THE REMOVAL OF THE SPERM PERINUCLEAR THECA AND ITS ASSOCIATION WITH THE BOVINE OOCYTE SURFACE DURING FERTILIZATION

The perinuclear theca (PT) is a unique cytoskeletal structure whose an terior part is intercalated between the inner acrosomal membrane and t he nuclear envelope of the mammalian sperm head and is important for s permiogenesis and stabilization of sperm structures (Oko and Maravei, Biol. Reprod. 50, 1000-1014, 1994; Oko and Maravei, Microsc. Res. Tech . 32, 520-532, 1995). Using immunofluorescence labeling of inseminated bovine oocytes and serial sectioning-ultrastructural analysis, we dem onstrate that the PT is removed from the sperm nucleus following the l oss of the sperm plasma membrane and the interaction of oocyte cortex with the PT. These events precede the development of the male pronucle us. The removal of the PT involves the elongated oocyte microvilli, ri ch in actin microfilaments, since it can be blocked by the microfilame nt-disrupting drug cytochalasin B. Reduction of disulfide bonds, which is a major factor supporting the disassembly of the sperm nucleus and accessory structures during mammalian fertilization, seems to exert l ittle effect on the PT in vitro, as evidenced by the treatment of isol ated bull sperm with the disulfide bond-reducing agent dithiothreitol. In vivo, intact bull sperm microinjected into mature oocytes do not u ndergo disassembly of the PT. Consequently, the decondensation of the sperm nucleus does not occur. These data suggest that the binding of t he PT to the oocyte microvillar region and its removal from the sperm nucleus constitute an early step in mammalian fertilization, which is required for the conversion of the sperm nucleus into a male pronucleu s. (C) 1997 Academic Press.