ASSEMBLY OF NUCLEAR-PORE COMPLEXES AND ANNULATE LAMELLAE PROMOTES NORMAL PRONUCLEAR DEVELOPMENT IN FERTILIZED MAMMALIAN OOCYTES

In addition to functional nuclear pore complexes engaged in nucleo-cyt oplasmic transport, the cytoplasmic stacks of pore complexes, called a nnulate lamellae, exist in numerous cell types. Although both annulate lamellae and nuclear pore complexes are present in fertilized mammali an oocytes, their relative roles in the process of fertilization and p reimplantation development are not known. Using epifluorescence and el ectron microscopy, we explored their fate during bovine fertilization. The assembly of annulate lamellae in bovine oocytes was triggered by sperm-oocyte binding and continued concomitantly with the incorporatio n of the nuclear pores in the nuclear envelopes of the developing male and female pronuclei, This process was also induced by the parthenoge netic activation of metaphase-II-arrested oocytes. Depletion of Ca2+, previously implicated in oocyte activation and in the insertion of por e complexes into the nuclear envelope, prevented the formation of nucl ear pore complexes, but not the assembly of annulate lamellae in oocyt e cytoplasm, Injection of the nuclear pore antagonist, wheat germ aggl utinin, into the cytoplasm of mature oocytes that were subsequently fe rtilized caused the arrest of pronuclear development, indicating the r equirement of nuclear pore complexes for normal pronuclear development . Treatment of the fertilized oocytes with the microtubule inhibitor, nocodazole, prevented gathering of annulate lamellae around the develo ping pronuclei, insertion of nuclear pores into their nuclear envelope s, and further pronuclear development. The formation of the male pronu clei was reconstituted in Xenopus egg extracts and reflected the behav ior of nuclear pores during natural fertilization. These data suggest that nuclear pore complexes are required for normal pronuclear develop ment from its beginning up until pronuclear apposition. Annulate lamel lae may be involved in the turnover of nuclear pore complexes during f ertilization, which is in turn facilitated by the reorganization of oo cyte microtubules and influx of Ca2+ into oocyte cytoplasm.