Characteristics of Small Antral Follicles in Heifers Carrying the Bovine High Fecundity Allele Trio

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The Ohio State University

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Trio, a bovine high fecundity allele, results in ovarian antral follicles that develop at a slower rate, acquire ovulatory capacity at smaller sizes, and have a 3-fold greater ovulation rate. Results from recent research indicate that, after primordial follicle activation, follicles in Trio carriers have slower progression through the primary stage. However, the effect of the Trio allele on small antral follicles remains unknown. Thus, the objective of the present study was to compare the morphological characteristics of small antral follicles of Trio carrier and non-carrier cattle. Ovarian tissue from Trio carrier (n=9) and non-carrier heifers (n=6) was collected, fixed, and paraffin embedded. Six sections of each ovary were stained with hematoxylin-eosin and evaluated under a bright field microscope. Small antral follicles (< 1 mm) with a clearly defined antrum and visible oocyte were imaged. The area of the follicle, oocyte, antrum, and granulosa cell layer was determined using Image J. Differences between genotypes were assessed by t-test or Wilcoxon's rank test. A total of 22 and 8 small antral follicles were identified from carrier and non-carrier heifers, respectively. Total follicle area was not different (P=0.56) between Trio carrier (181,522±24,963 µm^2) and non-carrier (209,295±35,429 µm^2) follicles. Similarly, there were no differences (P>0.3) in granulosa cell layer area (130,614±13,039 µm^2), antrum area (50,314±7,677 µm^2), or oocyte area (8,000±610 µm^2) between Trio carrier and non-carrier follicles. The percentage of the total follicle area occupied by the antrum (22.8% ± 1.6) or the granulosa cell layer (71.5%±1.3) was not different (P>0.20) between gentoypes. There was, however, a tendency (P=0.07) for the oocyte to occupy a larger percentage of the follicle area in Trio carrier (6.3%±0.8) than non-carrier (4.1%±0.6) follicles. In conclusion, presence of the Trio allele does not affect the morphological characteristics of small antral follicles.



trio, bovine, antral follicle, granulosa cell, oocyte, high fecundity, heifer