TISSUE RECONSTRUCTION FROM DISSOCIATED CELLS 217 



might obtain. Furthermore, the experimental procedures described 

 here in connection with work on histogenesis of aggregates were de- 

 signed for minimal interference with the normal range of cellular reac- 

 tions. There is growing evidence that exposure of cells to conditions 

 capable of profoundly interfering with certain aspects of their meta- 

 bolic functions can elicit reactions normally not displayed ( Fell, 1956; 

 Weiss and James, 1955; Murray, 1957; Wilde, 1958; Bridges and Pritch- 

 ard, 1958; FHckinger, 1959; Moscona, 1957b, 1959b; Barth and Barth, 

 1959; Benitez et al, 1959; Ebert, 1959; Selye et al, 1960). 



Of particular interest are the mechanisms involved in selective cell 

 groupings— the signals by means of which cells "communicate" and 

 "recognize" each other (Burnet, 1961) and are caused to converge 

 preferentially in distinct regions. This applies, naturally, not only to the 

 processes in cell aggregates but also to comparable events in the em- 

 bryo. Such cell activities must be presumed to involve interactions 

 across a distance, and therefore to require the presence of a suitable 

 continuum between the cells through which the signals may be trans- 

 mitted or exchanged. In considering a practical approach to these at 

 present largely hypothetical matters, the question arises of a possible 

 distinction between the specific cell-affecting stimuli and the means of 

 their transmission, i.e., between the signals and the cell-interconnecting 

 communication system. Such a distinction, if true, would be methodo- 

 logically very useful. This, however, may be an oversimplified expecta- 

 tion. Starting with the working assumption that the cell-linking con- 

 tinuum is represented by the extracellular material (ECM), it is, of 

 course, possible that this material functions merely to bind cells to- 

 gether and provides a mechanical substratum for their movements. 

 However, taking into account the available snippets of pertinent in- 

 formation referred to above, one is led to assume that this may be only 

 a part of its function— that, in linking and interconnecting cells, ECM 

 provides also the intercellular continuum by which stimuli may be 

 transferred and activities of cells coordinated. Such stimuli may have a 

 simple chemical basis, such as differentials in the concentration or diffu- 

 sion of cell-affecting molecules, or they may derive from the physico- 

 chemical characteristics of ECM (Grobstein, 1954; Moscona, 1959a). 

 Being a cell product, ECM may be endowed with cell-specific traits, 

 and its molecular architecture and composition may thereby vary with 

 differences in cellular derivation. Thus ECM may carry to the inter- 

 cellular environment some of the marks responsible for identities or 

 diversities of cells, and play a role in their mutual "recognition" and 

 histogenetic interactions. 



The purpose of these entirely speculative notions is primarily 

 heuristic. The chief justification for advancing them now is that, being 

 admissible in the light of available knowledge, they direct us to experi- 



