ULTRASTRUCTURAL ASPECTS OF CELL ELONGATION,CELLULOSE SYNTHESIS,AND SPORE DIFFERENTIATION IN ACYTOSTELIUM LEPTOSOMUM,A CELLULAR SLIME MOLD

Vegetative myxamoebae of Acytostelium leptosomum, a cellular slime mold, have the appearance of typical eucaryotic cells. The presence of dictyosomes has been established. Elongation of the cells during aggregation and culmination appears to be mediated by dense bundles of microfibrils traversing the cells longitudinally. Microtubules are present; however, they are randomly oriented and no correlation can be made with cell elongation or with the direction of the cellulose microfibrils within the stalk. A variety of vesicles, multivesicular bodies, and lysosome-like vacuoles seems to be involved in producing and transporting stalk material to the vicinity of the stalk. However, only rarely do the vesicles empty their contents directly to the outside of the cells. It seems rather that the fibrillar material of the stalk is assembled near or directly at the plasmalemma, and can then be seen to stream away and become an integral part of the stalk. An unusual structure, the H-body, is formed in great abundance during culmination indicating its possible involvement in stalk synthesis. The H-bodies are removed from the cells prior to spore formation together with other portions of the cytoplasm at least partly by a process involving autophagic vacuoles. These vacuoles, which are also present in the spores, appear to be part of a rather complex and extensive vacuolar apparatus including the food vacuoles, contractile vacuoles, lysosome-like structures, and possibly the H-bodies. The spore coat consists of a heavy outer wall with a fibrillar substructure and two thin, dense bands lining the inside of the plasmalemma. The fibrillar nature of both the outer spore wall and the stalk was accentuated by using barium permanganate to stain sectioned material.