Surface Ultrastructural Characteristics of Bovine Oocytes Following Heat Shock

Abstract

Surface morphology of the zona pellucida (ZP) and the vitellus of bovine oocytes treated by heat shock were evaluated by scanning electron microscopy (SEM). Bovine oocytes were recovered from slaughterhouse ovaries and matured in vitro for 24 h (young oocytes) or 40 h (aged oocytes). After maturation incubation, they were heat-shocked at 42 C for 30 min and then incubated at 39 C for 1, 2, 4, 8 and 16 h. The ZP was characterized by a mesh-like structure in untreated young oocytes, whereas in untreated aged oocytes the ZP meshwork became thinner. After heat shock, partial breakage (16/69) or coalescence (53/69) of the ZP meshwork was noted in young oocytes, and the altered surface of the ZP was not restored during incubations for up to 16 h. In aged oocytes, however, partial peeling off of the outer ZP surface was noted frequently after heat shock (6/10 at 1 h, 10/10 at 2 h, 7/10 at 4 h and 6/6 at 8 h). The vitelline surface of untreated young oocytes was predominantly covered by well-developed microvilli (MV). In aged oocytes, a mixed distribution pattern of MV and cytoplasmic protrusions (CP) was frequently noted (8/9). After heat shock, the observance of the mixed MV and CP increased in the young and aged oocytes and enlarged CP were observed more frequently in the aged oocytes. These observations suggest that a brief heat shock (42 C, 30 min) facilitates a breakage of the ZP surface and changes the vitelline surface from a MV-predominant to a CP-predominant pattern; furthermore its effect may manifest itself more quickly and to a greater degree in aged oocytes than in the young oocytes.