The physiological responses of anovulatory ewes to the introduction of rams – a review.

If pre-conditioned by a period of isolation from rams, prepubertally, seasonally or lactationally anovulatory ewes of many breeds can be stimulated to ovulate by the re-introduction of rams. The signal from the rams is partly pheromonal and activates neural connections between the main olfactory tract and the anterior hypothalamus. This leads to an increase in the frequency of pulses of luteinizing hormone (LH), a process which is essential for the induction of ovulation and is usually completed within a few minutes. The high pulse frequency stimulates follicular growth and oestradiol secretion by the ovaries. The subsequent build-up of oestradiol in the blood has two effects: in the short term (the first 2-12 h) it reduces the levels of follicle-stimulating hormone (FSH) and the amplitude of the LH pulses; in the long term (12-48 h) it induces preovulatory surges of both LH and FSH. The LH surge induces ovulation and the formation of a corpus luteum.
In some ewes, the corpus luteum is normal and a normal luteal phase follows the first ovulation. In other ewes, the first corpus luteum secretes little progesterone and regresses within 6 days. A second LH surge is then released, inducing a second ovulation and the formation of an apparently normal corpus luteum. If the ewes are treated with progestagen before the rams are introduced, all the corpora lutea formed after the first ovulation appear to be normal. This appears to be effected through both a delay in the onset of the LH surge and through a direct action of progesterone on the ovary.