144 PAUL WEISS 



But on close inspection there remains no property of a tissue that 

 could not be ultimately traced to the activity of either its own or other 

 cells. It is by virtue of properties acquired in cytodifferentiation that 

 cells can combine, interact, and arrange themselves in certain specific 

 ways, conditions permitting. "Histodifferentiation" implies merelv the 

 existence of conditions under which cytodififerentiated cells can realize 

 their individual and group faculties. This helps to clarify the meaning 

 of the term "tissue dedifferentiation." Evidently cells may lose their 

 tvpical associations and orderly arrangements just as we have seen be- 

 fore that they may lose internal differentiation products, without giving 

 up their essential character. Destruction of intercellular matrix, for 

 instance, can set cells free, and such cells mav resume motility, scatter, 

 and modulate in various ways. Their community has been dissolved, but 

 they themselves have retained their identity. To this extent, what is 

 called "tissue dedifferentiation" is merely tissue disintegration. Since 

 mobilization of cells is frequently attended by the dissolution of internal 

 differentiation products (e.g. myofibrils), the superficial picture becomes 

 one of general simplification and regression. There is no harm in con- 

 tinuing to speak of tissue dedifferentiation, so long as one bears in mind 

 that it need involve no true dedifferentiation of cells. 



Which Cell Parts Differentiate*? 



A cell is a highly complex and heterogeneous system. Then where 

 specifically do the changes occur that constitute differentiation? In the 

 cytoplasm, in the nucleus, or in both? Or rather, which ones of their 

 subdivisions undergo change, and which do not? 



It is customary in speaking of cytodifferentiation to emphasize mainly 

 the concomitant profound changes in the content of the cytoplasm. But 

 the nucleus likewise undergoes patent modifications, which become 

 evident in the divergence of nuclear characters and behavior among 

 different cell types. Some of these nuclear distinctions can be deduced 

 from differential reactions to histological stains, which in essence con- 

 stitute microchemical tests. In special instances even more direct signs 

 of nuclear differentiation have been observed, as in the extrusion of 

 formed secretion bodies from the nuclei of certain nerve cells (30), or 

 in the transfer of the major mineral content from nucleus to cytoplasm 

 during the maturation of neuroblasts (36). On the other hand, it is 

 a basic tenet of genetics that the chromosomes, or at least those parts 

 of them that constitute the genes, remain unaltered in the process of 



