Preantral Follicle Numbers and Size in Heifers Carrying the Bovine High Fecundity Allele Trio
The Ohio State University
The bovine high fecundity allele, Trio, results in overexpression of SMAD6 and a 3-fold increase in ovulation rate compared to wild-type non-carrier cattle. In addition, Trio carriers have similar number of antral follicles as non-carrier cattle; however, antral follicles develop at a slower growth rate and acquire ovulatory capacity at a smaller size. The present study was designed to test the following hypotheses: 1) Trio carrier cattle have similar preantral follicle numbers as non-carriers; and 2) preantral follicles of Trio carriers are smaller in size than non-carriers. Ovarian tissue from Trio carrier (n = 12) and non-carrier (n = 12) heifers were obtained by laparotomy and a 1 x 1 cm section was fixed and paraffin embedded. Sixty consecutive sections (6 µm) were obtained and every tenth (6 total) mounted and stained with hematoxylin-eosin. Follicle numbers were determined for each stage of development (primordial, primary, and secondary) using a 5 x 5 grid overlay. Follicle dimensions were determined from 10 random follicles of each follicle stage and heifer using ImageJ. Differences between genotypes were assessed by t-test or Wilcoxon's rank test. Number of primordial and secondary follicles were not different between genotypes (P > 0.1). Trio carriers had greater (P < 0.01) number of primary follicles per square mm (0.27 ± 0.04) than non-carriers (0.11 ± 0.01). Primordial follicle and oocyte volume were not different between genotypes (P > 0.2). Primary follicle and oocyte volume were 1.2-fold and 1.1-fold larger in Trio carrier than non-carriers (P < 0.05). Secondary follicle volume was not different (P > 0.5), however, oocyte volume was 1.8-fold larger in Trio carriers than non-carriers (P < 0.03). Granulosa cell number per cross section was not different between genotypes at any stage (P > 0.4). In conclusion, follicle numbers were similar for most preantral follicle stages, however, Trio carriers had greater number of primary follicles. In addition, primary follicle and oocyte and secondary oocyte size were greater in Trio carriers. These results suggest that, once activated, Trio carrier follicles, have reduced progression through the primary stage, hence the larger oocyte and greater number.
folliculogenesis, reproductive physiology, animal sciences, high fecundity allele