MITOSIS IN OEDOGONIUM - SPINDLE MICROFILAMENTS AND THE ORIGIN OF THE KINETOCHORE FIBER

Abstract

New ultrastructural observations of mitosis in the closed spindle of O. cardiacum were made using cells fixed with glutaraldehyde and tannic acid. Fine filaments 5-8 nm in diameter were attached to kinetochores from prophase through anaphase. Some were free in the early division nucleus while others emanated from forming kinetochores at prophase when few if any microtubules (MT) are inside the nucleus. During prometaphase, MT invade the nucleus from the poles and appear to interact with the microfilaments. Early in prometaphase, numerous MT were laterally associated with kinetochores, and the kinetochore fiber was formed 1st at 1 kinetochore of a pair. During metaphase and anaphase, the microfilaments were interspersed among the MT of these kinetochore fibers. There was an ill-defined matrix concentrated in the kinetochore fiber and MT were coated irregularly with osmiophilic material. Live mitotic cells of O. cordiacum were studied using time lapse cinematography. These microfilaments may constitute 1 structural component of the traction apparatus that moves chromosomes during metakinesis and anaphase; at least some (and possibly many) of the MT of the kinetochore fiber may be derived from those entering the nucleus at prometaphase.