34 ELECTRON-MICROSCOPIC STRUCTURE OF PROTOZOA 



present writing the questions cannot be answered, but vigorous 

 studies of pinocytosis in amebae are under way in many laboratories 

 (e.g., Holter, 1959; Brandt, 1958; Schumaker, 1958). 



Using fluorescent or radioactive-labeled proteins, investigators 

 have found that amebae are capable of taking up impressively 

 large quantities of protein from the medium, far more than could 

 be accounted for by simple ingestion of fluid. Apparently the 

 protein is adsorbed to the plasmalemma, and Brandt (1958) 

 suggests that surface binding weakens the membrane tension at 

 points which are consequently drawn into the cell by the contrac- 

 tile plasmagel system. Fluorescent proteins are visible some days 

 after uptake as multitudinous small fluorescing granules whose 

 changing densities in cells stratified by centrifugation suggest that 

 digestion is taking place. Radioactive glucose, administered with 

 unlabeled protein as a pinocytosis inducer, rapidly results in 

 general cytoplasmic labeling, showing absorption of glucose or 

 its breakdown products, although the plasmalemma normally is 

 nearly impermeable to glucose itself. The label also was 

 recovered from respiratory C0 2 and excretion products. 



While the above work is based on light-microscope studies, 

 several recent reports by electron microscopists illustrate the 

 morphology of some steps in the process of pinocytosis in amebae. 

 Brandt and Pappas (1960) used colloidal suspensions of thorium 

 dioxide particles and solutions of ferritin as both pinocytosis 

 inducers and electron-microscopically visible labels. The dense 

 particles of the label in both instances became concentrated on 

 the surfaces of fine hairs that typically coat the plasmalemma of 

 amebae (see Chapter 3). Particularly with thorium dioxide, the 

 hairy fringe became thicker and was densely impregnated with 

 particles; ferritin tended to concentrate in a layer at the tips of 

 the hairs. Convoluted tunnels extending deep into the cytoplasm 

 were lined by the impregnated fringe. Subsequent phases in the 

 history of the tunnels were not described in this first publication, 

 except for the notation that tiny vesicles containing densely 

 concentrated particles were observed some hours after exposure 

 to thorium doxide. In any event, morphological evidence now 

 clearly confirms the hypothesis that quantitatively significant 

 adsorption of dissolved or suspended substances occurs at the 

 plasmalemma surface. 



