580 GROWTH IN TISSUE CULTURE 6 



differ somewhat from Lewis', especially in a longer estimate for the telophase, 

 viz- prophase 19-25 min.; metaphase, 4-7 min.; anaphase, 3.5-6 min. and 

 telophase 7.5-14 min. Hughes' (1952) times for mouse spleen cells are, in agree- 

 ment with Lewis, nearly twice as long as for the chick cells. The effects of mitotic 

 inhibitors have been examined in living cells by phase contrast cinephotomicro- 

 graphy (Bucher, 1950; Lettre, 1954). 



VII. CONCLUSION 



Growth, thought of as a process affecting a whole multicellular organism, can 

 be broken down into a number of distinct but interrelated concepts. One of the 

 advantages of applying the methods of tissue culture to the study of growth is that 

 several of these components of "growth" can be experimentally, as well as con- 

 ceptually, separated. The cells themselves grow in size and complexity; popula- 

 tions of cells proliferate and interact. These changes can be seen and followed in 

 explanted cell cultures. The cells are very sensitive to changes in their environ- 

 ment, both the non-cellular environment and the adjacent cells. The external 

 environment of the cells can be quite closely controlled and the effects of varying 

 it made the subjects of quantitative and qualitative experiments. The still very 

 incompletely understood process of mitosis may be studied, and methods of 

 promoting and inhibiting cell division devised. Some of these studies are intimately 

 concerned with questions such as the differences and similarities between normal 

 and neoplastic growth. Growth is one of the responses of the cell or group of cells 

 to environmental factors. In reality, growth, metabolism, differentiation, and special 

 and general cell functions, are all closely connected processes. Tissue culture 

 methods are designed to place the environmental factors under experimental 

 control, so that specific effects at the cellular level can be recognized. 



The environment can be varied, or the type of cell. Insights into the properties 

 of individual cell types, separated from the parent organism, are gained by 

 growing cells originating from different tissues, normal and neoplastic, in cultures. 

 The dynamic cytology and histology made possible by tissue culture methods 

 supplement and amplify studies made on fixed cells. Modern methods for isolating 

 clones from single cells make feasible studies of the interactions of cells of known 

 lineage, of stabiUty and variation within particular cell strains, and of variations 

 in metabolic patterns. Methods are available for growing, not only cultures 

 from a single cell, but cultures containing 6- 10^ cells in a single vessel. In fact, 

 the methods of tissue culture and their applications are so diverse that no attempt 

 could be made to cover here all of those relevant to the study of growth. It is 

 hoped that the ground traversed and the examples given may suffice to demon- 

 strate some of the achievements and indicate some of the potentialities of this field. 



t^ 



