Oestrous Cycle - Sheep

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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.