242 Regeneration. 



Regeneration of Epiderm and Epithelium of Mucous Mem- 

 branes. — The epithelium on the surface of the skin and mucous 

 membranes is capable of very rapid and complete regeneration. 

 Because of the continual loss of cells from desquamation as they 

 grow old the tissue is always prepared for and actively engaged in 

 regeneration. Either because of cellular tactile irritability (v. p. 

 235) or because the removal of tissue resistance in defects mechan- 

 ically causes it to undergo a regressive development, the new tissue 

 is very apt to spread out upon the free surfaces and to some extent 

 to increase in volume, a flat cell thus arising from a cylindrical cell 

 at the border of the lesion. The new cells are always formed from 

 existing epithelium and can only be produced providing vitally 

 capable cells of the same tissue are present in the vicinity of the 

 lesion. 



The formerly accepted belief that epithelial cells could develop from 

 connective tissue has been shown to be erroneous. 



In case of superficial loss of epithelium (abrasions, epithelial ne- 

 croses) the new surface cells grow over the denuded papilhe from 

 the margin of the surrounding epiderm. Within three-quarters 

 of an hour after a wound has been inflicted, under the microscope 

 there may be observed enlargement of nuclei and direct and indi- 

 rect nuclear division in the cells of the stratum ]\Ialpighii, both in 

 the margins of the lesion and at some distance from it ; after eight 

 or ten hours the epidermal border is visibly thickened ; and small 

 lesions only a few millimeters in circumference may be covered by 

 several lavers of epithelium in the course of forty-eight hours. 

 The epithelium actually spreads over the surface of the wound, 

 either from active motile power or because the superficial cells are 

 dislodged by those growing back of them in the deeper layers. 

 If the part deprived of epithelium is covered by fibrin, blood or pus, 

 a granulating surface, the proliferating epithelium at the margin 

 forces its way beneath the coagulated crust over the cicatrizing 

 connective tissue ; in these cases the epithelial layer is often rid- 

 dled with leucocytes, and afterward in the clefts and spaces thus 

 produced new cells may grow, forming branched plugs made up of 

 concentrically laminated, keratosed, epidermal balls (horny pearls, 

 atypical epithelial proliferation of Friedlander). Occasionally the 

 epithelium may penetrate along fistulous passages ; as in a perforating 

 dental alveolar fistula (horse) the squamous epithelium may grow 

 through the alveolus from the mouth into the nasal cavity.* 



* Penetrative growth of epithelium at the border of ulcers and fistuljp 

 may determine a free atypical new formation of this tissue and thus give 

 rise to cancer. 



