Diaphragm development in cytokinetic vegetative cells of brown algae.

We studied development of the cytokinetic diaphragm (i.e., the membranous septum formed during cytokinesis) in vegetative cells of the brown algae Sphacelaria rigidula, Halopteris congesta and Dictyota dichotoma after cryofixation-freeze-substitution. Cytokinesis began by the gathering of organelles and large vesicles between daughter nuclei following telophase. Subsequently, a thin cytoplasmic strand was formed along this plane, where endoplasmic reticulum (ER), dictyosome vesicles and particular membranous elements, the flat cisternae, were accumulated. Their fusion formed a patchy diaphragm with irregular gaps. Fine tubular channels perforated the diaphragm during all stages of its formation. Following diaphragm completion, cell wall material was deposited in it. The new walls had ER-free plasmodesmata. In D. dichotoma, diaphragm development did not follow a definite pattern, i.e., centripetal or centrifugal, a phenomenon also confirmed in H. congesta. In contrast, in apical cells of S. rigidula the diaphragm started developing from the periphery, growing to some extent centripetally. In these cells, local cell wall deposition was greatest at the division site. In apical cells in which cytokinesis was experimentally inhibited a ring of wall material was usually deposited at the cytokinetic plane. [ABSTRACT FROM AUTHOR]