TISSUE RECONSTRUCTION FROM DISSOCIATED CELLS 207 



interfere with aggregation of the cell types tested, simple cationic re- 

 actions are not expected to be persistently inhibited. Yet the cells, 

 though brought together by rotation in a culture medium with optimal 

 calcium concentration, did not cohere. Therefore it seems likely that 

 cations, though unquestionably essential for normal cell-cohesion, rep- 

 resent only one of the requirements or the constituents in the mecha- 

 nism of cell-bonding. Whatever the nature of the other constituents, 

 under the conditions tested they were not available or not effective. In 

 view of the thoroughly adequate composition of the culture medium, 

 the possibility suggests itself that, in addition to external requirements, 

 processes which are intrinsic to the cell and sensitive to temperature 

 may be implicated in the formation of histogenetically effective cell 

 bonds. This brings us back to the discussion in the preceding section. 



The validity of these postulations is currently being studied. The 

 working concept adopted is that histogenetic bonding of cells, in aggre- 

 gates and in normal tissues, involves accumulation at the cell surface, 

 or between cells, of specific cellular products ( Moscona, 1959a ) . The 

 molecular make-up or the effective function of these extracellular ma- 

 terials includes divalent cations (see L, Weiss, 1960). At dissociation, 

 these materials are largely removed, and tissue reconstruction requires 

 their resynthesis. It is probable that in mature cells with specialized 

 functions, metabolic facilities for these synthetic processes are not 

 operational. In cells earlier in development, these processes are active 

 but are temperature-dependent; as they do not proceed effectively at 

 suboptimal temperatures, the cells remain dispersed when cooled. 

 These are, in all likelihood, gross oversimplifications. However, support 

 for the notion that bonding of cells depends on metabolic activities 

 seems to be forthcoming. There is recent information that RNA may be 

 involved in maintaining intercellular links in early embryonic tissue 

 ( Curtis, 1958; Brachet, 1959 ) . And there is strong evidence indicating 

 elaboration by embryonic cells of extracellular materials ( Weiss, 1945; 

 Grobstein, 1954 ) . In this connection it should be stated that cell-bond- 

 ing may be just one of the functions of such extracellular products, and 

 that they may also be involved in other cell interactions (Grobstein, 

 1954; Niu, 1956; Weiss, 1958; Edds, 1958; Moscona, 1959a; Moss, 1960). 



Molecular requirements and cell aggregation 



Additional insight into these problems was sought by defining more 

 closely some of the nutritional and molecular requirements of cells in 

 relation to aggregation. Early in this work it was found that an essential 

 prerequisite for orderly aggregation of freshly dissociated embryonic 

 cells was the presence of serum protein or of tissue extracts in the 

 culture medium. In the case of synthetic culture media consisting solely 



