Micronuclear Envelope Formation after Telophase of Mitosis in the Ciliate Euplotes eurystomus

Publisher Website
Google Scholar
Abstract
Euplotes has a diploid micronucleus and a highly polyploid macronucleus. During the cell division cycle, the micronucleus divides by intranuclear (closed) mitosis; the nuclear envelope does not break down during division. During telophase the daughter nuclei are separated by a large distance (about 35-50 .mu.m), while still contained within the original, greatly elongated nuclear envelope. EM observations of telophase and early interphase micronuclei reveal an unusual mechanism for completion of separation and compartmentalization of the daughter nuclei: the daughter nuclei form a new envelope inside the old one. Forks connecting the outer (old) and inner (new) envelopes are readily observed. At boundaries of the developing inner envelope where a fork is absent membrane vesicles are observed. These vesicles may be involved in formation of the inner envelope. In the outer envelope pore complexes are present, but are not as numerous as those of the developing inner envelope. After cell division the early interphase micronucleus is bounded by a single envelope, except for an occasional small stretch where a remnant of the old envelope can be detected. The developing inner envelope of the Euplotes micronucleus bears a striking resemblance to annulate lamellae and might serve as a good model system for the study of such membrane systems.