The effect of cysteamine on in vitro nuclear maturation and GSH content in equine oocytes

The aims of this study were to determine whether equine oocytes benefit from the addition of cysteamine during IVM and to compare the glutathione (GSH) content of mare oocytes after in vivo and in vitro maturation. In vivo matured cumulus-oocyte complexes (COCs) were collected from ovulatory follicles and intra-oocyte GSH concentration was determined after cumulus cell removal and MII stage assessment. Immature COCs were collected from ovaries isolated from slaughtered animals. They were matured either in 500 litre of control medium (TCM 199+20% fetal calf serum+50 ng/ml EGF), or control medium supplemented with 100 M cysteamine, or in defined medium (TCM 199+0.4% BSA), or defined medium supplemented with 50 ng/ml EGF with and without 100 M cysteamine for 30 h at 38.5degreesC and 5% CO 2 in air. The nuclear maturation rate after IVM of in vivo collected immature oocytes was similar to those isolated from ovaries of slaughtered animals. Oocytes matured in TCM 199 supplemented only with BSA exhibited a maturation rate significantly lower than other treatments (P<0.05). The addition of cysteamine did not influence the maturation rate in all treatments used in both in vivo collected immature oocytes and those isolated from slaughtered ovaries. The addition of cysteamine did not influence the GSH concentration at the end of maturation period. In vivo matured MII oocytes displayed a similar GSH content to oocytes matured in vitro in the presence of serum and EGF. The results demonstrated that in vitro maturation conditions are able to stimulate a significant increase in intra-oocyte GSH content, similarly to in vivo matured oocytes. Moreover, the addition of cysteamine affects neither the GSH content in MII oocytes nor the maturation rate of equine oocytes. It is concluded that the low in vitro developmental competence of equine oocytes is not due to the lack or defective GSH synthesis but to other factors that could be involved in cytoplasmic maturation, oocyte-sperm interaction and early development.