Follicular fluid steroid hormones and in vitro embryo development in Duroc and Landrace pigs

The Duroc sire line has a smaller litter size compared to the Landrace dam line and we have previously observed fewer surface follicles on Duroc ovaries one day after weaning. In that same study, a broader cumulus expansion and faster nuclear maturation were observed for Duroc oocytes at 20 h of in vitro maturation (IVM), while Landrace oocytes showed more advanced stages of cortical granule distributions. However, no differences between breeds were observed after the final IVM period. The aim of this study was to assess subsequent in vitro embryo production (IVP) in Duroc and Landrace. Furthermore, follicle diameter and steroid hormone levels in follicular fluid (FF) were measured to study possible relation to oocyte developmental competence. Follicular phase sow ovaries were collected one day after weaning and follicle size of the 10 largest follicles were measured per ovary before aspiration. Cumulus-oocyte complexes (COCs) were matured in vitro, and cumulus expansion was analysed by assessing individual COC areas at 0 and 20 h. Fertilization of Duroc and Landrace oocytes was performed with sperm from both a Duroc and a Landrace boar. A larger follicle diameter was observed for Landrace animals (5.7 vs. 4.8 mm, P < 0.0001) and individual COC area was additionally larger at 0 h after aspiration (P < 0.0001) compared to Duroc. Contrary, cumulus expansion from 0 to 20 h of maturation was broader for Duroc oocytes than for Landrace (407 ± 67% vs. 319 ± 31%, P < 0.0001). After fertilization, cleavage rate was higher for Duroc oocytes, and the highest blastocyst yield was obtained for Duroc oocytes fertilized with the Landrace sperm. Steroid hormone analysis of the follicular fluid showed differences in the pathways between breeds with a higher total level of estrogens (P = 0.01) and aromatase products/substrates ratio (P < 0.01) in Landrace compared to Duroc. In conclusion, results suggest that Duroc oocytes have a better in vitro oocyte developmental competence when cultured under the same in vitro conditions and breed differences in steroidogenesis were found in the early follicular phase.