Introductive Physiology of the mare and stallion

Oestrous cycle of the mare

Seasonal polyoestrus

• Multiple cycles during 1 season

• Long day (spring and summer)

April until September/ October

Stages of the oestrus cycle

Aneostrus

• not cycling

• winter months

Transition phase

• 50-70 days prior to first ovulation (false starts of cycling)

Cycling

• Dioestrus (16 days)

• Oestrus (5 days)

How does light affect oestrus?

Seasonality controlled by light- long day breeder

Light detected by eye > pineal gland

Activates brain centres

Decreased melatonin secretion with decreasing periods of darkness

Release of gonadotrophins (FSH and LH)

Activate ovary stimulating development of follicles

Secretion of oestrogen and progesterone

• Influence behaviour and receptivity of mare

• Alters uterine environment

Manipulation of seasonality

Artificial change to photoperiod i.e. put mares and stallions under lights at xmas

Physical change to photoperiod i.e. “shuttle” stallions from northern to southern hemispheres biannually

Oestrous cycle and relevant hormones

Mares reproductive tract

Follicle production

1st FSH peak- day 2-6: multiple follicles form

2nd FSH peak- days 8-12: follicles grow

days 16-18: dominant follicle grows at faster rate

LH- day 21: follicle lysis and ovulation

Ovulation

Ovulation via ovulation fossa

Follicle ovulates- oocyte (immature egg) released

• captured by Cumbria of infundibulum

• enters oviduct/ fallopian tube

• fertilisation,

NO- oocyte degenerates in oviduct

YES- fertilised embryo enters uterus and develops

Ovarian events following ovulation

Ruptured follicle fills with blood

• corpus haemorrhagicum forms (days 1-4)

Differentiates into corpus luteum (day 6-10)

Secretes progesterone

“Luteal phase”/ dioestrus

If not pregnant

• CL lysed by prostaglandin F2a

If pregnant

• CL remains

What hormone indicates the presence or absence of a corpus luteum?

Progesterone

What hormone rises rapidly after ovulation in the mare?

Progesterone

What hormone is useful for monitoring stage of cycle/ ovulation?

Progesterone

Progesterone

Progesterone assay

Stallion reproductive tract

Placenta