PINOCYTOSIS 163 



cribed its "segregation" into granules which he called phagosomes. 

 Recently, [17, 18] Xovikoff has devoted himself to the question of a 

 possible connection between pinocytosis and lysosomes. His hypothesis 

 is summarized in the diagram shown in Fig. 4, which he has kindly per- 

 mitted me to use. According to this interpretation the lysosomes are 

 derived from pinocytosis vacuoles and are regarded as the end products 

 of their transformation inside the cell. The exact nature of this assumed 

 transformation is not clear, and especially not the way in which the outfit 

 of hydrolytic enzymes which defines the lysosomes, should be acquired. 

 One interesting suggestion is based on the fact that at least one of the 

 characteristic enzymes of lysosomes, namely acid phosphatase, has been 

 found by Novikoff [18] to occur in those regions of the cell membrane of 

 Amoeba proteiis, to which food particles attach before engulfment. This 

 might indicate that an essential feature of the formation of lysosomes is the 

 transport of surface enzymes into the cell interior by means of pinocytosis, 

 and this would of course tie in verv nicely with Bennett's views on mem- 

 brane flow from the surface into the cell. 



Closely connected with the problem of a relation between pinocytosis 

 vacuoles and lysosomes is probably the similar claim of a connection 

 between pinocytosis vacuoles and mitochondria. This claim was put 

 forward by Gey and his coworkers [11] and has recently been revived by 

 Robineaux and Pinet [24] on the basis of their interference-microkine- 

 photographic investigations of protein uptake by macrophages. It seems to 

 me that the question is in reality one of being able morphologically to 

 differentiate between lysosomes and mitochondria, which may be difficult 

 without the aid of the electron microscope and cytochemical technique. 



The other school of thought links the fate of the pinocytic vesicles to 

 the endoplasmic reticulum. This goes back to the work of Palade [19] who 

 claimed the continuitv of the cell surface with the membrane system of 

 the reticulum. I should like to present to you this view by means of 

 another schematic diagram, taken with the author's kind permission, from 

 Siekevitz's [28] Ciba lecture (Fig. 5). 



The essential part of the diagram for us at the moment is that depicting 

 the migration of pinocvtic vesicles toward, and fusion with, the intracellular 

 spaces of the endoplasmic reticulum. This process is an important feature 

 of Bennett's [3] and Palade's [19] \iews regarding the connection between 

 surface membrane and the endoplasmic reticulum, but its reality was not 

 strictly proven and it is therefore also marked off" as hypothetical in 

 Siekevitz's diagram. Very recently, howe\"er, Palay [20] has described an 

 example of the process. He studied fat absorption by the cells of the in- 

 testinal wall of rats, and found that fat droplets that enter these cells by 

 means of pinocytic vesicles could be observed again as inclusions in the 

 perinuclear space. If this observation can be shown to be valid (and we 



