Lecture 13: Equine Repro: Seasonality & Estrous cycle (Macpherson)

Question 0

what type of cycle is the mare’s estrus?

Answer 0

seasonally polyestrus

Question 1

what is the age of puberty in a mare?

Answer 1

12 - 24 months

Question 2

at what age does the stallion go through puberty?

Answer 2

4 - 5 years

Question 3

what influences the reproductive cyclicity in horses?

Answer 3

length of day ie photo period

Question 4

how does the photo period influence cyclicity?

Answer 4

• 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.

Question 5

what happens with cyclity as photo period ie day length increases?

Answer 5

as photo period increases, melatonin secretion decreases, GnRH increases and reproductive cyclicity ensues!

Question 6

horses are referred to as “— day breeders”

Answer 6

long day breeders

Question 7

repro activity can be divided into 4 phases:

Answer 7

1. winter anestrus
2. anovulatory receptivity - vernal transition (spring)
3. ovulatory receptivity
4. anovulatory receptivity - autumnal transition

Question 8

months of greatest cyclicity

Answer 8

summer - June and July

Question 9

cycle during the summer: increased photo period effects what? describe the pathway that leads to cyclicity

Answer 9

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

Question 10

what do we see as spring months occur?

Answer 10

will see waves of follicles. typically will see 3 - 4 transitional cycles before they ovulate.

Question 11

estrous behavior is stimulated by?

Answer 11

estrogen

Question 12

LH stimulates…

Answer 12

production of estrogen in follicles

Question 13

what are transitional ovaries?

Answer 13

little follicles that develop in beginning of cycle but do not ovulate

Question 14

when does the first ovulation occur?

Answer 14

april

Question 15

how long is “heat” period?

Answer 15

4 - 7 days

Question 16

why will some mares show prolonged levels of “heat”

Answer 16

because of low estrogen levels

Question 17

what will the tract look/feel like when in estrous

Answer 17

“wagon wheel” shape consistent with uterine edema

Question 18

where do follicles develop in the ovary?

Answer 18

ovulation fossa

Question 19

what is the PHYSIOLOGIC breeding season?

Answer 19

April to September

Question 20

what is the OPERATIONAL breeding season?

Answer 20

February to July

Question 21

how did we (humans) “mess up the system?”

Answer 21

by establishing a universal birth date of January 1 (originated in the horse racing industry).

Question 22

what is the most effective method of starting a mare’s cycle early?

Answer 22

increase the photo period.

• simulate daylight with lighted barns and fields.
• this ACCELERATES the time to first ovulation, it does NOT shorten transition.

Question 23

length of estrous cycle in days?

Answer 23

18 - 21 days

Question 24

what are the 2 phases within the estrous cycle?

Answer 24

follicular phase and luteal phase

Question 25

follicular phase

Answer 25

• estrus
• 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

Question 26

luteal phase

Answer 26

• diestrus
• 14 days
• begins with ovulation
• you then get a corpus hemorrhagicum and eventually a corpus luteum (CL)

Question 27

describe pathway for autumnal transition

Answer 27

shorter day period effects –> pineal gland increases melatonin –> hypothalamus decreases GnRH –> anterior pituitary decreases FSH, LH –> ovary decreases estrogen and progesterone

Question 28

what drives the estrous cycle?

Answer 28

hormones!! via the hypothalamic-pituitary-gonadal axis

Question 29

what happens if mare does not get pregnant after ovulation?

Answer 29

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.

Question 30

what is odd about the autumnal transition?

Answer 30

mare will continue to make HUGE follicles, but they just hang there because LH levels are not

Question 31

GnRH

Answer 31

• produced and secreted by the hypothalamus
• released into the hypothalamic-hypophyseal portal system.
• GnRH stimulates release of FSH and LH

Question 32

FSH (follicle stimulating hormone)

Answer 32

• 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.

Question 33

LH (luteinizing hormone) - target organ and 4 functions

Answer 33

• targets the ovary
• functions:
  1. follicular maturation
  2. subsequent follicular estrogen production
  3. ovulation
  4. luteinization of recently ovulated follicle

Question 34

when does LH secretion begin? when does it peak?

Answer 34

secretion begins a few days prior to estrus and peaks THE DAY AFTER ovulation. this is unique to the mare!

Question 35

what inhibits LH?

Answer 35

progesterone - secreted by CL in diestrus, feeds back negatively on the anterior pituitary to inhibit secretion of LH following ovulation.

Question 36

estrogen - what secretes it? when is its peak?

Answer 36

• secreted by the developing follicle

- peak levels during estrus

Question 37

function of estrogen:

Answer 37

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.

Question 38

what does estrogen inhibit?

Answer 38

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.

Question 39

progesterone: what is it secreted by?

Answer 39

the CL. this is the dominant hormone during diestrus and early pregnancy

Question 40

function of progesterone

Answer 40

• “drives” the cycle

- has a dominant effect over behavioral and physical effects of estrogen if both are present.

Question 41

when are peak progesterone levels?

Answer 41

approx day 5 following ovulation when the CL ismature.

Question 42

what does progesterone inhibit?

Answer 42

ovulation.
- by inhibiting the release of preovulatory surge of LH (negative feedback).
- inhibits GnRH secretion from hypothalamus.

Question 43

PGF2-alpha (prostaglandin): what is it secreted by?

Answer 43

released from the endometrium of the uterus

Question 44

PGF2-alpha: functions

Answer 44

causes lysis of the CL and stimulates contractility of smooth muscle throughout the body.

Question 45

what will exogenously administered PGF2-alpha do?

Answer 45

it is exogenously given to bring a mare back into heat faster. symptoms of administration are profuse sweating and possibly colic. these symptoms generally reside within an hour of administration.