Changes in nucleolar ultrastructure associated with differentiation in the root tip

Abstract

The development of the nucleolus in the root apex of Plantago ovata has been studied primarily at the ultrastructural level. Nucleolar volumes in the quiescent center and selected regions of the root cap, cortex, and epidermis correlate qualitatively with cytoplasmic basophilia and mitotic frequency. The small nucleoli of the quiescent center have been shown by means of serial sections to contain intrusions of heterochromatin into the internal fibrillar zone. The number of intrusions corresponds to what would be expected if each represents the attachment of a nucleolar organizing chromosome. Differences in the chromosomal-nucleolar junction following different fixation procedures are described. One per cent collidine-buffered OsO 4 routinely produces a sharp nucleolar image with the fibrillar and ribosomal regions clearly distinguished. Other fixatives such as acrolein-glutaraldehyde followed by OsO 4 produce a more diffuse nucleolar image but make much clearer the differences between chromatin and nucleolar heterochromatin. Where cortical and epidermal nucleoli are largest (10–12 times the volume of those in the quiescent zone) the ribosomal regions are intermingled with the fibrillar regions and the general structure is less compact. Often present is a large central lacuna containing fibrils of a chromatin-like character as well as what appear to be free ribosome-like particles. As division ceases and cells increase in size, the vacuoles occupy an increasing proportion of the cell volume, the nucleoli return to a size and arrangement like that in the quiescent center.