Ultrastructure of Mitotic Cells

Abstract

The study of chromosomal behavior during mitosis has always been one of the most fascinating topics of cytologic research. The early light-microscopic examination of fixed and stained cells provided evidence for the existence of spindle fibers, structures that apparently connected the kinetochores of the condensed chromosomes with the polar centers (chromosomal fibers), and that spanned the distance between polar centers (continuous or interpolar fibers). Based on these observations spindle models were formulated in which the chromosomes were “pulled” or “pushed” by contraction or dilatation of spindle fibers toward the poles (reviewed by Schrader, 1953). Since spindle fibers generally could not be visualized in living cells by bright field microscopy, several authors maintained that the fibers were artifacts caused by fixation (see Schrader, 1953). However, it could finally be demonstrated with the polarization microscope that fiber structures are a reality (Inoue, 1953). Zones of weak birefringence between chromosomes and spindle poles and between the poles suggested the existence of anisotropically arranged rod-like elements in these areas. Improved electron-microscopic fixation procedures, particularly the introduction of glutaraldehyde as a fixing agent in the early sixties, resulted in the demonstration of microtubules (MTs) as the predominant structural elements of the mitotic spindle.