In sheep, there have been few and conflicting
data regarding the necessity of the corpus luteum (CL) for the maintenance of
pregnancy. The aims of the present study were to examine the effect of
luteectomy on and after Day 50 of pregnancy on maternal plasma progesterone
concentrations and the progression of pregnancy, to determine the minimum
placental progesterone support required for the maintenance of pregnancy, and
to evaluate the effect of luteectomy on lambing performance. In Experiment 1,
four ewes luteectomized on Day 50 of pregnancy aborted 2–7 days after
surgery, whereas pregnancy progressed and parturition occurred between Days
143 and 149, with live lambs, in three of four ewes and in four ewes
luteectomized on Days 60 and 70 of pregnancy respectively. The mean ( SEM)
progesterone concentrations on the day before and one day after luteectomy
decreased from 4.87 0.85 to 0.42 0.06 ng mL –1
(P<0.01), from 4.57 0.51 to 0.80 0.12 ng mL
–1 (P<0.02) and from 6.05
0.52 to 1.67 0.11 ng mL –1
(P<0.01), respectively, for the ewes luteectomized on
Days 50, 60 and 70 of pregnancy. The fall in progesterone concentrations was
90%, 80% and 71%, respectively, for the ewes
luteectomized on Days 50, 60 and 70 of pregnancy. In Experiment 2, pregnancy
progressed in four ewes luteectomized on Day 70 and parturition occurred
between Days 146 and 149, with live lambs. The mean progesterone
concentrations declined (P<0.01) from 6.9 0.7 ng mL
–1 on the day before luteectomy to 2.1 0.3 ng mL
–1 the day after surgery. The concentrations of
progesterone in blood collected every 3 h during a 24-h period were stable on
Days 60 and 80 of pregnancy, but they were lower
(P<0.03) on Day 80 than on Day 60 of pregnancy, for
each time period examined. In Experiment 3, the gestation length and
birthweights of single, twin and triplet lambs were not different between the
control intact ewes (n = 111) and the ewes
luteectomized on Days 70–80 of pregnancy (n
= 71). Lamb mortality was not different between the two groups
(7.2% v. 8.4%, control v. luteectomized).
In conclusion, these results showed that (1) the sheep
CL is necessary to maintain pregnancy until at least Day 60,
(2) progesterone withdrawal induced by luteectomy on and
after Day 50 of pregnancy must be of a critical magnitude to provoke abortion,
(3) after Day 60 of pregnancy, the CL and the placenta
together secrete more progesterone than required for pregnancy maintenance,
(4) there is no apparent 24- hour rhythm in maternal
plasma progesterone concentrations before and after luteectomy, and
(5) luteectomy at mid pregnancy has no apparent effect
on gestation length, lamb birthweight or lamb mortality.
Studies on the Anemia of Disseminated Malignant Neoplastic Disease. II. Study of the Life Span of the Erythrocyte
