Cumulus cell expansion and ultrastructural changes in in vitro matured bovine oocytes under heat stress.

Cumulus-oocyte complexes (COCs) from cows were matured under normal (38.5°C) and elevated temperatures (41°C) simulating heat stress and their maturation was assessed based on measurement of cumulus expansion in both groups. There was a significant reduction (P<0.01) in maturation rate in the heat stressed oocytes. The ultrastructural events associated with in vitro oocyte maturation and changes associated with elevated temperature were also studied by transmission electron microscopy (TEM). Normal maturation cellular events were marked by migration of Golgi and mitochondria from the cortical regions, and conversely by a migration of cortical granules from the inner regions to a sub-perivitelline zone. Heat stressed oocytes (41°C) were not only marked by a reduction in rate and less cumulus cell expansion, but also by a reduction in cortical granule migration. The mitochondria appeared swollen with cristolysis. Ribosomal disruption and an abundance of free ribosomes were also seen. Changes in the cumulus cells include nuclear chromatin margination, condensation and karyolysis, formation of nuclear and cell membrane blebs, and typical membrane bound vesicles enclosing cell fragments indistinguishable from apoptosis. Evidently, heat stress can be associated with reduced cytoplasmic events of oocyte maturation, thereby decreasing the oocyte competence and can be associated with apoptosis of the cumulus cells and therefore compromise the survival of the oocyte itself.