122 - The Cell 



tides suspended in their tiuid con Lent. They 

 are relatively small, ranging from 1 to 2 mi- 

 crons in the common amoebae (Fig. 6-8) 

 down to 0.1 to 0.01 micron, in certain other 

 cells (Fig. 6-9). Pinocytosis may represent an 

 important mechanism by which cells actively 

 take in dissolved substances, particularly 

 macromolecular organic matter, from the ex- 

 ternal medium. 



* * 



**. % i »* % 



% \ 



%^ 



•#<■' 



n 



-. 



Fig. 6-9. Pinocytosis, observed by electron microg- 

 raphy, in an absorptive cell in the lining of the small 

 intestine of a suckling mouse. Note that the cell mem- 

 brane is deeply and complexly folded and that the 

 pinocytic vacuoles appear to originate from the deep 

 ends of the in-pocketings. The descriptive term "brush 

 border cells" is often applied to cells of this type. 

 (Courtesy of Sam. L. Clark, Jr., School of Medicine, 

 Washington University, St. Louis. Total magnification, 

 18,000.) 



The contents of a pinocytic or phagocytic 

 vacuole cannot be considered as part of the 

 protoplasm, however, since these materials 

 are segregated within the vacuolar mem- 

 brane. Moreover, it still is somewhat problem- 

 atical as to how intravacuolar materials 

 may be delivered into the protoplasm proper. 

 Water and other relatively small molecules 



undoubtedly escape by diffusion and osmosis, 

 since these vacuoles shrink and swell accord- 

 ing to the tonicity of their fluid content (p. 

 131). Thus proteins and other macromolecu- 

 lar substances may escape from the vacuole 

 after they have been digested, or hydrolyzed, 

 into their micromolecular constituents (p. 

 129). In other cases, however, it seems prob- 

 able that the macromolecules may be ad- 

 sorbed upon the vacuolar membrane and 

 that subsequently the membrane may frag- 

 ment into pieces that become incorporated 

 into the endoplasmic reticulum of the cell. 



A close relationship between pinocytosis 

 and phagocytosis has been demonstrated re- 

 cently by studies on amoebae, made by Heinz 

 Holter and collaborators of the Carlsberg 

 Laboratory in Copenhagen. The amoebae 

 were placed in a protein solution that in- 

 duces very active pinocytosis for about 30 

 minutes, whereupon this activity comes to a 

 halt. Such amoebae, now placed in a densely 

 populated suspension of small ciliated proto- 

 zoans, were able to phagocytize only a very 

 few of these favorite food organisms, al- 

 though ordinarily they would have ingested 

 the ciliates in great numbers. Conversely, 

 amoebae that had phagocytized to exhaus- 

 tion in a dense suspension of ciliates showed 

 little or no tendency to pinocytize when sub- 

 sequently placed in protein solutions. In each 

 case, apparently, the amoebae had used up 

 almost all of their available supply of cell 

 membrane materials during the period of 

 very active pinocytosis or phagocytosis. Con- 

 sequently scarcely any was left to be utilized 

 for further activity. 



The same Danish workers also observed 

 that if glucose, as well as protein, is included 

 in the solution being pinocytized, the protein 

 is taken into the cell preferentially — to a 

 tenfold extent. Such glucose as is taken in, 

 however, freely diffuses forth from the pino- 

 cytic vacuoles into the surrounding cyto- 

 plasm, whereas the protein does not. The 

 protein, in fact, appears to be adsorbed and 

 bound to a mucoprotein component (p. 

 117) of the vacuolar membrane; subsequently 



