Equine Estrous Cycles & Seasonality Flashcards
What is unique about the equine ovary?
entirely internal development of follicles, oocytes, and CL with eruption of oocytes through the ovulatory fossa
- cannot palpate CL or stigma
What are 5 unique aspects of the equine female reproductive tract?
- larger uterine body
- cervix is low in cartilage
- less prominent intercorneal ligament
- transverse fold = main barrier to contamination
- clitoral fossa contains small sinuses where samples can be taken for venereal diseases
When does puberty most commonly occur in horses? What 5 things affect onset?
12-24 months —> able to detect progesterone from the CL of the first ovulation
- environment - season
- social factors
- body size
- nutrition
- genetics
When is it recommended to breed horses? What happens if this is not done?
wait until 3 y/o for complete maturation of stature and reproductive capacity
high rate of pregnancy loss
How can the high rate of pregnancy loss in mares younger than 3 years old be circumvented?
embryo transfer from young mare into a mature mare
What is the seasonality of horse breeding?
long-day seasonal breeders
- 85% seasonally polyestrous
- 5% year-round polyestrous
- 10% irregular
Where are horses most likely to cycle year-round?
equatorial regions
What are the 4 stages of equine seasonality? What causes them to be long-day breeders?
- breeding season
- fall transition
- winter anestrus
- vernal/spring transition - longer daylight, but not enough to cycle
melatonin created from the pineal gland in dark periods inhibits GnRH secretions
What is equine winter anestrus? What 2 hormonal changes are observed? What is not seen?
short days = reproductive incompetence
- minimum or absent GnRH
- low LH, estrogen, and progesterone
minimal change in FSH —> few small follicles are still produced, but stay small and unable to go pasted recruitment due to low GnRH
Long vs. short days, equine cycling:
What 2 physical changes to the reproductive tract is seen in mares during the winter? Behavior?
- few follicles - <5-10 mm, mostly stromal tissue in ovaries
- flaccid reproductive tract
variable - permissively receptive due to lack of progesterone
What happens in mares during the vernal/spring transition? How long does this last? What behavior is seen?
daylight begins to increase from January to March —> 1-2 months (55-60 days)
erratic estrus behavior - inconsistent, wait until after the first ovulation of the year for breeding
What 2 hormonal changes are seen in mares during vernal/spring transition? How do ovaries typically appear?
- gradual increase in GnRH, FSH, and LH, resulting in follicle growth
- low LH due to follicle regression without ovulation
(steroidogenically incompetent)
active
What marks the end of the vernal transition in mares?
first ovulation of the year —> onset of seasonal cyclicity
Breeding season, mare:
melatonin induces seasonality!
How long does the equine estruos cycle last? Estrys and diestrus?
21 days —> 18 with one follicular wave - 24 with two follicular waves during estrus and diestrus
- ESTRUS (follicular) = 5-7 days
- DIESTRUS (luteal) = 14-15 days
When does ovulation occur in mares?
24-48 hours before the disappearance of estrous behavior
- if behavior is seen, it may be too late!
What is considered day 0 in the equine estrous cycle?
day ovulation occurs (24-48 hours before disappearance of estrous behavior)
Estrus vs. diestrus in mares:
diestrus behavior due to increase in progesterone!
What is characteristic of estrus in mares? What changes in the ovaries, uterus, and cervix is expected?
absence of CL with estrogen dominance (edema!) —> sexually receptive
- OVARIES = lack CL, growing follicles
- UTERUS = relaxed, edematous
- CERVIX = relaxed, pink, sits on the floor
What behavioral signs are seen in mares in estrus? What are these signs associated with? Why is this significant?
- stallion receptivity (+ Flehmen)
- raising tail
- winking
- passive urination
- squatting
absence of progesterone —> may be seen during anestrus (winter)!
How do follicles typically grow during estrus in mares? What causes them to grow?
3-5 mm/day
production of inhibin and estrogen by dominant follicle decreases FSH and induces LH surge —> dominant follicle!
What receptors are found in mare dominant follicles?
granulosa LH receptors - able to ovulate more efficiently in response to LH surge
What is the expected size of light breeds, Drafts/Friesians, and ponies/minis preovulatory follicle?
30-45 mm
> 40-50 mm
<30 mm
What is the average size of the equine ovulatory follicle? When are they seen? What forms following stage?
40-45 mm (>35 mm)
24-48 hours prior to the end of behavioral estrus
corpus hemorrhagicum formation
What are 3 signs of estrus in mares on ultrasound? What system is stimulated?
- increased coiling of glands
- endometrial folds thicken
- edema pattern obvious throughout uterus
immune system
How can edema be scored during estrus in mares?
- 0 = no edema, typical in diestrus or anestrus
- 1 = slight edema (“spoke wheel”), more evident in uterine horns
- 2 = moderate edema, peak of estrogen effect
- 3 = heavy edema, exaggerated, pathological (can be seen in vernal transition)
increased echogenicity around follicle seen
How does cervix appearance compare in estrus and diestrus?
ESTRUS = pink, moist, relaxed
DIESTRUS = pale, dry, light (less blood flow)
When is diestrus seen in mares?
14-15 days from the end of behavioral estrus —> 15-16 days from ovulation
(most consistent phase)
What behavior changes are seen in diestrus in mares? What causes this?
- squealing
- laid-back ears
- switching tail
- kicking or biting stallion —> NOT receptive
presence of progesterone
What is characteristic of diestrus in mares? What are 3 common physical exam findings?
presence of CH <72 hrs after ovulation or a CL —> produces progesterone
- CH/CL
- toned, round uterine lacking edema (no estrogen!)
- toned (high, tight, white, and dry) cervix
What 3 effects does progesterone production during diestrus have on the mare’s reproductive tract?
- inhibits LH release and GnRH production (NOT FSH) - inhibits secondary ovulations, postpones follicular maturation and growth (1-2 follicular waves)
- immune suppression
- glandular development and secretion of uterine milk (histotroph) to prepare for pregnancy
What is the major exception to progesterone inhibition of secondary ovulations in mares?
diestrus ovulations are possible, but rare
- due to the decreased sensitivity to inhibin
When does luteolysis occur in mares? What are the 5 steps?
day 14 of diestrus
RAPID
- luteal CL produces progesterone
- downregulated endometrial P4 causes an increase in estrogen receptors in the endometrium
- this induces endometrial oxytocin receptor production
- oxytocin binding causes PGF2a production from the endometrium
- PGF2a travels through systemic circulation to CL, resulting in vasoconstriction of utero-ovarian vessels and luteal ischemia
(decline in PGF2a seen 4 hours after release, with a more significant release within 24 hours)
When does fall transition occur in mares?
decreasing day length around September to December for 1-2 months
What 2 things happen in the ovaries during fall transition in mares?
- ovulation fails and may become hemorrhagic (Autumn follicles) —> can look like a follicle on U/S, but has “dots” due to the presence of blood and fibrin
- abbreviated luteal function - luteotropic and luteolytic function have decreased efficiency resulting in a spontaneously prolonged luteal phase —> can’t maintain CL, don’t have mechanism to lyse
