EXPERIMENTAL ANALYSIS 



Recent studies of the trephocytes, which 'have the function of carrying 

 nutrients to all parts of the organism' and 'produce and ultimately 

 liberate nutritive growth substances', have been made by Lieb- 

 MAN^^ 68 69^ _ Vcrtebratcs have two types of trephocyte, the lympho- 

 cytes and the mast cells, and Liebman^^ draws particular attention to 

 their probable importance in protein synthesis and growth, and to the 

 fact that 'it is apparently common that mitotically active tissues contain 

 lymphocytes'. The role of the thymus in regulating growth has been the 

 subject of much speculation (Rowntree;''^ Tyler*^), and one finds 

 such suggestions as that 'the thymus and other organs of the lymphoid 

 system are to be regarded as a "cy to-regulatory system" inasmuch as 

 they would decisively determine processes of cell division and conse- 

 quently growth, by storing or making available nucleoproteins' (Stern 

 and Willheim'^). 



The addition of more than a small amount of embryo extract to 

 cultures of macrophages causes their rapid disintegration and death. 

 This could be due to over-stimulation by factors similar to those 

 elaborated by the macrophages themselves and does not invalidate the 

 hypothesis that the growth-promoting actions of both types of extract 

 may be similar. Evidence suggesting that an external supply of labile, 

 macro-molecular trephones or components of the 'embryonin' type 

 (Fischer; '^2 Fischer and Astrup'^^) may not be indispensable for pro- 

 liferation of fibroblasts is given by Laser,'* who found that, in spite of 

 smaller areas, fibroblast cultures in serum alone could reach greater 

 dry weights in a three to four day period of observation than cultures 

 in media containing embryo extract. This does not necessarily mean that 

 mitosis has contributed to a growth effect which may be mostly due to 

 increase in cell size. In addition, however, Baker'^^ has described a 

 medium in which rapid and prolonged growth of fibroblasts took 

 place. This contained only serum, Witte peptone, haemin, insulin, 

 thyroxin, cysteine, ascorbic acid, glutathione, vitamins A and D, 

 glucose and salts. It is conceivable that fibroblasts are capable of 

 producing the 'embryonin' type of factor, but usually only in subliminal 

 amounts, and that a medium such as Baker's stimulates its production 

 to a level above the threshold required to induce mitosis. That such 

 factors are indeed present in fibroblasts, but not readily available, is 

 suggested by Fischer's''^ "^"^ demonstration that fibroblast cultures 

 which are repeatedly 'wounded' release growth-promoting substances 

 from some cells at the expense of which others can proliferate. 

 Such 'wounded' cultures grow much faster than undamaged controls. 

 The macrophages seem able to produce and release the 'growth- 

 promoting factors'. 



The circulating leucocytes, and especially the lymphocytes, are 

 short-lived cells which have about them something of the character of 



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