Electron microscopy of chromosomal changes in ambystomal somatic cells during the mitotic cycle

This sequence of electron micrographs shows the progressive changes undergone by the dispersed chromosomal fibrils of the interphase nucleus as they begin to condense, in early prophase, into clumps centered on three types of small nuclear structures. These bodies do not seem to have been described previously in detail in connection with mitosis, although they were mentioned as a group in a preliminary note by Bloom (1963). In the present report they are divided into three groups indicated as F 1 , F 2 and F 3 and are labelled 1, 2 or 3 in the accompanying electron micrographs. The clumps enlarge by continued accretion of more and more fibrils concentrated about these three types of bodies until the chromosomes of metaphase are formed. The inference that the bodies depicted here form focal points for the condensation of dispersed chromosomal filaments rests solely on the repeated finding of the progressive aggregation of the fibrils about them. Further studies are in progress on factors mediating these changes and on the nature of the F bodies.