CONICET | Buscador de Institutos y Recursos Humanos

Abstract In a previous paper we have described a pronounced increase of apoptotic nuclei in rat corpus luteum of pregnancy whose programmed chromatin degeneration was induced by an antiprogesterone drug. These observations encouraged us to study the apoptotic nuclear membrane using freeze-fracture techniques, which allows us to observed en face the nuclear envelop and thus permit us nuclear pore counting. This study was complemented with the TUNEL assay (TdT-mediated dUTP nick-end labelling), an apoptotic specific assay using paraffin sections and light microscopy. Electron microscopy was done on ultra thin sections as routinely used . For freeze-fracture, fixation and cryoprotection were done according to a routine procedure, fracture of the freezed tissue and replicas were performed in a Balzers BAF 301 apparatus. Platinum-carbon replicas were examined in a Zeiss 900 electron microscope. Nuclear pores were counted using a conventional morphometric analysis. Changes in nuclear pores begin in an uncommon form. They display an intense reduction in number showing an even distribution on the nuclear membrane, never forming aggregations sharply separated from pore-free areas characteristic of usual apoptotic models. Thin-sections and electron microscopy indicates, coincidently with the freeze-fracture finding, small irregular aggregation of marginated heterochromatin condensations. After nuclear fragmentation and micronuclear formation, pores behave in the usual manner, mainly the migrations of the nuclear pores toward the still observed free chromatin areas. The present results support the hypothesis that nuclear pore complexes are dynamic structures, which permit their migration toward nuclear membrane areas devoid of chromatin aggregations that might block the nuclear-cytoplasmic transport in such areas.