Membrane Structure of Dictyosomes, Large Vesicles and Plasma Membranes in a Green Alga, Micrasterias crux-melitensis

Abstract

Freeze-fracture replicas of the cisternal membranes of dictyo-somes, large vesicle (LV) membranes and plasma membranes in the green alga, Micrasterias crux-melitensis, were examined by electron microscopy. Intra-membrane particles were evenly distributed in the cisternal membranes in resting cells. In growing cells, the particle density decreased from the centre to the periphery of the cisternal membranes and was greatly diminished at the vesiculated parts. The particle density in LV membranes was about twenty percent that in cisternal membranes, but equivalent generally in the plasma membranes of growing daughter semicells. The possibility of the direct supply of LV membranes to the plasma membrane is discussed. In the plasma mem-branes of fully grown cells, rosette complexes with a hexagonal array were observed. Each rosette consisted of six particles. These complexes were assumed to correlate with microfibril formation in the cell wall.