Oestrous Cycle - Horse

Oestrous Cycle

• Seasonal breeder (long day)
• Oestrous cyclicity from spring-autumn. During the winter, the mare is normally anoestrus.
• Winter anoestrus is follwed by a transition to regular cyclic activity. During this time, the duration of oestrus may be irregular or very long.
• Before the first ovulation, behavioural oestrus may not be accompanied by the presence of palpable follicles on the ovary.
• Some long oestrus periods in spring are anovulatory.

• Average cycle length is 20-23 days. The cycles are longest in spring and shortest from June-September.
• Oestrus lasts 6 days.
• Dioestrus lasts 15 days.
• Ovulation occurs on the penultimate day or last day of oestrus.
• The diameter of a ripe follicle ready to ovulate is 3-7cm. The day before ovulation, the tension in the follicle is reduced. Palpation of a large, fluctuating follicle is a reliable sign of impending ovulation.

• The onset of oestrus after foaling is known as the 'foal heat'.
• Occurs on day 5-10 post-parturition.
• Sometimes shorter than normal, lasting 2-4 days.
• The first two post-parturient cycles are a few days longer than normal.

• During Oestrus, a single egg is usually released. Thus, the mare is monotocious.
• Ovulation seems to occur more frequently from the left ovary.
• All ovulations occur from the ovulation fossa.
• Due to the reversed structure of the ovary, corpora lutea may only be seen sometimes at the ovarian hilus. However, because the ovary is curved and mostly covered by fimbrae of the oviduct, corpora lutea cannot be identified by rectal palpation.
• Only fertilized eggs can pass into the uterus.
• Non-fertilized eggs remain in the Utero-Tubal junction for months, where they finally disintegrate.

Cyclic Changes in the Ovaries

• Just before the onset of oestrus, several follicles enlarge to 1-3cm.
• By the first day of oestrus, one dominant follicle is significantly larger than the others, with a diameter of 2.5-3.5 cm.
• During oestrus, this dominant follicle matures and ruptures once it has attained a diameter of 3-7 cm.
• Several hours before ovulation, the ripe follicle becomes much less tense and can be recognised as an indentation on the surface of the ovary.
• There is usually haemorrhage into the follicle and the coagulum hardens within the next 24 hours.
• After ovulation, the other follicles regress.
• During the first 4-9 days of dioestrus, there are no follicles over 1cm present on the ovary.
• For 3 days after ovulation, the leutinising mass can be felt, but later it normally has the same texture as the ovary.
• The corpus luteum reaches full size at 4-5 days after ovulation but does not protrude from the ovarian surface.
• On section of the ovary, the corpus luteum will appear brown and later yellow. It is triangular or conical in shape, with the narrow end on the ovulaiton fossa. The centre of the corpus luteum normally contains dark brown fibrin.
• The corpus luteum of the cycle (non-pregnant) starts to regress on about the 12th day of the cycle. At the time of regression, there is a fall in blood progesterone concentration.

• During winter anoestrus, both ovaries are usually small and bean-shaped. They normally measure:
  • 6cm pole-pole
  • 4cm from the hilus to the free border
  • 3 cm side-side
• During the cycle, the size of the ovary depends on the number and size of the follicles.
• During oestrus, the ovary may contain 2-3 follicles. These can each measure 4-7 cm. These, combined with other less developed follicles, give the ovary a huge size.
• During Dioestrus, there is an active corpus luteum and atretic follicles present. However, these only give the ovary a size a little larger than in Anoestrus.

Endocrine Changes During the Oestrous Cycle

• Biphasic secretion of FSH with surges every 10-12 days.
  • One surge after ovulation and a second in mid-late Dioestrus (~10 days before the next ovulation).
• This increase in FSH secretion is unique to the mare.
  • Primes the development of a new generation of follicles, one of which will ovulate during the next oestrus.
• The pattern of LH secretion is also unusual.
  • No dramatic LH surge prior to ovulation.
  • LH gradually increases and elevated levels then persist for 5-6 days either side of ovulation.
• Oestrogens reach peak values during oestrus.
• Progesterone concentrations follow the changes of the corpus luteum closely.

Uterine Cytology

This is often performed routinely before breeding commences on stud farms. Endometrial and inflammatory cells are examined. Evidence of bacteria in the absence of inflammatory cells is normally due to contamination.

• Oestrus/Active Cycling
  • A high number of columnar epithelial cells with oval, basal nuclei containing finely stippled chromatin.
  • Columnar, finely vacuolated cytoplasm.
  • There should be no inflammatory cells or infectious agents visible. Rarely neutrophils may be present if it is a foal heat or the first oestrus cycle in maiden mares.

• Anoestrus/Ovarian Inactivity
  • Clumps of epithelial cells with crowded or overlapping, oval, hypochromatic ‘bland’ nuclei without granular chromatin or nucleoli.
  • Unsubstantial cytoplasm with indistinct boundaries.