The Beginnings of Multicellular Organization 31 



Finally, there is a great deal of evidence to suggest that, in Dicty- 

 ostelium discoideum, the formation of an aggregate requires the presence 

 of a specially endowed cell, which has been called the "initiator cell." 

 Thus at the time of aggregation, the population of myxamoebae is differ- 

 entiated into initiator cells, which provide the signal to start aggregating 

 (and in whose absence aggregation does not occur), and the responder 

 cells, which answer the signal. 



CELL INTERACTIONS DURING DEVELOPMENT 



The development of a multicellular system is accompanied, indeed 

 is largely directed, by a hierarchy of cell interactions. One cell or one 

 tissue may stimulate or inhibit the development of another by exchange of 

 appropriate chemical agents. It is this matrix of interactions that ensures 

 the harmonious organization of the whole, i.e., that the components will 

 develop at the right time, in the right place, and in the right amount. As 

 we will see in Chapter 7, the use of cell interactions reaches its highest 

 degree of complexity and refinement in vertebrate embryos. However, 

 the beginnings of these control mechanisms operate in primitive organ- 

 isms such as the slime molds. 



A case in point is the way in which the cells aggregate. A large body 

 of evidence has been accumulated to show that outlying cells are attracted 

 and caused to aggregate by specific chemical agents produced at the 

 center. (Attraction of cells by a chemical compound is called chemotaxis.) 

 For example, if the myxamoebae are permitted to aggregate under a 

 flowing stream of water (Fig. 15), a center forms, but the cells upstream 

 are unaffected. Only those downstream are attracted by and move toward 

 the center. This is just what you would expect if the center were produc- 

 ing a chemical agent that was being carried downstream by the current. 





Direction of ■ 

 current 



Fig. 15. Myxamoebae ag- 

 gregate under a flowing 

 stream. 



Cells upstream from 



aggregative center 



are unaffected 



■ Cells downstream 

 elongate and move 

 toward the center 



