Formation of spindle fibers, kinetochore orientation, and behavior of the nuclear envelope during mitosis in endosperm

Abstract

The formation of kinetochore (chromosomal) and continuous fibers, and the behavior of the nuclear envelope (NE) was described in studies combining light and electron microscopy. Microtubules (MTs) “push” and “pull” the NE which becomes progressively weaker before breaking. It breaks to a certain extent due to mechanical pressure. Clear zone MTs penetrate into the nuclear area as dense bundles and form continuous fibers. These MTs also attach to some kinetochores during this process. Some kinetochore fibers seem to be formed by the kinetochores themselves which are also responsible for further development and changes of kinetochore fibers. Formation of kinetochore fibers is asynchronous for different chromosomes and even for two sister kinetochores. Often temporary “faulty” connections between different kinetochores or the polar regions are formed which usually break in later stages. This results in movements of chromosomes toward the poles and across the spindle during prometaphase. The NE, whose fine structure has been described, breaks into small pieces which often persist to the next mitosis. Old pieces of NE are utilized in the formation of new NE at telophase. Several problems concerning the mechanism of chromosome movements, visibility of the NE, etc., have also been discussed.