Flashcards in Lecture 13: Equine Repro: Seasonality & Estrous cycle (Macpherson) Deck (46):
what is the age of puberty in a mare?
12 - 24 months
what type of cycle is the mare's estrus?
at what age does the stallion go through puberty?
4 - 5 years
what influences the reproductive cyclicity in horses?
length of day ie photo period
how does the photo period influence cyclicity?
- light perceived by retina transmits signal to the pineal gland
- pineal gland secretes melatonin (melatonin increases with darkness)
- melatonin inhibits GnRH secretion from the hypothalamus.
what happens with cyclity as photo period ie day length increases?
as photo period increases, melatonin secretion decreases, GnRH increases and reproductive cyclicity ensues!
horses are referred to as "--- day breeders"
long day breeders
repro activity can be divided into 4 phases:
1. winter anestrus
2. anovulatory receptivity - vernal transition (spring)
3. ovulatory receptivity
4. anovulatory receptivity - autumnal transition
months of greatest cyclicity
summer - June and July
cycle during the summer: increased photo period effects what? describe the pathway that leads to cyclicity
increased photo period -> pineal gland decreases melatonin -> inhibition of melatonin acts on hypothalamus increases release of GnRH -> anterior pituitary increases FSH, LH -> ovary increases estrogen and progesterone
what do we see as spring months occur?
will see waves of follicles. typically will see 3 - 4 transitional cycles before they ovulate.
estrous behavior is stimulated by?
production of estrogen in follicles
what are transitional ovaries?
little follicles that develop in beginning of cycle but do not ovulate
when does the first ovulation occur?
how long is "heat" period?
4 - 7 days
why will some mares show prolonged levels of "heat"
because of low estrogen levels
what will the tract look/feel like when in estrous
"wagon wheel" shape consistent with uterine edema
where do follicles develop in the ovary?
what is the PHYSIOLOGIC breeding season?
April to September
what is the OPERATIONAL breeding season?
February to July
how did we (humans) "mess up the system?"
by establishing a universal birth date of January 1 (originated in the horse racing industry).
what is the most effective method of starting a mare's cycle early?
increase the photo period.
- simulate daylight with lighted barns and fields.
- this ACCELERATES the time to first ovulation, it does NOT shorten transition.
length of estrous cycle in days?
18 - 21 days
what are the 2 phases within the estrous cycle?
follicular phase and luteal phase
- 4 - 7 days
- dominant follicle(s) produces estrogen
- follicle ovulates at 40 - 50 mm (REALLY big!) 24 to 48 hours prior to the end of estrus
- her behavior = receptive
- 14 days
- begins with ovulation
- you then get a corpus hemorrhagicum and eventually a corpus luteum (CL)
describe pathway for autumnal transition
shorter day period effects --> pineal gland increases melatonin --> hypothalamus decreases GnRH --> anterior pituitary decreases FSH, LH --> ovary decreases estrogen and progesterone
what drives the estrous cycle?
hormones!! via the hypothalamic-pituitary-gonadal axis
what happens if mare does not get pregnant after ovulation?
endometrium of the uterus will secrete prostaglandin to lyse the CL. This can be done exogenously and is known as short cycling. It is done so she can get back into heat faster.
what is odd about the autumnal transition?
mare will continue to make HUGE follicles, but they just hang there because LH levels are not
- produced and secreted by the hypothalamus
- released into the hypothalamic-hypophyseal portal system.
- GnRH stimulates release of FSH and LH
FSH (follicle stimulating hormone)
- enters circulation and targets ovaries for follicular recrutiment.
- FSH release in mare is biphasic with primary surge occurring in mid-luteal phase.
- inhibin and estrogen are produced by the developing follicle and feedback negatively to the anterior pituitary to inhibit FSH secretion.
LH (luteinizing hormone) - target organ and 4 functions
- targets the ovary
1. follicular maturation
2. subsequent follicular estrogen production
4. luteinization of recently ovulated follicle
when does LH secretion begin? when does it peak?
secretion begins a few days prior to estrus and peaks THE DAY AFTER ovulation. this is unique to the mare!
what inhibits LH?
progesterone - secreted by CL in diestrus, feeds back negatively on the anterior pituitary to inhibit secretion of LH following ovulation.
estrogen - what secretes it? when is its peak?
- secreted by the developing follicle
- peak levels during estrus
function of estrogen:
1. estrogen stimulates receptive reproductive behavior (in absence of progesterone).
2. feeds back positively on LH to stimulate release of MORE LH to promote follicular maturation and ovulation.
what does estrogen inhibit?
estrogen, along with inhibin, feeds back negatively on FSH and inhibits further FSH release.
It also has a negative feedback on GnRH in the hypothalamus.
progesterone: what is it secreted by?
the CL. this is the dominant hormone during diestrus and early pregnancy
function of progesterone
- "drives" the cycle
- has a dominant effect over behavioral and physical effects of estrogen if both are present.
when are peak progesterone levels?
approx day 5 following ovulation when the CL ismature.
what does progesterone inhibit?
- by inhibiting the release of preovulatory surge of LH (negative feedback).
- inhibits GnRH secretion from hypothalamus.
PGF2-alpha (prostaglandin): what is it secreted by?
released from the endometrium of the uterus
causes lysis of the CL and stimulates contractility of smooth muscle throughout the body.