284 CELLS, TISSUES, AND ORGANISMS 



In addition to these views, there are still others sometimes more 

 casually mentioned or implied. Goss (1957) has, ascribed a formative 

 role to the skin, as well as the mechanical one noted above. The epi- 

 dermis has been said to function in the accumulation of the blastema 

 (for example, Thornton, 1954), to guide regenerative events in some 

 way (Trampusch, 1959), and to function in morphogenesis (for ex- 

 ample, Komala, 1957; compare Holtzer, 1958). Finally, Faber (1960) 

 has recently suggested that the apical epidermis may play a role in the 

 establishment of an apical proliferation center among the mesenchy- 

 matous cells which is important for regeneration of distal structures. 



The structure of the epidermis in the amphibian regenerate 



The sheet of epidermis that comes to cover the amputation wound 

 of the limb of the adult newt within the first day after amputation is 

 thin— about one or two layers. It then thickens, and by the fifth 

 day it is approximately five to ten layers thick, whereas in non- 

 regenerating skin it is only about three or four layers. The number of 

 layers may reach about 15 after 10 to 14 days— a time when histolysis, 

 edema, and cellular accumulation are very high. In subsequent stages 

 the thickened epidermis, now called the apical cap, thins slowly. Dur- 

 ing the stage of the medium or late bud it contains about six to ten 

 layers of cells, and fewer later on. The thickening reflects a period of 

 heightened mitoses preceding a similar outburst in the underlying 

 mesenchymatous accumulation ( Inoue, 1956 ) . 



Initially the epidermal cells are elongated tangentially to the 

 wound surface, but by approximately the end of a week the cells of all 

 layers except the outermost cornified ones are cuboidal; indeed, the 

 basal layers may even on occasion be low columnar in shape. The 

 cuticular surface is smooth and contains no papillae such as are ob- 

 served in normal skin. Papillae are differentiated relatively late in 

 regeneration. 



The transition from adult epidermis to apical cap at the original 

 amputation line is quite abrupt; a "lip" of regenerate epidermis sud- 

 denly protrudes into underlying tissue to delimit the margin of the 

 apical cap. On one side are skin glands, underlying basal membrane 

 and connective tissue of the dermis, but they are absent on the regen- 

 erate side. Ide-Rozas ( 1936 ) believed that the epidermal cells undergo 

 embryonalization in contact with the wound and the cells of regenera- 

 tion. The basal epidermal cells are affected first and then the upper 

 layers. The process disappears when the mesenchymatous cells dif- 

 ferentiate. According to him, the epidermal cells retain their shape as 

 they become embryonic but acquire some of the morphology of the 

 underlying cells. The nucleus enlarges, stains less deeply, and contains 



