122 CONTROL MECHANISMS IN CELLULAR PROCESSES 



produce. One of the most puzzling and central problems of biology 

 is the understanding of the mechanisms of this differentiation. 



If differentiated cells have a complete set of genes, how is it that 

 they perform only restricted functions? Weissman ( 1892 ) long ago 

 formulated the theory of nuclear diiferentiation, a progressive loss 

 of genie function during development bv different cell strains. The 

 problem was not approached experimentally until recent times, when 

 Briggs and King (cf. King and Briggs, 1956) started their experi- 

 ments on nuclear transplantation into enucleated eggs. These au- 

 thors seem to have demonstrated that nuclear potentialities actually 

 change irreversibly during development. The proof that this change 

 is related to the restricted function of nuclei in differentiated cells 

 is not absolutely convincing yet. Fischberg et al. (1959) have 

 shown, on the contrary, that nuclei from differentiated cells of ad- 

 vanced stages of development, transplanted into eggs, were able to 

 direct the formation of a complete organism. 



Whichever is the case, irreversible nuclear differentiation is not a 

 necessary condition for cellular differentiation. A beautiful example 

 of this is lens regeneration in Trituriis (Wolff, 1895). Pigmented 

 iris epithelium, undoubtedly a highly differentiated type of cell, 

 during lens regeneration loses its pigment, starts to divide and soon 

 forms a lens cell able to synthesize highly specific lens proteins. 

 Botanists can testify to many cases where an apparently differen- 

 tiated cell can revert into a meristematic cell, able to produce a 

 whole new plant. This can be seen in the formation of adventitious 

 meristems from parenchvmatous cells, normally not functioning in 

 any further cell multiplications (Sinnott, 1946). A verv instructive 

 case can be found in peat-moss {Sphagnum) (Zepf, 1952). The 

 leaves of Sphagnum are composed of two types of cells, small chloro- 

 phyll-bearing and large, water-storing cells. The latter, in a fully 

 grown leaf, become dead cell walls. At the apex of the leaf bud, 

 an apical cell divides unequally, giving one cell destined to be 

 chlorophyll-bearing and another to be a water-storage cell. Each 

 type of cell develops further according to its prospective poten- 

 tiality. If there is any functional differentiation of nuclei, it must 

 have occurred at the time of unequal cell division. Zepf could, how- 

 ever, under proper conditions, induce regeneration of a whole plant 

 from both types of cell. If there was any nuclear differentiation it 

 could not have been irreversible. 



