Lecture 7: Equine Reproduction

How do male and female reproductive systems develop?

Both sexes start the same. Androgenizing influence leads to male development, while the lack of influence leads to female development.

What are the testicles?

Paired structures that exit the abdominal cavity via inguinal canals and are suspended in the scrotum by the spermatic cord.

What is the function of the pampiniform plexus?

A counter-current cooling mechanism that helps regulate testicular temperature.

What does the cremaster muscle do?

It contracts in cold temperatures and relaxes in warm temperatures to regulate testicular position.

What is the function of Sertoli cells?

They aid in the development of spermatocytes (sperm-forming cells).

What do Leydig cells produce?

Testosterone, the primary male sex hormone.

What is the pathway of sperm from production to ejaculation?

Seminiferous tubules → Straight tubules → Rete testis → Efferent duct → Epididymis (caput, corpus, cauda) → Vas deferens

What are the male accessory sex glands?

• Vesicular gland

• Prostate gland

• Bulbourethral gland

• Ampulla of the deferent duct

Why are sperm immotile before mixing with seminal plasma?

They require seminal fluid to activate movement and protect them.

What are the three main parts of the penis?

• Root – Attachment to the skeletal system

• Body/Shaft – Contains corpus cavernosum penis and corpus spongiosum penis

• Glans – Includes the corona glandis

What is the tunica vaginalis in the testis?

serous membrane sac that surrounds the testis and epididymis

What is smegma?

A white or yellowish substance made of skin cells, oils, and moisture that can accumulate in the genital area. It can be cleaned with a hose and soap

What do germinal cells develop into?

they undergo spermatogonia and turn into sperm

What is castration?

The removal of both testes.

What is cryptorchidism?

A condition where one or both testes fail to drop into the scrotum.

How is castration on a horse performed?

It is a referred field surgery and is left to heal by second intention (healing naturally without sutures).

What sedation is used before castration?

1.1 mg/kg xylazine to induce drowsiness.

What drug is used for recumbency (laying the horse down)?

1.1 mg/lb ketamine, which maintains good blood pressure.

What is in the “Triple Drip” used for anesthesia?

• 5% Guaifenesin

• 500 mg xylazine

• 1 gram ketamine

What is the vulva in mares?

external genitalia

Where is the vagina in mares?

Between vulva and cervix

What is the function of the cervix?

leads to uterus, responds to hormones

Describe the uterus in mares?

“Y shaped”, consist of two uterine horns and a body

What is the oviduct in mares?

site of fertilization

What are the ovaries in mares?

Paired, ovulation occurs at the ovulation fossa

How does aging affect the mare’s vulva conformation?

As a mare ages, the anus sinks in, allowing fecal contamination of the vulva, increasing infection risk.

What is a Caslick operation?

A surgical procedure that closes the upper ⅓ to ⅖ of the vulva to prevent contamination.

How is a Caslick operation performed?

1. Block vulvar edges

2. Trim a fresh edge on both sides

3. Suture vulva closed, leaving ~2 inches open for urination

What does seasonal polyestrous mean in mares?

Mares cycle in late spring and shut down in the fall.

What is the transitional heat cycle?

The first heat cycle of the breeding season, often irregular.

What is the pineal gland, and where is it located?

An unpaired structure in the brain near the fourth ventricle that secretes melatonin in response to light.

How does photoperiod affect melatonin secretion?

• Decreasing daylight → Increases melatonin → Suppresses GnRH

• Increasing daylight → Decreases melatonin → Removes GnRH inhibition

How does melatonin affect a mare’s reproductive cycle?

High melatonin levels suppress GnRH, preventing the estrous cycle; low melatonin levels allow cycling to begin in spring.

How does daylight affect the estrous cycle?

Longer days decrease melatonin levels, allowing GnRH secretion from the hypothalamus to increase, which triggers the estrous cycle.

What does GnRH do?

Stimulates the pituitary gland to release FSH and LH.

What is the role of FSH (Follicle-Stimulating Hormone)?

Recruits follicles for maturation

What is the role of LH (Luteinizing Hormone)?

Stimulates follicular maturation, estrogen production, ovulation, and luteinization.

What happens when follicles mature?

They produce estrogen and inhibin, which negatively regulate FSH, while estrogen positively regulates LH to trigger ovulation.

What happens after ovulation?

The follicle becomes a corpus luteum (CL), which secretes progesterone, blocking LH secretion and stopping estrus behavior.

What hormone terminates the corpus luteum?

Prostaglandin F2α (PGF2α), released by the endometrium 13-16 days post-ovulation, restarts the cycle.

What are veterinarians checking for in a rectal exam?

• Uterine tone

• Cervix (feels like a pencil)

• Follicles (developing on the outside of the ovary)

What is the universal birthday for racehorses?

January 1st, regardless of actual birth date.

Why do breeders want foals born early in the year?

To ensure the foal is physically mature and competitive with others in its age group.

When is breeding season for horses?

