Cell interactions during Growth and Morphogenesis 85 



Synergistic Inductions 



Synergistic inductions are those in which two different tissues mu- 

 tually interact so that each causes the other to develop in a way that it 

 would not do when alone. For example, the developing kidney contains 

 many small S-shaped secretory tubules connected to a fan-like network 

 of branched collecting tubes which serve to transport material collected 

 by the secretory tubules into the ureter to be voided as urine. Figure 41 

 is a schematic diagram of kidney-tubule development. The tubes and 

 tubules originate from two different rudimentary tissues, starting at about 

 the eleventh day of embryonic life; these are the nephrogenic cord, a 

 loose collection of mesodermal cells, and the ureteric bud, a compact 

 tubular tissue also derived from mesoderm. The development of the secre- 

 tory tubules from the former and the collecting tubes from the latter has 

 been found to depend on the intimate association of both. Removal of 

 either from the embryo stops the development of the other. 



It is possible to separate the ureteric bud and the associated nephro- 

 genic cord from a mouse embryo and cultivate the tissues on a blood clot 

 overlain with a nutrient solution. Under these conditions, tubes and 

 tubules develop as they do in the embryo. If the two tissues are separated 

 from each other and cultivated, tubes and tubules never develop. Many 

 unsuccessful attempts have been made to extract material from either 

 tissue, that, when added to a culture of the other, would make it develop 

 in normal fashion. If, however, the two tissues are separated by a very 

 thin membrane with tiny holes, the synergistic induction does occur. Al- 

 though the membrane is thick enough to prevent the two kinds of cells 

 from touching each other across the membrane, blobs of cytoplasm could 

 conceivably pass between the two. When opposed across thicker mem- 

 branes, synergistic development does not occur. 



This type of induction, termed direct-contact induction, is a common 

 phenomenon in developmental systems. It requires the reacting tissues to 



Fig. 41. Formation of kidney tubules (after Arey). 







Collecting tubuli 



