Effects of reduction or replacement of fetal calf serum by other compounds during in vitro maturation of bovine oocytes

The use of fetal calf serum (FCS), although widely employed during in vitro production (IVP) of bovine embryos, has limitations. FCS is an undefined media and may have harmful effects on the quality of embryos. For this reason, in recent years, research efforts aimed at improving IVP of bovine embryos, have focused at the replacement of FCS by alternative compounds in culture media. In this study, fatty acid free bovine serum albumin (BSA-FAF) and embryonic fluid (EF) were used separately or in combination, in different concentrations, to replace or reduce the concentration of FCS during in vitro maturation (IVM). For this purpose, bovine oocytes were in vitro matured under different treatments and were assigned to one of the following groups (G) according to the protein supplementation: G1 (control) = 10% FCS, G2 = 8mg/mL BSA-FAF, G3 = 10% EF, G4 = 6mg/mL BSA-FAF + 5% FCS, G5 = 6mg/mL BSA-FAF + 3.5% FCS + 1.5% EF, G6 = 6mg/mL BSA-FAF + 1.5% FCS + 3.5% EF, G7 = 6mg/mL BSA-FAF + 5% EF, and G8 = 5% FCS + 5% EF). After 24 hours of IVM, oocytes were classified according to the meiotic progression and migration of cortical granules (CG) to the periphery. Maturation rates were evaluated by chi-square test (chi 2) or, when appropriate, by Fisher's exact test, and orthogonal contrasts were performed to isolate effects of the compounds. The group supplemented with BSA-FAF (G2) had a lower number of oocytes that reached MII (75%) compared to the groups G1, G4, G5 and G8 (88.9%, 89.6%, 87% and 86.8%, respectively), with no difference in relation to G3 (79.8%), G6 (82.9%) and G7 (82.9%). Moreover, G3 also showed inferior nuclear maturation rate when compared to G4. Regarding cytoplasmic maturation, the rates were reduced to 43.9%, 43.2%, 43.1% and 36.5% in G2, G7, G6 and G3 groups, respectively, compared to the control group (G1; 62.4%). On the other hand, in the groups G8, G4 and G5, maturation rates were not affected by reduction of FCS, where 59.3%, 51.3% and 50.8% of the oocytes displayed CG arranged peripherally, respectively. The results obtained by the orthogonal contrast test are in accordance with the ones from the evaluation of the nuclear maturation and cortical granules migration. These data show the need of FCS on the MIV, even in low concentrations, and the possibility of decrease its concentration by associating it with BSA-FAF and/or EF. Therefore, we concluded that it is possible to reduce the concentration of FCS in IVM medium to a concentration of 3.5% without affecting nuclear and cytoplasmic maturation rates.