculation:

  1. Sperm-free portion: Secreted during initial arousal.

  2. Sperm-rich portion: A small amount produced quickly.

  3. Sperm-poor portion: Often constitutes the largest total volume.

Anatomy of the Female Reproductive System

• Ovaries: Located in the abdomen behind the kidneys and supported by the mesovarium.

  • Functions: Production of ova (eggs) and hormones (estrogen, progesterone, relaxin).

  • Physical Characteristics:

  • Bitches: Oval, approximately $2\,cm \times 1.5\,cm$.

  • Mares: Bean-shaped, approximately $6\,cm \times 4\,cm$.

  • Cows: Oval, approximately $3\,cm \times 2\,cm$.

  • Follicles: Capsules containing oocytes (undeveloped eggs) that produce estrogen.

  • Corpus Luteum: Formed from a ruptured follicle after ovulation; produces progesterone.

• Uterine Tubes (Oviducts): Connect ovaries to the uterus. Features fimbria (finger-like projections) at the funnel-like opening to catch released ova. This is the site of fertilization.

• Uterus: A muscular organ with a body and two horns. Its innermost layer, the endometrium, forms the placenta during pregnancy for fetal support.

• Vagina: Muscular canal for coitus and birth passage.

  • Cervix: The connection point between the vagina and uterus.

  • Hymen: A delicate membrane covering the vaginal opening that typically disappears after breeding or birth.

• Vulva and Vestibule: The outer termination of the tract.

  • Vulva: Includes labia and the clitoris.

  • Vestibule: Contains the urethral opening and sphincter muscles that allow for receptive behavioral signaling. These areas have a dense sensory nerve supply.

Oogenesis and Ovulation

• Oogenesis: The formation of oocytes. In domestic species, this is completed before or shortly after birth. Females are born with a finite supply that decreases with age via follicular atresia (degeneration of follicles).

• Ovulation: The release of a mature ovum in response to a reproductive cycle.

  • Hormonal Stimuli: FSH and LH cause follicle growth. After puberty, a surge in LH triggers the follicle to break through the ovary wall, often with slight bleeding.

  • Spontaneous vs. Induced: Most species are spontaneous ovulators. Cats, rabbits, and ferrets are induced ovulators, requiring the stimulus of copulation to trigger the release of LH.

• Post-Ovulation: The follicle transforms into the corpus luteum. If no pregnancy occurs, it typically degenerates, except in the bitch, where it persists for $2\text{--}3\,months$ regardless of pregnancy status.

The Estrous Cycle: Phases and Classifications

• Functional Rhythm: The estrous cycle (or heat cycle) occurs in all post-pubertal females.

• Classifications:

  1. Monestrous: One cycle per year (common in wild animals).

  2. Polyestrous: Frequent cycles throughout the year (cow, sow, mare).

  3. Seasonal Polyestrous: Periodic cycles during specific seasons (sheep).

  • Bitches and Queens: Exhibit $2\text{--}3$ periods of estrus per year.

• Phases of the Estrous Cycle:

  1. Proestrus: Preparation stage. FSH leads to follicular growth and high estrogen. Includes increased blood supply to the tract and vaginal wall thickening. Signs include: cornified cells (bitches/queens), bloody discharge (bitches), swollen vulva (bitches/sows), or stringy mucus (cows/mares).

  2. Estrus: Period of sexual receptivity and peak estrogen. Follicles are mature. Ovulation typically occurs toward the end of this phase in response to an LH surge.

  3. Metestrus/Diestrus: Rapid growth of the corpus luteum under progesterone influence. The endometrium thickens, glands develop, the cervix closes, and the uterus relaxes.

  4. Anestrus: Reproductive tract is inactive and functionless. Provides a period for uterine repair in bitches.

Estrous Cycle Phase Durations

• Cow: Proestrus: $3\,days$ | Estrus: $12\text{--}24\,hours$ | Metestrus/Diestrus: $16\text{--}18\,days$

• Mare: Proestrus: $3\,days$ | Estrus: $4\text{--}7\,days$ | Metestrus/Diestrus: $9\text{--}15\,days$

• Sow: Proestrus: $3\,days$ | Estrus: $2\text{--}4\,days$ | Metestrus/Diestrus: $12\text{--}17\,days$

• Ewe: Proestrus: $2\,days$ | Estrus: $1\text{--}2\,days$ | Metestrus/Diestrus: $10\text{--}15\,days$

• Bitch: Proestrus: $9\,days$ | Estrus: $9\,days$ | Metestrus/Diestrus: $2\text{--}3\text{ months}$ | Anestrus: $2\text{--}5\text{ months}$

• Queen: Length of heat is $3\text{--}7\,days$; interval between cycles is slightly less than $3\,weeks$. They are induced ovulators and may stay in heat continuously if they do not mate.

Essential Female Reproductive Hormones

• From the Anterior Pituitary:

  • FSH: Necessary for follicle growth and estrogen production.

  • LH: Triggers ovulation and corpus luteum development.

• From the Posterior Pituitary:

  • Oxytocin: Causes smooth muscle contraction; aids in parturition and stimulates lactation in mammary glands.

• From the Ovaries / Placenta:

  • Estrogens: Mediate secondary sex characteristics (fat distribution, mammary development), regulate the cycle, facilitate mating/sperm transport, and prepare the body for birth.

  • Progesterone: Produced by the corpus luteum (and the placenta in late-stage mares/ewes); essential for maintaining pregnancy. In bitches, follicle production starts just before ovulation.

  • Relaxin: Produced by ovaries, uterus, and placenta; relaxes pelvic ligaments and the cervix just before parturition.