Equine Estrous Cycle (Week 5 ASCI 333)

Equine estrous cycle length

21 days; truly cyclic in nature

Estrus duration

4-7 days, seasonally affected; mid-season 7 days, late season 5 days

Individual mare estrous behavior

Variable and complex; never expect mares to show identical behaviors; most mares will be true to themselves

How to observe mare estrous behavior

Observe over time and record consistently to develop a feel for each individual mare

Teasing

Mare behavior assessed by exposing mare to stallion and recording behavioral responses; correlates extremely well with estrous cycle

Teasing schedule

Must be done regularly at minimum Monday, Wednesday, Friday (MWF); daily teasing is best practice

Why is teasing important

Determines breeding success and is very important for breeding decisions; must record to maintain "at a glance" teasing history

Hot (in heat, horsing) behavior

Classic estrous behaviors indicating mare will allow stallion to breed; produced in response to rising estrogen levels

Classic hot mare behaviors

Squatting, tail raising, vulvar eversion (winking)

Out (out of heat, cold) behavior

Classic behaviors of mare that refuses stallion and will not allow breeding; progesterone-mediated

Classic cold mare behaviors

Agitated when approached by stallion; sharp, short responses indicating rejection

Indifferent behavior

Neither hot nor cold; not displaying classic heat behaviors but not aggressively rejecting stallion; indicative of approaching heat or waning heat

Photoperiod effect on mare cyclicity

Increased day length decreases pineal production of melatonin, which relieves inhibition of GnRH release from hypothalamus

Estrous cycle hormone marker

Marked by cyclical rise and fall of estradiol (estrogen) levels

Two phases of estrous cycle

Follicular phase (20% of cycle) and luteal phase (80% of cycle)

Follicular phase definition

Extends from regression of corpus luteum to ovulation; short duration (20% of cycle)

Follicular phase dominant structures and hormones

Follicles are dominant ovarian structures; estradiol (estrogen) is dominant hormone

Luteal phase definition

Extends from ovulation to regression of corpus luteum; long duration (80% of cycle)

Luteal phase dominant structures and hormones

Corpus luteum is dominant ovarian structure; progesterone is primary reproductive hormone produced by CL

Follicular activity during luteal phase

Follicles continue to grow and regress throughout the luteal phase

Proestrus

Beginning of follicular phase when follicles are actively forming and developing; progesterone declines dramatically due to luteolysis; estradiol secretion begins to increase

Estrus

Period when mare is receptive to stallion displaying classic heat behavior; estradiol levels peak; ovulation occurs in last 24-48 hours of estrus

Metestrus

Period between ovulation and formation of functional CL; transition from estrogen dominance to progesterone dominance; progesterone production increases

Diestrus

Longest stage of estrous cycle (14-16 days); fully functional CL producing maximal progesterone; mare shows rejection behavior toward stallions

When does progesterone reach maximum after ovulation

About 5 days post-ovulation

PGF2α secretion timing

Around day 13 post-ovulation if mare is not pregnant; secreted by endometrium and causes luteolysis

Primary importance of follicular phase

Luteolysis and resultant decrease in progesterone

How does decreased progesterone affect hypothalamus

Negative feedback on hypothalamus reduced, resulting in GnRH secretion

FSH and LH during follicular phase

Increase in response to GnRH, producing and supporting follicular development

Hallmark of GnRH release from hypothalamic surge center

Requires high estrogen levels combined with low progesterone levels

Preovulatory LH surge mechanism

Dominant follicle produces estradiol that reaches threshold triggering positive feedback on hypothalamus; induces GnRH surge which triggers LH surge from pituitary

What does LH surge cause

Final follicular maturation and triggers ovulation

Progesterone target organs

Hypothalamus, uterus, and mammary glands

Progesterone effects on hypothalamus

Negative feedback on GnRH neurons, suppressing GnRH, LH, and FSH secretion; thought to decrease number of GnRH receptors on anterior pituitary

Progesterone effects on uterus

Strong positive influence on endometrium causing uterine glands to secrete materials into uterine lumen; inhibits myometrium reducing contractility and tone

Progesterone effects on mammary glands

Promotes alveolar development

Antral follicles

Always present on ovaries throughout cycle; developing follicles that vary in size

Four developmental stages of antral follicles

Recruitment, selection, dominance, and atresia

Recruitment

Group of small antral follicles begins to grow and produce estradiol; some become atretic while others continue developing

Selection

From surviving pool of recruited follicles, subset is selected to continue development; follicles continue to grow or become atretic; estradiol production continues to increase

Dominance and follicular deviation

Single follicle emerges and becomes dominant in mare; estradiol secretion increases dramatically and inhibin is produced

When does follicular deviation occur

When two largest follicles are on average 22.5 mm and 19.0 mm in diameter, approximately 3 days after FSH peak

How do inhibin and estradiol affect other follicles

Work synergistically to suppress FSH, preventing further growth of subordinate follicles

Dominant follicle characteristics

Develops greater LH receptor expression and higher concentrations of estradiol, free IGF-1, activin-A, and inhibin-A about 1 day before deviation

Atresia

Process that results in destruction of follicles; majority of follicles are in some stage of atresia at any given time; vastly greater number become atretic than ovulate

Three events of luteal phase

Luteinization (formation of luteal cells), growth and maturation of CL (elevated progesterone production), luteolysis (destruction of CL)

Luteal phase follicular waves

Unique to the horse

Corpus hemorrhagicum

Formed 1-3 days after follicular rupture and collapse with hemorrhage at rupture site

CL cellular composition

Mixture of large luteal cells (LUC, formerly granulosa cells) and many small luteal cells (SLC, formerly thecal cells)

Basement membrane deterioration before ovulation

Separates granulosa from theca interna cells; deteriorates due to collagenase action allowing cells to begin intermixing

Follicle implosion during ovulation

Follicle implodes and is thrown into folds; theca and granulosa cells mix and transform into luteal tissue

Number of corpora lutea per cycle in mares

Only one CL forms per cycle (monovular)

CL palpability in mares

In almost all cases, corpora lutea are buried within ovarian cortex and very difficult to palpate per rectum

When is CL easily identified by ultrasound

In cases where a central cavity exists (remnant of follicular antrum)

Luteolysis definition

Destruction of corpus luteum at end of luteal phase

What organ is required for luteolysis

Uterus; produces luteolytic agent prostaglandin F2α (PGF2α)

Oxytocin-PGF2α interaction in luteolysis

Neuroendocrine-mediated event; hypothalamic oxytocin pulse generator produces intermittent oxytocin secretion; estrogen upregulates endometrial oxytocin receptors; oxytocin initiates PGF2α secretion from uterus

Positive feedback loop in luteolysis

Uterine PGF2α stimulates oxytocin release from CL itself; luteal oxytocin further stimulates PGF2α secretion from uterus

When does pulsatile PGF2α secretion commence

Around day 13-14 post-ovulation if pregnancy is not detected

Result of PGF2α on CL

Causes "lysing" of CL with almost immediate drop in circulating progesterone levels; permits mare to start displaying estrus

PGF2α effects on smooth muscle

Causes contraction of smooth muscle including uterus

Luteolysis results

Dramatic reduction in progesterone production affecting HPO axis; irreversible regression of CL to corpus albicans (CA); mare enters new follicular phase

Reproductive cyclicity significance

Estrous cycles repeat regularly during breeding season allowing multiple attempts at fertilization and pregnancy