Ultrastructure of the cell envelope and septation process in Caryophanon latum as revealed by thin section and freeze-etching techniques

Abstract

With optimal conditions of thin-sectioning and freeze-etching, the cell wall of Caryophanon latum was composed of a thick peptidoglycan layer plus two external wall layers. The freeze-etched appearance of the external surface of the outer layer was smooth and lacked structural detail. The numerous cross septa within a trichome were formed by the symmetrical and concurrent ingrowth of the cytoplasmic membrane and the peptidoglycan layer. The site of septum initiation was identifiable by a dart-shaped ingrowth of the peptidoglycan layer rather than by the presence of a mesosome. However, small simple mesosomes were occasionally seen associated with the developing septum. The peptidoglycan in the septum had thickened to at least double the thickness of the wall peptidoglycan layer by the time of septum completion. The external wall layers did not participate in septum formation but did participate in trichome separation. The separation of the septal peptidoglycan was completed during the early ingrowth of the external wall layers. A unique cross-sectional view of a developing septum closing like an iris diaphragm as seen in a freeze-etched preparation was observed.