Heat stress (HS) during summer was found to impair the reproductive potentials in buffalo. The present study was designed to evaluate the effect of seasons on: a) Number of the ovarian follicles and number and quality of their enclosing oocytes, b) Cumulus cell expansion and nuclear maturation rates of in vitro matured buffalo oocytes In experiment 1: During whole
year, buffalo ovaries were collected and the number of ovarian follicles was counted, oocytes were recovered/ovary and classified according to their morphology into 4 Grades. In Experiment 2: Grade 1 and 2 cumulus-oocyte-complexes (COCs) were in vitro matured (IVM), cumulus-cell expansion and nuclear maturation were determined. In Experiment 3:Mitochondrial activities in non-matured and matured buffalo oocytes were assayed using Mito-Tracker Red stain. Results showed that the number of ovarian follicles and the quality of oocytes were diminished in summer. Also, summer season significantly (P<0.05) decreased cumulus-cell expansion and nuclear maturation of IVM buffalo oocytes. Matured buffalo expressed clustering homogenous distribution, while, non-matured ones showed polarized mitochondrial distribution.