The influence of the ovulation rate on ultrasonically determined ovine corpus luteum morphometry and progesterone concentrations in cyclic and early pregnant sheep

The objective of the present study was to describe morphology and function of the Corpora lutea (CL) during the oestrous cycle and early pregnancy in sheep with different ovulation rates. In total 40 Booroola. Mutton Merino crosses heterozygous carriers (FecBFec+) and non-carriers (Fec+Fec+) of the Booroola-fecundity gene (FecB)] with ovulation rates 1 to 4 were examined. During the oestrous cycle (n = 20) and the first month of pregnancy (n = 20) blood samples were taken daily (radioimmunoassay of progesterone) and an ultrasonic ovary diagnosis was conducted. The ewes were scanned transrectally with a 7.5 MHz linear probe lying in a dorsal position. During every examination the CL could be detected. The number and the diameter of the CL were documented and the total volume of luteal tissue per ewe was calculated. The effect of the ovulation rate on CL-morphology (diameter and total volume of luteal tissue per ewe) and peripheral progesterone concentrations were assessed by one-way ANOVA. On day 6 and 7 post ovulationem in cyclic and early pregnant sheep 42% of the diagnosed CL had a cavity. On day 11 (cyclic sheep) and day 10 post ovulationem (early pregnant sheep) this number decreased to 22% (p < 0.05). Both conditions of the CL (compact or with a central cavity) are similar in function and should be regarded as appearances of the same basic process. From the third day onwards the ovulation rate influenced significantly (p < 0.05) the development of the outside diameters of the CL. However, the ovulation rate had no effect on the total volume of the luteal tissue per sheep and on the progesterone concentrations. Yet, in sheep with the ovulation rate 1 significantly lower progesterone concentrations were determined than in sheep with the ovulation rates 2 to 4. In sheep with the ovulation rates 2 to 4 the peripheral progesterone concentrations did not differ significantly. In cyclic and pregnant sheep there is a positive correlation (r = 0.75, p < 0.05) between the progesterone concentration and the total volume of luteal tissue. Considering the smaller diameters of the preovulatory follicles it seems that the development of the CL continues until a threshold-value of progesterone and/or of the total luteal tissue is exceeded. Ewes with low ovulation rates reach this threshold-value with only a few but large CL. With increasing ovulation rate the CL tend to have smaller diameters.