Recovery of interphase nuclei from extreme structural alterations in centrifuged fern protonemal cells

Nuclear ability to recover morphologically and physiologically from tremendous elongation induced by centrifugation was investigated in single-celled protonemata of the fern Adiantum capillus-veneris. Basipetal centrifugation at a strength of ca. 2000 g for more than 1 h caused an extremely long (more than 500 microns), thread-like chromatin-containing extension of the nuclear envelope in the centrifugal direction. The nucleolus was either located inside a terminal widening of the extension, or it was detached from the thread and located outside the nucleus. Such enucleolation occurred in about half of the nuclei during 3 h of basipetal centrifugation. After centrifugation, the nuclei recovered morphologically by completely reducing the chromatin thread. One day after the end of centrifugation, all nuclei again contained a nucleolus, indicating that nucleolar regeneration had taken place in the interphase nucleus. Almost all basipetally centrifuged cells were still able to divide under white light conditions. A strong delay in the division time course may be attributed to recovery processes of the nuclear internal order. The remarkable ability of the Adiantum interphase nuclei to recover morphologically and physiologically is assumed to be due to processes which also drive the maintenance of the functional nuclear structure during interphase.