72 Cellular Differentiation 



habited the dorsal area of the embryo after gastrulation. Only those cells 

 that remain in the inner core of the neural tube continue to reproduce 

 actively and to resemble their parents. Those cells that are pushed toward 

 the periphery cease dividing and are transformed into the types illustrated 

 in Fig. 36. It should be reaHzed that the drastic morphological changes 

 shown are reflections of equally drastic changes in the metabolic capabil- 

 ities of these cells. 



The extent to which differentiated cells can differ from one another 

 can be appreciated from current work on the reaggregation of tissue cells. 

 Cartilage tissue was cut from a chick embryo and the cells were dispersed 

 by treatment with an enzyme that destroyed the material cementing them 

 together. A suspension of the loose cells in nutrient broth was then 

 pipetted into a glass chamber. The cells promptly reaggregated and 

 became cemented together so as to construct a tissue of uniform texture, 

 a perfectly normal cartilage tissue. When dispersed kidney cells and 

 cartilage cells from the chick were intermixed and pipetted into the cham- 

 ber, the two cell types ignored each other. They formed separate ag- 

 gregates, the one producing something approaching normal kidney tissue 

 and the other apparently normal cartilage. In other words, the kidney and 

 cartilage cells recognized their mutual differences and refused to become 

 cemented one to the other in a mixed tissue. Now, dispersed cartilage 

 cells from chick and mouse embryos were mixed. The two kinds of cells 

 could be distinguished by size and nuclear structure. The cells came to- 

 gether and were cemented into a single cartilage tissue in which chick 

 and mouse cells were homogeneously interspersed. Mouse kidney and 

 chick cartilage cells ignored each other, as did the homologous chick mix- 

 tures. Two conclusions are permissible: 



1. With respect to at least one criterion (the ability to stick together 

 when they meet and so form a stable aggregate and later a homogeneous 

 tissue), chick cartilage cells are very much like mouse cartilage cells and 

 very unlike chick kidney cells. Thus, in this instance, embryological differ- 

 ences within the same animal can transcend evolutionary differences be- 

 tween two animals as diverse as a bird and a mammal. 



2. The ability of two cells to be cemented together is a very specific 

 process. While the mechanisms are still unknown, apparently the same 

 ones are used regardless of species (otherwise mouse and chick tissues 

 could not stick together ) . 



The Genetic Basis of Cellular DifFerentiation 



When we say that parent cells give rise to progeny that display a 

 wide difference in structural and metabolic properties, it is the ears of the 



