PINOCYTOSIS 159 



Schumaker [27]. Brandt induced pinocytosis in amoebae by means of a 

 fluorescent protein, and could directly demonstrate that a fluorescent film 

 is adsorbed to the surface of the amoeba and that the same film coats the 

 inside of the pinocytosis channels and pinocytosis vacuoles. Since then, 

 other examples for the surface adsorption of pinocytosis-inducers have 

 been found [21, 15]. With regard to the nature of the adsorbing sites 

 the hypothesis has been advanced by Marshall, Schumaker, and Brandt 

 [15] and by Chapman- Andresen [6] that they are furnished by the mucous 

 coat that is known to occur at the surface not only of amoebae but of many, 

 perhaps all, pinocytic cells. I believe that Dr. Marshall will tell you more 

 about this problem. 



Fig. 2. Electron micrograpli ot jejcnum of 3-clay-iikl mouse (original magni- 

 fication X 6500) showing pinocytic vesicles of varying size originating from the 

 brush border. (After Clark and Wochner [9a].) 



According to this well-supported hypothesis, therefore, we may assume 

 that the adsorption of a suitable inducer brings about a surface reaction 

 by which the adsorbed substance, together with some of the solvent, is 

 transported into the cell. This is in good agreement with the morphological 

 observations. The question is now: What is the physiological significance 

 of such a process .'' Is the physiologically predominant feature of pino- 

 cytosis the specific uptake of the absorbed substance, often high-molecular, 

 or is it the accompanying uptake of the fluid ? 



This question cannot be answered in a general way on the basis of our 

 present knowledge. Important features of the process are certainly dif- 

 ferent after the induction, for instance, by salts and by proteins, and my 

 personal guess is that different cells in different situations will be found to 

 utilize the possibilities of pinocytosis for different physiological means. 



