Variation in the follicle pool of weaned sows: granulosa cell gene expression in healthy small antral follicles

Over the last decades, pigs have been genetically selected to produce larger litters. This resulted in decreased piglet birth weights and increased within-litter variation in piglet birth weight which are both related to higher piglet mortality.
The aim of this study was to identify the underlying mechanism of variation in the follicle pool by studying differential gene expression of granulosa cells of large and small follicles within the same antral follicle pool at the time of weaning (=onset of the follicular phase).
The right ovary of 29 multiparous sows was collected within 2 hours after weaning and immediately snap-frozen. All visible antral follicles were measured and classified as either healthy or atretic by a cleaved caspase-3 immunostaining. The variation in follicle size of the 10 largest healthy follicles was used to select a total of eight sows for further analysis; four sows with the largest variation in follicle size of the 10 largest healthy follicles and four sows with the smallest variation in follicle size. Granulosa cells of the 3 largest and 3 smallest healthy follicles of each of the 8 animals were isolated using laser-capture microdissection and differential gene expression was analyzed by individual hybridization with porcine whole-genome microarrays.
Average follicle size of the 3 largest and 3 smallest healthy follicles of the sows selected for large variation in follicle size was 7.1±0.8mm vs. 3.9±0.9mm and was 6.1±0.8 vs. 4.9±1.1mm for the sows selected for small variation in follicle size. A core set of 33 genes was differentially expressed in large vs. small follicles in both the large variation follicle pools as the small variation follicle pools. The role of these core set genes will be analyzed to investigate the underlying mechanism of variation in the healthy antral follicle pool.
Surprisingly, genes involved in the final steps of the steroid synthesis pathway were already expressed in early antral follicles. Steroid profiles in serum and follicular fluid have been measured using LC-MS and data are currently analysed.