The nucleolar cycle in the myxomycete Physarum polycephalum. I. Ultracytochemical characteristics of the mature nucleolus and of the persisting nucleolar material during the mitotic stages

The interphase nucleolus, in the myxomycete Physarum polycephalum, consists of granular and fibrillar zones, as in all eukaryotes studied so far. Each of these latter regions, however, is observed to be composed of a coarse filamentous element which is generally folded into a globular or cup-shaped structure characterized by a lighter core portion. These globular structures are bleached by EDTA and also react with both bismuth and phosphotungstic acid, thus showing that they contain chromatin. Similarly shaped nucleolar components are observed till late prophase and these show cytochemical properties identical to those seen during interphase. Following dispersion of the nucleolus at prometaphase, these globular nucleolar regions persist as characteristic remnants which contain DNA, RNA, and basic proteins. At anaphase, numerous similar bodies migrate with the chromosomes and can be demonstrated to become part of the chromatin masses which occupy the two poles of the dumbbell-shaped telophase nucleus. It i...