Ultrasonographic study of the effects of the corpus luteum on antral follicular development in unilaterally ovulating western white-faced ewes

dThe objective of this study was to examine the local effects of the corpus luteum (CL) on ovarian antral follicle development by looking at follicle populations and dynamics in ovaries with or without CL, in unilaterally ovu lating ewes, using a retrospective analysis of daily ultrasonographic recor ds. The present report summarises the data from the first luteal phase of t he breeding season (August-October; n = 4), a luteal phase in the mid-breed ing season (November-December; n = 5), the last luteal phase of the breedin g season (January-March; n = 5), and the luteal phase after GnRH-induced ov ulations in mid-anoestrus (May-June; n = 4) of western white-faced ewes. Me an daily numbers of 3 mm follicles that did not grow any larger were signif icantly reduced in the CL-containing ovaries of ewes at all periods of stud y except for the transition to anoestrus. With all scanning periods combine d, daily numbers of 3 mm follicles not growing further increased (P < 0.05) between day 6 and 15 after ovulation in the CL-containing ovaries. Based o n mean data for the whole periods of observation, the non-CL-bearing ovarie s of ewes in the transition to anoestrus had fewer (P < 0.05) follicles gro wing from 3 to greater than or equal to5 mm in size before regression compa red with the mid-breeding season and mid-anoestrus. The lifespan of follicl es reaching >5 mm in diameter was shorter (P < 0.05) in the CL- compared wi th non-CL-containing ovaries of anoestrous ewes induced to ovulate with GnR H ((6.5 +/- 1.3) and (9.0 +/- 1.0) days, respectively). Circulating concent rations of progesterone were lower during both transitional periods (into a nd out of anoestrus) and mid-anoestrus than during the mid-breeding season (P < 0.001), and were less during anoestrus than during both transitional p eriods (P < 0.05). It was concluded that CL/luteal structures locally suppr essed the growth of ovarian antral follicles to the 3 mm size-range except during the transition to anoestrus, but that there was no inhibitory effect of the CL on the growth of ovarian follicles to larger diameters. The pres ence of CL/luteal structures did not affect the length of the lifespan of f ollicles reaching greater than or equal to5 min in diameter nor the number of ovulations per ovary in cyclic ewes, but shortened large follicle lifesp an in anoestrous ewes. Variations in peripheral concentrations of progester one across the breeding season and between the breeding season and anoestru s did not alter the lifespan of large antral follicles. In the transition t o anoestrus and during mid-anoestrus, the presence of the CL in an ovary ap peared to maintain follicle development to ovulatory sizes and to increase the rate of turnover of large antral. follicles, respectively. (C) 2001 Els evier Science B.V. All rights reserved.