NEOPLASTIC DISEASE 



cell surfaces with which they are in contact. No organized 

 tissue structure would be possible without controls of some 

 sort maintaining cells in normal morphological and functional 

 relations. To a large extent these proximity controls seem to 

 be intrinsic to the cells themselves. Moscona (1956) in 

 Weiss's laboratory has described the development of organoid 

 structures when single-cell suspensions disaggregated from 

 chick embryo tissues were held under tissue culture con- 

 ditions on plasma clot. The cells re-aggregated into clusters, 

 estabhshed a tissue-hke association and resumed their 

 characteristic histiotypic development. Weiss (1955) has 

 discussed many examples from embryonic development in 

 which special relationships are established between cell 

 types. In such a case as the migration of the potentially 

 pigment-producing cells of the neural crest to their allotted 

 positions in the body, the process is presumably a result of 

 successive 'recognition' of cell surfaces on the path of migra- 

 tion and of the final appropriate niche for permanent 

 lodgment. 



A more direct approach to the nature of these proximity 

 relationships is provided by the work of Abercrombie, 

 Heaysman and Karthauser (1957) using interference cine- 

 photomicrography to study the movement of cells in tissue 

 culture. With normal fibroblasts, movement is associated 

 with undulations of the cell membrane. When contact with 

 another cell occurs, the activity ceases at the point of contact 

 and, if the cell is in contact with a sufficient number of 

 surfaces, movement ceases over the entire membrane. The 

 cell has virtually then become part of an established tissue. 

 Malignant cells in tissue culture are not subject to contact 

 inhibition. In mixed cultures they can be seen to move freely 

 over the surfaces of normal cells. Obviously this quality may 

 represent an important reason for the selective survival ad- 

 vantage of the cancer cell and equally provide a clue to the 

 nature of the controls which are abrogated in somatic 

 mutation towards malignancy. 



194 



