Ultrastructure and Differentiation in Chara Sp. II. Mitosis

Abstract

The ultrastructure of some dividing cells of Chara are described. No centrioles have ever been detected in vegetative cells. Asymmetric cell division, forming a predetermined pattern of cells, was apparently not preceded by any characteristic grouping of cell organelles. The nucleoli became dispersed during pre-prophase, and most of the nucleolar material appeared intimately associated with the chromosomes throughout division, although some seemed excluded from the nucleus at late telo-phase. Polar zones of endoplasmic reticulum were formed in early prophase, and attachment of microtubules to daughter chromosomes slightly preceded the formation of a very precisely aligned metaphase plate. The chromosome arms were also apparently all aligned in the plane of this plate. A marked concentration of mitochondria was normally seen at the poles of dividing cells in the tips of laterals throughout division. As anaphase progressed, interzonal microtubules increased in number and clustered together in the phragmoplast region. Nuclear envelopes reformed early in telophase. Cell plate initiation was characterized by the presence of a network of membranous tubules in the plate region. Vesicles, apparently derived from the Golgi bodies, contributed material to the plate. Fibrillar material was frequently encountered at the poles of dividing cells. Some spindle microtubules are suspected to "end" at small diffuse bodies. It is suggested that sites of synthesis of polymerization of microtubules occur at the kinetochores or centromeres and poles, since this could help explain metakinesis and anaphase movement of chromosomes.