In vivo ovarian temperature promotes the in vitro growth and developmental competence of oocytes derived from bovine early antral follicles

In cattle, the culture temperature used for the in vitro growth (IVG) of immature oocytes is generally 38.5 or 39.0 °C, which is close to the normal temperature in the vagina or rectum. However, the temperature in the in vivo ovarian tissue is approximately 1 °C lower (37.5 °C) than that in the vagina or rectum. Therefore, the generally accepted culture temperature may not be optimal for the IVG of bovine oocytes. Herein, we investigated the effects of culture temperature on the IVG of oocyte-cumulus granulosa complexes (OCGCs) derived from early antral follicles (0.5–1 mm in diameter). OCGCs were subjected to 12 days of IVG at temperatures of 37.5, 38.5, and 39.0 °C. OCGC viability and antrum formation were evaluated every 4 days. Estradiol-17β (E₂) and progesterone (P₄) production from OCGCs during the 1st, 2nd, and 3rd 4-day periods was evaluated by enzyme immunoassay. Viable OCGCs after IVG were subjected to in vitro maturation (IVM), in vitro fertilization, and embryo culture. Then, the nuclear status and diameter of oocytes after IVM, rates of cleavage and blastocysts, and cell number in blastocysts were evaluated. In addition, the mRNA expression of heat shock proteins (HSPs) in the granulosa cells and reduced glutathione (GSH) levels in oocytes after IVG were measured. The viability of OCGCs did not differ among the groups, whereas the rate of antrum formation on day 12 of IVG culture was highest in the 37.5 °C group (P < 0.05). P₄ production did not differ among the groups; however, E₂ production during days 8–12 tended to be higher in the 37.5 °C group than in the other two groups combined (P < 0.1). The mRNA expression of HSP70 and 90, and the GSH levels of oocytes, did not differ among the groups. The oocyte diameter after culture was larger in the 37.5 °C group than in the 39.0 °C group (P < 0.05), and that in the 38.5 °C group was intermediate between the other two groups. The rates of nuclear maturation and cleavage did not differ among the groups. However, the blastocyst rate was higher in the 37.5 and 38.5 °C groups than in the 39.0 °C group (P < 0.05). The cell number in the blastocysts in the 38.5 °C group was smaller than the in vivo-grown oocytes, while that in the 37.5 °C group and the in vivo-grown oocytes did not differ. In summary, OCGCs in the 37.5 °C group showed healthy morphology and steroidogenesis, as well as better growth and developmental competence of oocytes. Therefore, culture conditions close to the in vivo ovarian tissue temperature would be optimal for the IVG of immature bovine oocytes.