THE MORPHOLOGICAL APPROACH 67 



A. Pinocytosis 



Pinocytosis (cell-drinking) was first described by Lewis (1931) in rat 

 macrophages in tissue culture in autologous sera. It has since been found in 

 many different types of cell cultures, and has even been studied in cells 

 within the animal (Algire, 1957). Small droplets, apparently projecting into 

 the cell, have been found in electron microscope pictures (Palade. 1956), and 

 these may represent the same phenomenon at a smaller size level. It has been 

 proposed that this process may be an important mechanism of virus entry into 

 the cell (Bang and Gey, 1951). The relationship of pinocytosis to phago- 

 cytosis, which may also occur in a number of cells (Fell and Brieger, 1947; 

 Lewis and Lewis, 1924) besides the macrophage, is not known. Pinocytosis 

 may be greatly increased in tissue culture with various types of sera (Rose, 

 1955), or with sera which had been injected into the host from which the cells 

 were taken (Gonzalez-Ramirez et al., 1956). 



B. Mitochondria 



Although mitochondria may be altered by substances which damage the 

 cell and have been found to vary tremendously with different physiological 

 activities of cells, little is known of specific physiological effects. The capacity 

 of coenzyme A to block the fragmenting effect of 6-mercaptopurine on mito- 

 chondria and to cause the mitochondria to become longer and thicker may 

 be one such effect (Biesele, 1955). 



C. Nucleus 



The appearance of a normal nucleus has been subject to much discussion. 

 The picture presented here is now generally accepted. The nucleolus, how- 

 ever, has in tissue culture been shown to vary considerably. A grossly rather 

 coiled appearance which was seen clearly in early, relatively thick sections 

 studied in the electron microscope (Borysko and Bang, 1951; Bernhard et al., 

 1952), has been shown at higher resolution to consist of many fine dots or 

 cross sections of threads grouped together in varying concentration. Of par- 

 ticular interest, however, are the studies on the appearance of the nucleolus 

 in tissue culture when the cultures are treated with various purines (Hughes, 

 1952a; Lettre and Siebs, 1954). With the addition of adenosine, the nucleolus, 

 which in actively growing cultures is normally homogeneous, changes to a 

 fragmented and frequently coiled structure. This change may be related to 

 nucleolar changes which have been described in infections with the virus of 

 fowlpox (Bang et al., 1951) and rabbit fibroma (Bernhard et al., 1955). 



D. Secretion 



The processes of cell secretion are of obvious importance for virus release. 

 For example, the fact that the mammary cell surface is constantly broken 



