I EFFECTS OF ENVIRONMENT 549 



and the use of perfusion chambers. Cuhures made in Carrel or Earle type flasks, 

 or in other flasks, l^ottles or tubes of various shapes, can be maintained for pro- 

 longed periods in the same vessel, and may be modified by agitation or rotation or 

 by the introduction of "flying coverslips" into flasks or tubes. Descriptions of the 

 principal technical procedures for tissue culture are to be found in various 

 excellent handbooks {e.g. Strangeways, 1924a; Fischer, 1930; Parker, 1950; 

 Cameron, 1950; White, 1954; Hanks, Scherer, Fawcett, Leighton and Porter, 



1955)- 



Great experimental variability is possible with the tissue culture technique. 



Variations can be made in (a) the physical substrate (plasma, cellophane, paper, 

 glass); (b) in the type of cell population ("pure"[clonal]lines of cells, cells of one 

 type or mixed cell cultures, organized cultures) ; and (c) in the nutrient medium. 

 The possibility of manipulation of all these factors makes for a very flexible ex- 

 perimental system. This very flexibility makes necessary a number of safeguards 

 against unintended variations. It also necessitates the adoption of special criteria 

 for evaluating each type of culture. Tissue culture techniques are so diverse that 

 certainly no single set of rules can be set up for determining success in all possible 

 cell systems. Growth in one of its manifestations is in many cases an important 

 criterion of successful cultivation, but, as has already been mentioned, and will 

 be further discussed below, this is a far from simple criterion. Cells in vitro, as in 

 vivo, are engaged in a multitude of activities; a few or many of these inay contrib- 

 ute to the overall process called "growth". 



I. EFFECTS OF ENVIRONMENT ON GROWTH IN TISSUE CULTURE 



(a) Cell autonomy and interdependence. Single cell clones 



In tissue culture one can study either populations of similar cells, mixtures of 

 cells as they come from the living host, or different cell types deliberately culti- 

 vated together. As has been stressed, the cells and their environment together 

 form the tissue culture, both contributing to the behaviour of the cells, including 

 their growth. The complete environmental needs of most types of cells still elude 

 our understanding, and therefore our full control. The basic knowledge is grad- 

 ually being built up which will enable a variety of cell types to be maintained or 

 grown under fully standardized conditions, so that they may be submitted to 

 exact and reproducible experimental procedures. Even now it is possible to 

 manipulate the environment in many ways which affect the growth and behaviour 

 of the cells. 



One of the most interesting kinds of study which these techniques make possible 

 deals with the mutual interaction of the cell and its environment. In the parent 

 animal, the environment of the cell is composed of its "milieu exterieur" or "milieu 

 extra-organique", and its "milieu interieur" or "milieu intra-organique" of Claude 

 Bernard (1865). Tissue culture makes possible a defined and controlled "milieu 

 exterieur" for the study of growth. 



Consideration of the course of cellular differentiation in embryogenesis makes 

 it no surprise that one type of cell should affect the growth and development of 



Literature p. 581 



