Oestrous Cycle - Sheep

Oestrous Cycle

• Short day breeders (oestrous cycles commence in response to decrease in day length).
• In Britain, sheep cycle from October-February, during which time there are 8-10 recurrent oestrous cycles.
• Ewe lambs and yearling ewes have shorter breeding seasons than older ewes.
• The length of the oestrous cycle is about 17 days.
• Duration of oestrus in most British breeds is ~30 hours and is at least 10 hours in immature ewes.
• In merino sheep, behavioural oestrus (heat) may last 48 hours.
• Ovulation occurs towards the end of oestrus.

Cyclic Changes in the Ovaries

• Ovaries are smaller than those of the cow and their shape is more spherical.
• During anoestrus they measure approximately:
  • 1.3 cm pole-pole
  • 1.1. cm from the fixed edge to the free margin
  • 0.8 cm from side to side
• Transrectal ultrasound is difficult, because the ovaries are difficult to access. Therefore, folliculogenesis is much harder to visualise than in the cow.
• The ewe is similar to the cow, with 3-4 follicular waves in each oestrous cycle.
  • If there are 3 follicular waves, two will occur during the luteal phase and one during the follicular phase.
• Even during anoestrus, dominant follicles will reach the same size as those present during cyclical activity.
• At the onset of oestrus, one or more follicles reach a size of 1cm.
  • Follicular walls are thin and transparent.
• Rupture of the follicle at ovulation is preceded by the elevation of a small papilla above the general surface.
• Ovulation occurs through rupture of this papilla ~24 hours after the onset of heat.
• There is rapid development of the corpus luteum.
  • Development is linear from day 2-12 after ovulation.
• By day 5 of dioestrus, the corpus luteum is 0.6cm in diameter.
• It then attains a maximum size of 0.9cm diameter and has a central cavity.
• Over the period of dioestrus, the colour changes from blood red to pale pink. The size stays constant until the onset of the next oestrus.
• At the onset of the next oestrus:
  • Regression of the corpus luteum of the previous cycle is rapid
  • Colour changes to yellow and then brownish-yellow.
• The luteolytic mechanism is similar to in the cow.
  • At the end of dioestrus, under the influence of oestradiol and progesterone, there is an increase in the number of uterine oxytocin receptors.
  • At the same time, the corpus luteum produces oxytocin, which stimulates PGF2α. PGF2α then acts to stimulate the release of oxytocin in a feedback loop mechanism.
• The first corpora lutea formed after the first ovulation at the start of the breeding season have a shorter lifespan than subsequent ones.
• In twin ovulations, the two corpora lutea can occupy the same or opposite ovaries.
• During pregnancy:
  • The corpus luteum is 0.7-0.9 cm in diameter.
  • It is pale pink in colour
  • The central cavity that is seen in cyclic corpora lutea has disappeared and is filled with white tissue.
• Ovulation with corpus luteum formation but without heat may occur in the anoestrus period. This is known as spurious ovulation.
• The number of ova shed at each oestrus is dependent on genetic and nutritional factors.

Endocrine Changes during the Oestrous Cycle

• Just before the onset of oestrus, there is a rise in oestrogens, particularly oestradiol 17β.
• This is follwed by an LH surge, which peaks at ~14 hours before ovulation.
• At the same time as LH peaks, there is a rise in FSH.
• There is also a second peak in FSH two days after ovulation.
• Progesterone concentrations closely follow changes in the corpora lutea.
  • Maximum progesterone concentrations are lower than those of the cow.
• Prolactin fluctuates throughout the oestrous cycle.
  • Concentrations rise during oestrus and ovulation
  • This pattern reflects the role of prolactin in formation of the corpus luteum.