EXPERIMENTAL ANALYSIS 



HuEPER et alii,^^ are among those who have made determined 

 attempts to dissociate the different effects of embryo extracts on growing 

 tissue ciihurcs. They recognized that a growing culture may be affected 

 by numerous endogenous and exogenous factors, and they distinguish 

 carefully between (i) survival without proliferation, (ii) 'cell multiplica- 

 tion without increase in protoplasmic matter, due to the presence of 

 substances stimulating mitosis and the absence of the proper nutritive 

 elements in the culture medium' and (iii) proliferation with formation 

 of new protoplasm. They f )und no evidence of significantly different 

 activities in extracts from diff'erent anatomical regions of the embryo, 

 and concluded that the value of embryo extracts for supporting the 

 growth of tissue cultures lies in their great complexity, in that they 

 contain 'not only substances for the stimulation of cell division, but also 

 nutritive material which can be assimilated by the explanted cells and 

 used f )r the synthesis of new protoplasmic matter'. It is indeed pre- 

 cisely this complexity that has reduced so many investigators to despair. 

 Success in d( termining whether any one (or a few) of the components 

 of an extract is responsible for the growth-promoting effect has been 

 very elus'vt . Fischer^^ ''^ believes that the effect of embryo extracts in 

 promotir g cell division is to a great extent indirect, through accelerating 

 cell migration (Willmeri^^o Gaillard^"^). Doljanski and Gold- 

 HABER^^i bdieved that, because migratory activity can be stimulated 

 by embryo extract after mitosis has been inhibited by X-radiation, 

 migrati m and cell division are separately affected. The expeiiments of 

 Medawar^^ 1^2 on fibroblast inhibitors have shown the importance of 

 cell mo\T ment in the growth of connective tissue. Medawar believes 

 the inhib'tors he has studied act on cell movement through a surface 

 active e ff( ct and that this is the basis for the differential action on 

 fibroblasts and not on epithelial cells, in which amoeboid movement 

 does not accompany cell division. Movement of epithelial sheet as a 

 whole precedes cell division in cultures of kidney epithelium (Ephrussi 

 and LiTVAC^^^). 



Substances necessary for fibroblastic growth 



The importance of the distinction between 'survival of tissue and active 

 growth' was already emphasized by Carrel and Ebeling^^* in 1923 and 

 Fischer has made a descriptive classification of the substances required 

 for the growth of fibroblasts into (i) low molecular 'accessory growth 

 factors' necessary for cell nutrition (Astrup et alii^^^) and (ii) high 

 molecular, labile substances associated with a nucleoprotein fraction 

 and called 'embryonin' (Fischer and Astrup ''^). By dialysing plasma 

 and embryonic extract, ^^ he freed an otherwise wholly adequate 

 medium from 'accessory growth factors', so that it no longer supported 

 life or growth. Supplementation of the dialysed medium with synthetic 



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