Dynamics of nuclear envelope and karyosphere in late phophase i of oocyte meiosis

Late meiotic prophase I is crucial for oocyte maturation. Cells at this stage still possess a nucleus, traditionally called germinal vesicle. Inside it, a circular layer of condensed chromatin surrounds the inactivated nucleolus. This structure is known as karyosphere. The transition between prophase and metaphase I includes major chromatin rearrangement and gradual nuclear envelope breakdown. Our aim was to correlate oocyte karyosphere status with its nuclear envelope changes indicated by altered localization of structural proteins. For this purpose, lamins, nucleoporin 160, actin and alpha-tubulin were detected immunocytochemically, actin was stained with phalloidin and chromatin with Hoechst 33258. We observed the following processes: Initially, lamins and nucleoporin were localized in the nuclear periphery while actin and tubulin were restricted to the cytoplasm. With the formation of karyosphere, part of lamin A/C and nucleoporin immunoreactivity was transferred to it, forming two concentric spheres. In oocytes with advanced chromatin condensation, the inner sphere showed a brighter reaction, nuclear envelope was visibly deformed, and actin and tubulin were detected in the nucleus in addition to cytoplasm. These results indicate a temporal correlation between karyosphere dynamics and the increasing permeability of oocyte nuclear envelope.