Term uterus and maintenance of pregnancy
Keywords: ovine, sheep, placenta, corpus luteum endocrinology, pregnancy
An illustration of a term uterus and the ovaries. Although this was a triplet pregnancy, the ovaries were very small (arrows) and contained corpora lutea that were significantly smaller than those in cycling ewes. They appeared to be in a state of regression similar to that shown on the lower left inset, a CL from another term ovine pregnancy.
Image size: 1499 x 1691px
In ewes, the corpora lutea are only essential for the maintenance of pregnancy in the first third of a 145 to 150 gestation. After about 60 days (with some variance) neither ovariectomy nor luteolyic treatment with prostaglandins will cause abortion. This is because pregnancy is maintained by the placenta. This situation is distinctly different to that in goats where luteal tissue is essential throughout pregnancy.
With single, twin and triplet lambs in one study, neither gestation length, birth weights nor incidence of dystocia were affected by ovariectomy at 70 days of gestation. The lack of dystocia is not surprising, because ovine corpora lutea are not essential sources of relaxin during pregnancy (as they are in pigs); that role falls to the placenta in sheep.
Although corpora lutea are present at the end of gestation and actively contribute to serum progesterone concentrations, the essential role of luteal tissue in late gestation remains an enigma.
References:
Al-Gubory,K.H. et al 1999. Effects of luteectomy on the maintenance of pregnancy, circulating progesterone concentrations and lambing performance in sheep. Reprod.Fert.Devel. 11:317 - 322
Bryant. G and Chamley, W.A. 1976 Plasma relaxin and prolactin immunoactivities in pregnancy and at parturition in the ewe. J. Reprod Fert. 48:201-204
