204 CELLS, TISSUES, AND ORGANISMS 



aggregation patterns. This resemblance applied both to internal archi- 

 tecture and to external diagnostic features— as if the properties or sig- 

 nals that guide cells in establishing these collective systems were 

 operationally homologous in histogenetically similar cells, regardless 

 of differences in generic derivation. 



We shall return to this problem later. The points to be stressed here 

 are that the patterns established by aggregating cells represent critical 

 equilibrium products between cell-intrinsic factors, group properties, 

 and environmental effects; that they are characteristic for a given cell 

 population and consistently diagnostic for a specified set of conditions. 

 As such, they provide reliable base lines for precise testing of se- 

 lected variables— cellular and environmental— chosen to examine vari- 

 ous aspects of cell bonding and histogenetic interaction. 



A particularly striking example of correlation between cell-de- 

 pendent factors and aggregation patterns is the effect of age, e.g., of the 

 developmental stage of the cells in testing. By comparing the aggrega- 

 tion of cells from similar tissues at different stages of development, it 

 was found that cells from progressively older embryos showed a con- 

 tinuous decline in mutual cohesiveness ( Moscona and T. Weis, 1961 ) . 

 This age-dependent change expressed itself as a progressive decease in 

 the size of the aggregate, so that cells dissociated toward the end of 

 embryonic development were mostly unable to become functionally 

 reconnected. Figures 6 and 7 illustrate the aggregation patterns of dis- 

 sociated neural-retina cells from chick embryos at various stages of 

 development, tested under identical conditions. Such developmentally 

 progressive changes in cohesiveness and aggregability were found to 

 occur consistently and in a highly regular manner in cells from all the 

 tissues tested, though the rates of change and the resulting patterns 

 differed according to the type of tissue. 



How can we interpret this new information: that cells dissociated 

 from older, more differentiated tissues are less capable of cohering and 

 reconstructing formative frameworks than cells from younger tissues? 

 No firm statements can presently be made; however, it is not unlikely 

 that these changes, as a feature of development, are closely related to 

 the main theme of differentiation, i.e., to the progressive restriction and 

 channeling of metabolic processes in maturing cells toward specific 

 functions. If so, the possibility suggests itself that the competence of 

 dispersed cells to become reconnected into a functional continuum de- 

 pends on metabolic sequences or products which are available in young 

 cells but become altered or shut off as the synthetic economy of the 

 cells is restrictively committed. This would point to a possible involve- 

 ment of cellular products in the mechanisms of cohesion and histoge- 

 netic bonding of cells— a proposition of considerable conceptual and 

 practical interest. 



