ULTRASTRUCTURAL AND CYTOCHEMICAL STUDIES OF PLASMA MEMBRANE REGENERATION IN THE MYXOMYCETE BADHAMIA UTRICULARIS

Plasmodia of myxomycetes (plasmodial slime molds) are widely used as model systems in cell biological studies because of their interesting structural and physiological features, macroscopic size, and ease of culture. The following aspects of plasma membrane regeneration in the plasmodia of Badhamia utricularis are investigated: (1) Comparison of fixation methods for preserving the fine structure of plasmodial strands and cytoplasmic droplets; (2) Structural interpretation of events during experimentally induced plasma membrane regeneration; (3) Identification of intracellular compartments involved in membrane regeneration.; Several electron microscopy fixatives are compared for their ability to preserve the structure of cytoplasmic constituents, produce detailed high contrast membrane images, and ameliorate artifacts often encountered in the fixation of myxomycete plasmodia. An osmium tetroxide/glutaraldehyde-tannic acid/methylamine tungstate fixation schedule gives the best overall results. This fixation also has the advantage of rapidly arresting structural changes that occur during plasma membrane regeneration.; Experimental injury of B. utricularis plasmodial strands results in exudation of a cytoplasmic droplet that rapidly regenerates a plasma membrane at its surface. Regeneration involves fusion of membrane bound vesicles near the droplet surface. Several morphologically distinct layers also arise within the droplet. Some of these layers are separated from each other by flattened, membrane bound vacuolar spaces that result from vesicle fusion. The extent and duration of vesicle fusion events may be regulated in part by gelation of the cytoplasmic droplet.; Endocytotic uptake of several types of colloidal gold particles, as well as E. coli cells, is used in conjunction with phosphotungstic acidchromic acid cytochemical staining to label the intracellular compartments involved in endocytotic processes and plasma membrane regeneration. Endocytotic vesicles and secondary lysosomes are major vesicle types contributing to the regenerated plasma membrane. These compartments are also contributors to the internal vacuolar spaces of cytoplasmic droplets. The membranes bounding these internal spaces may have the properties of plasma membrane as indicated by cytochemical staining.