112 OVARIAN FOLLICLE RESERVE IN NELLORE AND ANGUS HEIFERS WITH LOW AND HIGH FOLLICLE COUNT

Abstract

Cattle from Bos indicus breeds recruit and develop more follicles per oestrous cycle compared with cattle from Bos taurus breeds. The number of antral follicles in the ovaries is a decisive factor for the success of reproductive technologies such as ovum pick-up and embryo transfer. The reasons why indicine cattle recruit and develop more follicles are not known. Our objectives were to evaluate the ovarian reserve of Nellore and Angus heifers and determine at which stage of follicle development Nellor ovarian reserve differs from Angus ovarian reserve. To determine if animals were in the low or high follicle count group (LFC or HFC), antral follicles (≥3 mm) were scanned with an ultrasound device (US; Mindray Vet DPS 2200, Sao Paulo, Brazil) equipped with a 7.5-MHz probe Day 1 after ovulation (Day 0) for 3 consecutive oestrous cycles. Eighteen Nellore heifers (11 LFC and 7 HFC) and 25 Angus heifers (12 LFC and 13 HFC) were used in the experiment. Oestrous cycles were synchronized with 2 doses of PGF2α  ± 11 days apart. Animals were slaughtered ~24 h after ovulation of the third oestrous cycle. Ovulation was determined by oestrous observation, absence of the dominant follicle on the ultrasound, and presence of the corpus luteum after slaughter. One ovary from each animal was used for histology. The ovary was sliced longitudinally in 6 parts, fixed in 10% paraformaldehyde for 24 h, and stored in 70% ethanol until processing. Two longitudinal slices were blocked in paraffin. One section (8 μm thick) was stained after every 50 sections, totaling 10 sections per longitudinal slice of ovarian tissue. Sections were stained using eosin and hematoxylin. Follicles were counted and classified under a stereo microscope with a 40× objective; only follicles with a visible nucleus were counted. Follicles were classified as primordial, primary, and secondary. The area of each section was measured using the hand tool on the ImageJ software. Area measurements were expressed as pixels squared. The density of each follicle classification was calculated as follicle/log10 [pixel²]. Data were analysed using PROC GLM procedure of SAS 9.2 considering the effects of breed and group (LFC or HFC). Angus heifers had more primordial follicles (P < 0.02) when compared with Nellore heifers, but there was no difference within groups. There was no difference in the number of primary follicles within breeds or groups. Secondary follicle numbers tended to be higher (P = 0.09) in Nellore heifers when compared with Angus heifers and was higher within groups (P < 0.02), with HFC Angus and Nellore heifers having more follicles than LFC heifers. It is possible that HFC heifers have a mechanism that is developing more follicles beginning at the secondary stage, especially Nellore heifers. The development of more secondary follicles may contribute to the depletion of primordial follicles observed in Nellore heifers.

This research is funded by Sao Paulo Research Foundation (FAPESP) grant #2011/50964-0. Scholarships for Favoureto, Queiroz, Loureiro, Ereno, and Pupulin are also from FAPESP.