TISSUE RECONSTRUCTION FROM DISSOCL\TED CELLS 209 



cells. It provides the framework within which the compacting of cells 

 into coherent aggregates takes place (Moscona, 1959a). This ECM is 

 of mucoidal nature and resembles in many respects the intercellular 

 ground substance of normal embryonic tissues. The possibility that it 

 represents the postulated cellular products prerequisite for cell-bonding 

 and aggregation suggests itself strongly. Work toward clarifying this 

 question is now in progress. 



Histogenesis in aggregates 



Let us pass on now to firmer ground and review briefly the be- 

 havior of cells after their merger into multicellular aggregates. The first 

 question is whether the discrete cells, bunched together at random by 

 mechanical forces, are able to establish developmentally functional as- 

 sociations and to reconstruct normal tissue patterns. The answer to 

 this is affirmative. 



A characteristic feature of freshly formed aggregates is that the 

 cells, though closely packed, are in constant motion relative to one an- 

 other—like bees in a swarm. Cell contart does not lead in this case to 

 static cohesion or immobilization (but see Abercrombie, 1958; Weiss, 

 1958) but, rather, sets off new kinetic activities. As mentioned later, 

 these activities are in all likelihood instrumental in bringing about the 

 progressive ordering of the randomly assembled cells and their organi- 

 zation into histogenetic fabrics. In early aggregates the cells are packed 

 without definable order; later, as their kinetic activitv subsides, thev 

 are found arranged in histologically identifiable tissues. 



In Figure 8, photograph h shows a twelve-hour aggregate pro- 

 duced by cells from the cartilage-forming zone of 4-day chick embryo 

 hmb-buds. At the time of dissociation the cells are fibroblastic in shape; 

 in early aggregates they show no clear arrangement or indication of 

 their prospective fate. Yet in a few more hours this lump of cells be- 

 comes typical cartilage ( c in Figure 8 ) . Similarly, aggregates of liver- 

 forming cells from 7-day chick embryos become organized into typical 

 liver parenchyma. It may be argued that cartilage and liver are tissues 

 with a relatively simple, easily rebuilt architectural organization. How- 

 ever, cells from tissues considerably more complex structurally display 

 even more remarkable capacities for tissue restitution. 



The neural retina of the 7-day chick embryo consists of several 

 layers of cells ordered according to their prospective roles in the differ- 

 entiation of this tissue. At this stage of development the cells are al- 

 ready diversified in appearance, and the over-all organization of this 

 tissue appears quite complex. When brought into suspension, retinal 

 cells form aggregates within which they promptly become arrayed in 

 concentric layers. Each of these centers— or rosettes— represents essen- 



