In vitro maturation and early developmental capacity of bovine oocytes cultured in pure follicular fluid and supplementation with follicular wall

Mammalian oocytes mature in follicular fluid (FF), surrounded by follicular cells. In the present study, in vitro maturation of bovine oocytes cultured in FF from dominant follicles 15–17 mm in diameter (with various forms of heat pretreatment) and supplementation with follicular wall from follicles 3–5 mm in diameter (FW1) were examined. Heat pretreatment of FF was as follows: (1) no treatment (FF1); (2) 56 °C for 30 min (FF2); and (3) 100 °C for 20 s (FF3). After IVM in FF1, oocytes underwent IVF and IVC and embryo development was assessed (up to the morula stage). The rate of oocyte maturation was decreased in pure FF1 versus control (44.5% versus 62.8%, P < 0.001). In the control medium, FW1 did not significantly affect nuclear maturation. By contrast, the addition of FW1 to FF1 increased the rate of matured oocytes approximately two-fold (85.9% versus 45.6%, P < 0.001). Furthermore, the maturation rate in the FF + FW1 system declined (from 85.9 to 71.0%, P < 0.001), whereas that in the FF system increased (from 45.6 to 71.6%, P < 0.001) with increased temperature of the FF treatment. Supplementation of the control medium with FW1 increased the yield of morulae (42.6% versus 13.7%, P < 0.001). However, the stimulatory effect of FW1 on the morula rate was much higher in pure FF1 (72.5% versus 31.7%, P < 0.001). These findings indicated, for the first time, the stimulatory impact of FW1 on in vitro maturation and early developmental capacity of bovine oocytes cultured in pure FF from dominant follicles. We also inferred that bovine FF constituents affecting bovine oocyte maturation and the meiosis-promoting ability of the FW were heat-labile.