288 CELLS, TISSUES, AND ORGANISMS 



keratinized ones. At times these enlarged spaces contain particles of 

 various size, nerve fibers, and wandering cells. Wandering cells, 

 squeezed between adjacent epithelial cells, often have large and 

 numerous osmiophilic inclusions and finger-like projections. A feature 

 that helped to distinguish them from epidermal cells is the absence of 

 the distinct plaques characteristic of desmosomes (Figure 1). Epi- 

 dermal cells sometimes contain as much debris as the phagocytes. 



The electron microscope also revealed that numerous epithelial 

 cells possess a cisternal type of endoplasmic reticulum which we have 

 not seen in non-regenerating epithelium. Such a diflFerence bespeaks 

 either a qualitative or quantitative difference in the physiology of the 

 cell. In normal epidermis the reticulum consists of small round or oval 

 profiles scattered in the cytoplasm (see Figure 10 of Salpeter and 

 Singer, 1960). Numerous granules similar to those containing ribo- 

 nucleoprotein (Palade and Siekevitch, 1956) are found free in the 

 cytoplasm, with some adhering to membranes of the reticulum. In 

 regenerating epithelium the endoplasmic reticulum ranges from the 

 vesicular type, as in the normal cell, to one consisting of many parallel 

 elongated or distended profiles. Granules of ribonucleoprotein are 

 abundant along the membranes of the reticulum. It has been con- 

 sistently suggested that this type of endoplasmic reticulum is associ- 

 ated with cells engaged in the secretion of a protein-rich product 

 (Palade and Porter, 1954; Howatson and Ham, 1955; reviewed by 

 Haguenau, 1958). And so it may be that the more elaborate reticulum 

 in the epidermal cells of the regenerate denotes a more active synthe- 

 sis of protein or other substance. In contrast, the activity of the non- 

 regenerating epidermis may be more modest. 



The cells of the regenerate epithelium also frequently have a 

 cortical zone which may be quite broad ( Figures 2 and 3 ) . It contains 

 no endoplasmic reticulum or mitochondria but consists rather of a uni- 

 form, spongy material which is somewhat indistinct. The cortical zone 

 is encountered most frequently among basal cells and least frequently 

 among the outer ones. We have seen such a zone in normal epithelium, 

 but it is less striking and much less frequent. The significance of the 

 zone is unknown; it may represent an area of accumulating secre- 

 tion which is being discharged gradually through the cytoplasmic 

 membrane. 



The relation between epidermis and blastema 



The undersurface of the epithelium during the stage of formation 

 of the early regenerate is devoid of a basal membrane, which ends 

 rather abruptly at the original wound level and may even turn inward 

 (Singer and Andrews, 1956). In the Hght microscope, cytoplasmic 