February 15 – July 15

How is photoperiod manipulation used for breeding?

Horses are exposed to 14-16 hours of light (200 foot-candles) to simulate spring and induce cycling.

What is oral synthetic progesterone used for in breeding?

It is used to induce ovulation

What gonadotropins are used in breeding management?

hCG and deslorelin

What is Prostaglandin F2α used for in breeding?

It shortens the cycle by terminating the corpus luteum, causing the mare to return to estrus sooner.

How must Thoroughbreds be bred according to the Jockey Club?

Natural cover only (no artificial insemination or embryo transfer allowed).

How is semen collected in horses?

By teasing the stallion and using an artificial vagina made of latex.

What steps are taken to minimize contamination during artificial insemination?

• Wash vulva with mild soap.

• Use a sterile shoulder-length glove.

• Use a sterile pipette and sterile lubricant.

What is the breeding dose in horses?

• 500 million progressively motile spermatozoa.

• Volume <50 mL.

Where should semen be deposited for successful fertilization?

In the mare’s uterus, 24 hours before to 16 hours after ovulation for the best chance of fertilization.

What is capacitation in sperm?

A maturation process that allows sperm to fertilize an egg. It is aided by the uterine environment and takes about 6 hours.

Where does fertilization occur in the mare?

In the ampulla of the uterine tubes.

What is the zona pellucida, and what happens after fertilization?

• The zona pellucida surrounds the oocyte.

• After fertilization, it prevents additional sperm from entering the egg.

What is maternal recognition of pregnancy, and why is it important?

The conceptus signals the mare’s body to prevent prostaglandin release.

Blockage of the uterine horns can disrupt this process.

Why are twin pregnancies bad in horses?

Can cause dystocia, abortion, or one embryo dying.

How can twin embryos be managed in a mare?

1. Administer PGF2a to terminate pregnancy and restart cycle.

2. Manually squish one embryo to allow the other to survive.

What are endometrial cups, and when do they form?

Structures in the uterus that appear around day 38 and produce equine chorionic gonadotropin (PMSG) to support pregnancy.

What are the three fetal membranes in a horse pregnancy?

• Amnion

• Chorion

• Allantois

What are the four structures inside the umbilical cord?

• Two umbilical arteries

• One umbilical vein

• Urachus (tube that connect belly button to bladder, drains the babys pee)

How should the umbilical cord be cared for after birth?

Dip in iodine or chlorhexidine multiple times to prevent infection.

How long is equine gestation?

335-342 days (~11 months).

What are signs that a mare is close to foaling?

• Relaxing of tail tone, vulva, and muscles around tailhead

• Mammary development

• Waxing (colostrum buildup on teats)

How long should it take for a foal to be born after the mare’s water breaks?

Within 20 minutes (foal is in a diver’s pose).

When should the placenta pass after foaling?

Within 2 hours after birth.

Why does foaling often happen at night?

Predators are asleep, making it safer.

What are the key milestones for a newborn foal?

• Suck reflex present at birth

• Standing within 1 hour

• Nursing within 1 hour of standing

• Meconium (first feces) passed within 1-3 hours

How can you help a foal pass meconium?

• encourage colostrum intake

• administer enemas

Why is colostrum important for a newborn foal?

Provides antibodies (passive immunity). Foal must nurse or be tube-fed within the first 24 hours.

Why are pregnant mares vaccinated 6 weeks before birth?

To boost the mare’s immune response, ensuring antibodies are passed to the foal through colostrum.

How do breeding farms ensure colostrum is available for foals?

They maintain frozen colostrum banks in case a foal cannot nurse from its mother.

Why are IgG levels tested in foals within the first 24 hours?

To check if the foal received enough antibodies from colostrum for immune protection.

What is the treatment for Failure of Passive Transfer (FPT) in foals?

Hyperimmune plasma IV is administered to provide antibodies if the foal did not absorb enough from colostrum.

What is dystocia?

Any disruption in the normal timeline of fetal delivery.

What is a red bag delivery, and why is it dangerous?

Premature placental separation → baby is not getting oxygen.

What is a breach birth, and how is it fixed?

Foal’s back legs come out first → must be pulled out manually.

What is flexor tendon laxity?

A foal’s weak muscles make tendons loose; resolves as muscles strengthen.

What is navel ill, and how is it treated?

Infection of the umbilical stump. Treated by dipping in disinfectant ; can spread to joints if untreated.

What are contracted tendons, and how are they treated?

• Tendons are too tight, causing the foal to buckle over.

• Treated with oxytetracycline (antibiotic) to relax muscles.

Why is semen extender used for shipping?

To preserve sperm viability and provide nutrients during transport.

How is sperm count managed before shipping?

Sperm count is measured in mL and diluted with semen extender to ensure proper concentration.

Why are plastic syringes used for semen handling?

Sperm does not tolerate rubber, so only plastic syringes are used.

How is semen shipped safely?

Semen is shipped in a Styrofoam box with an infusion rod or a frozen semen shipper to maintain proper temperature.