Reproductive System of the Horse

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  • Title: Reproductive System of the Horse

  • Course: APSC 451

  • Department: Equine Production

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  • Mare’s Reproductive System: Lies horizontally in abdominal and pelvic cavities, consists of the following components:

    • Vulva

    • Vagina

    • Cervix

    • Ovaries

    • Oviduct

    • Uterus

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  • Diagram of Mare's Reproductive System:

    • Vagina

    • Cervix

    • Horn of Vulva

    • Vestibule

    • Bladder

    • Body of Uterus

    • Uterus

    • Ovary

    • Fallopian Tube

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  • Vulva:

    • External opening of the female reproductive tract.

  • Vagina:

    • A muscular tube (6-8 inches) lined with mucus membranes.

    • Connects the vulva to the cervix.

    • Elastic tissues accommodate stallion’s penis and foaling.

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  • Cervix:

    • Provides entrance to the uterus, approximately 4 inches long.

    • Flaccid and open during estrus to facilitate semen passage.

    • Closed when the mare is not in estrus, serving as a barrier.

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  • Ovary:

    • Primary reproductive organ, two located at the upper end.

    • Contains all egg cells a mare will produce in her lifetime; each egg is contained in a follicle.

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  • Oviducts (Fallopian Tubes):

    • Site of fertilization.

    • Egg is discharged when the follicle ruptures, trapped by the infundibulum.

    • Oviduct carries fertilized egg to the uterus.

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  • Uterus:

    • Largest female reproductive organ, responds as fetus grows.

    • Multi-layered, hollow, Y-shaped structure.

    • Two branches (horns) and a base (uterine body).

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  • Parts of Uterus:

    • Uterine body

    • Uterine horn

    • Oviduct

    • Infundibulum

    • Ovary

    • Broad ligament

    • Cervix

    • Vagina

    • Vulva

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  • Uterus Structure:

    • Three distinct layers:

      • Serous Layer: Outer layer.

      • Myometrium: Middle layer with muscular tissue for parturition.

      • Endometrium: Innermost layer with rich blood supply.

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  • Estrous Cycle:

    • Normal cycle lasts 21-22 days:

      • Estrus Phase: Mare receptive to stallion, lasts 6-8 days.

      • Diestrus Phase: Mare not in heat, rejects stallion, lasts 14-15 days.

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  • Cyclic Behavior:

    • Primary factor: increased hours of light.

    • Artificial lighting can regulate cycles.

    • Hormones control cycle.

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  • Hormonal Control:

    • Increased light stimulates hypothalamus to produce GnRH.

    • GnRH leads to pituitary secretion of FSH and LH.

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  • Follicle Development:

    • FSH stimulates ovaries to develop follicles.

    • Developing follicle secretes estrogen, stimulating sexual activity.

    • High estrogen triggers LH release from pituitary.

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  • Ovulation:

    • LH causes follicle to rupture, releasing the ovum.

    • After ovulation, estrogen levels drop; corpus hemorrhagicum forms from the ruptured follicle.

    • Corpus luteum formed from luteinization of the hemorrhagicum.

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  • Progesterone Function:

    • Produced by granulosa cells in the corpus luteum.

    • Shuts down LH release, maintains pregnancy.

    • Cervix closes; mare loses interest in stallion.

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  • Cyclic Reset:

    • If mare does not conceive, progesterone influences uterus for 12-14 days.

    • Uterine endometrium secretes prostaglandin, causing corpus luteum regression.

    • FSH levels rise, restarting the cycle.

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  • Pregnancy Recognition:

    • If conception occurs, embryo migration inhibits prostaglandin secretion.

    • Corpus luteum remains functional, maintaining progesterone levels for pregnancy.

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  • Hormonal Pathway:

    • Diagram showing relationship between sunlight hours, pineal gland stimulation, hypothalamus, GnRH, pituitary hormones (FSH, LH), ovarian hormones, and roles in reproduction. Gondotropin Releasing Hormone

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  • Breeding Behavior:

    • Mares are seasonally polyestrous, cycling several times/year.

    • Cycling begins in January-February; peaks in June-July.

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  • Seasonal Cycling:

    • Cycling irregular by September-October; ceases in November (anestrous).

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  • Estrous Cycle Initiation:

    • Begins at puberty (15-24 months), normal cycle is 21-22 days.

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  • Fertilization:

    • Ovum viable for 8-12 hours post-ovulation.

    • Sperm can survive 24-48 hours in oviduct; takes 4-6 hours to reach ovum.

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  • Embryo Development:

    • Fertilized egg travels to uterus in 5-6 days.

    • By days 16-18, the embryo implants in one horn.

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  • Gestation Period:

    • Average gestation length: 335 days (+/- 2-4 weeks).

    • Influenced by season, nutrition, and the fetus' sex.

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  • Stallion’s Reproductive System:

    • Main functions: produce sperm and testosterone, introduce sperm into mare.

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  • Stallion's Anatomy:

    • Includes the prostate, vesicular glands, bladder, vas deferens, urethra, glans penis, scrotum, testicles, and epididymis.

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  • Components of Male Reproductive System:

    • Two testes, accessory sex glands, tubules for sperm transport, and scrotum.

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  • Detailed Male Anatomy:

    • Vesicular glands, prostate, ampulla, deferent duct, epididymis, bulbourethral glands, glans penis, and retractor penis muscle.

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  • Testes Characteristics:

    • Two testes suspended by spermatic cord, ovoid shape (80-140 mm long, 56-80 mm wide).

    • Housed in scrotum, separated by a septum.

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  • Testicular Descent:

    • Testes should descend into scrotum by 3 weeks gestation or 2 weeks post-birth.

    • If not, it results in cryptorchidism.

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  • Penis Structure:

    • Organ of copulation, consisting of three parts: root, body (shaft), and glans penis (enlarged end).

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  • Sperm Production:

    • Occurs in seminiferous tubules of testes, billions produced daily.

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  • Epididymis Function:

    • Transports, matures, and stores sperm from testes.

    • Deferent ducts transport sperm to accessory glands, adding fluids to semen.

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  • Leydig Cells:

    • Produce testosterone, present in interstitial tissues of seminiferous tubules.

    • FSH and LH also involved in stallion's reproductive processes.

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  • Mating Process:

    • Involves three basic processes: erection, emission, and ejaculation.

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  • Sperm Maturity:

    • Full production begins at over a year old, reaches capacity at about 4 years; remains constant until age 20.

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  • Factors Influencing Sperm Production:

    • Season, testicular size, age, frequency of ejaculation.

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  • Seasonality in Stallions:

    • Stallions are seasonal breeders, sperm production year-round.

    • Best months for testicular size: May, June, July.

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  • Sperm Count Variation:

    • Sperm count drops significantly in December-January compared to June-July.

    • Stallions should be exposed to 16 hours of light daily to optimize breeding conditions.

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