Pinocytosis 



H. HOLTER 



Carhherg Laboratory, (j)peuluigen, Deiiinark 



Dr. Porter's report has shown very clearly how intimately the endo- 

 plasmic reticulum and its morphology are related to the problems of protein 

 synthesis. The releyance of pinocytosis in this session is much more doubt- 

 ful ; but since my paper has been included here I shall try to make it as 

 relay ant as I can. 



Perhaps I may be allowed, for the benefit of those who are not quite 

 familiar with the process of pinocytosis, to recapitulate very briefly its 

 main morphological features. 



Ths term "pinocytosis", coined by Warren Lewis in 193 1 [14], was 

 originally intended to designate a process of active drinking by cells. In 

 pinocytosis, fluid is taken up discontinuously, in droplets that are engulfed 

 or sucked in by the cell, and the primary products of this acti\ity are fluid- 

 filled vacuoles or yesicles, by which a certain amount of the surrounding 

 medium is transferred to the interior of the cell. Pinocytosis therefore 

 amounts to an uptake of substances, but let me say at once that the term 

 "uptake" should be understood primarily in a spatial sense, as long as 

 the contents of the pinocytosis vesicles remain separated from the cyto- 

 plasm by a vacuolar membrane ; whether or not the process also amounts 

 to an uptake in the physiological sense, depends therefore on the later fate 

 of the vacuoles. 



Morphologically, pinocytosis displays quite a variety in mechanism and 

 a very wide range of dimensions. In tissue culture cells, the classical object, 

 pinocytosis is brought about by the activity of membranous ruftle pseudo- 

 podia which, by undulating movements, form folds that enclose a certain 

 amount of fluid. The second variety of pinocytosis, very beautifully dis- 

 played by amoebae, is that of invagination (Fig. i). In this process the cell 

 surface forms cavities, in extreme cases long narrow tubes, from which 

 the pinocytosis vacuoles are pinched off^. Pinocytosis by invagination is 

 fairly variable with regard to the shape of the cavities formed, and extremely 

 variable with regard to dimensions. In amoebae the average diameter of 

 the primary pinocytosis vacuoles is about 1-2 /t, while in many other 

 objects it is only discernible by means of the electron microscope, the 

 diameter of the yesicles being from 100 A upwards (Fig. 2). 



