Interactions between the Nucleus and Cytoplasmic Organelles during the Cell Cycle of Euglena gracilis in Synchronized Cultures III. Association between the Nucleus and Chloroplasts at an Early Stage in the Cell Cycle under Photoorganotrophic Conditions (Part II)1

Abstract

Three-dimensional models of chloroplasts before and during their association with the nucleus in Euglena gracilis Z were constructed based on serial sections of cells taken during an early phase of the cell cycle in synchronized cultures under photoorganotrophic condition. Before association, the cell contained two to three chloroplasts which were composed of elongated tubular bodies extending in different directions toward the cell periphery. These tubular arms of chloroplasts were drawn toward the nucleus, and folded, with concurrent coalescence, into a single giant body surrounding the nucleus. A DAPI-fluorescence photomicrograph of a giant chloroplast revealed that the chloroplast-nucleoids were in the form of a continuous strand lying throughout the chloroplast body, and that some protuberances from the nucleoid strand were in especially close proximity to the nuclear periphery. A new mode of the chloroplast-nucleus connection was observed. The nucleus had conspicuous protrusions, whose distal ends were connected with the chloroplast. The outer membrane of the nuclear envelope was continuous with the outer chloroplast membrane, and at some sites, an open space was formed between the inner nuclear membrane and the inner chloroplast membrane.