THE MITOTIC CYCLE 



be necessary for the special function of inducing cell division to occur. 

 Both of the possible approaches to the study of the components of media 

 for the growth and maintenance of tissue cultures, the analytic and the 

 synthetic, have been made. Attempts have been made to determine, by 

 analytical methods, which components of media known to be growth 

 supporting are in fact essential. Chemically defined media have been 

 synthesized (White ;*^ ^° Morgan et alli^^). More often, a combination 

 of both methods has been used {e.g., Fischer). 



Leucocyte secretions 



The first media for cultivating animal tissues in vitro were composed of 

 lymph, serum, plasma, tissue extracts, or some combination of these 

 biological fluids. Early in the history of the tissue culture technique, a 

 clear and simple distinction seemed possible between (i) media which 

 could support the proliferation of leucocytes — probably monocytes and 

 lymphocytes, indistinguishable in cultures from the tissue macrophages 

 — consisting of dilute serum or plasma alone; and (ii) media for normal 

 connective tissue (fibroblasts) and epithelial tissues, in which it seemed 

 essential to include extracts of embryonic tissue (Carrel and 

 Ebeling.^2 ^^ Some tumours can also grow in serum alone (Stern^*), 

 but while normal connective tissue can remain alive and healthy for 

 prolonged periods with no other nutrient (Carrel^^), if increase by 

 mitotic division is to take place, some additional source of 'growth- 

 promoting factor', such as embryo extract (Carrel^^ ^'^) or extract of 

 leucocytes (Carrel^^) or of spleen (Carrel ;^^ Carrel and Ebel- 

 ING^^ 60j js necessary. Fibroblasts will grow if they are placed together 

 in the same culture medium with macrophages (Carrel and 

 Ebeling;^^ FischerI^). From these observations the conclusion was 

 drawn (Carrel^^) that the macrophages are themselves able to pro- 

 duce from serum some factor or factors of which the normal fixed 

 tissues require an exogenous source. These leucocyte secretions were 

 named 'trephones' (Carrel^^ ^^ ^^) and their action was originally 

 regarded as purely nutrient (Carrel ;^^ Carrel and Ebeling;^^ des 

 LiGNERis^^). LuDFORD^^ belicves that their action is to stimulate enzymes 

 which break down the serum proteins to units of a size assimilable by the 

 fibroblasts and Fischer^^ regards the leucocytic trephones, not as 

 nutrients, but as specific growth hormones with the special function of 

 stimulating mitosis. 



Claude Bernard (quoted by Fischer^') and Renaut (1893, quoted 

 by Carrel and Ebeling^^ ^°) were among the first to suggest that the 

 main function of the leucocytes was to transport food material to the 

 fixed tissue cells, and des Ligneris^^ discusses the 'trephocytes' which 

 can make and generate Carrel's trephones from serum and plasma and 

 act as intermediaries for the nutrition of more differentiated cells. 



170 



