Histological and steroidogenic changes in dominant ovarian follicles during oestradiol-induced atresia in heifers

Histological and steroidogenic changes within dominant ovarian follicles (DFs) undergoing atresia following systemic administration of oestradiol benzoate (ODB) were characterized in beef heifers. At 5.6 ± 0.1 days after the onset of oestrus, heifers received 1 mg ODB i.m./500 kg body weight (ODB; n = 15) or served as controls (n = 15). Timing of treatment initiation was designated as hour (h) 0 on day (d) 0, and coincided with the presence of the DF of the first follicular wave (DF1). Within treatments, the DF1 was collected following ovariectomy in four animals at h 12, h 36 or after ultrasonic detection of a new wave (NW) of ovarian follicular development. In heifers of the NW groups (n = 7 per treatment), blood samples were collected at intervals of 20 min for 12 h beginning at h -12, 0, 24 and 48 to characterize circulating LH patterns. Administration of ODB suppressed (P 0.1) among treatments. A prominent characteristic of oestradiol-induced atresia of the DF1 of the oestrous cycle in heifers was a loss in oestrogenic function associated with reduced LH support. However, the timing of new follicular development may be influenced by a factor(s) other than the status of the DF undergoing oestradiol-induced atresia.