Effect of heifer age on the granulosa cell transcriptome after ovarian stimulation

Genomic selection is accelerating genetic gain in dairy cattle. Decreasing generation time by using younger gamete donors would further accelerate breed improvement programs. Although ovarian stimulation of peripubertal animals is possible and embryos produced in vitro from the resulting oocytes are viable, developmental competence is lower than when sexually mature cows are used. The aim of the present study was to shed light on how oocyte developmental competence is acquired as a heifer ages. Ten peripubertal Bos taurus Holstein heifers underwent ovarian stimulation cycles at the ages of 8, 11 (mean 10.8) and 14 (mean 13.7) months. Collected oocytes were fertilised in vitro with spermatozoa from the same adult male. Each heifer served as its own control. The transcriptomes of granulosa cells recovered with the oocytes were analysed using microarrays. Differential expression of certain genes was measured using polymerase chain reaction. Principal component analysis of microarray data revealed that the younger the animal, the more distinctive the gene expression pattern. Using ingenuity pathway analysis (IPA) and NetworkAnalyst (www.networkanalyst.ca), the main biological functions affected in younger donors were identified. The results suggest that cell differentiation, inflammation and apoptosis signalling are less apparent in peripubertal donors. Such physiological traits have been associated with a lower basal concentration of LH.