102 HYPERTROPHY, HYPERPLASIA, METAPLASIA, REGENERATION 



it is later characterized when the papillae of the new cutis are formed 

 and the new cells have adjusted themselves to these and to each other. 

 In stratified epithelium it is the deeper layers from which the new 

 cells chiefly arise. While arnitotic division has been observed in the 

 restitution of surface epithelium, this occurs in the more superficial cells 

 and is believed not to be concerned in the regenerative process. 



Regeneration of gland epithelium after injury is of frequent occur^ 

 rence, though this capacity varies considerably in different glands. In 

 many cases it appears to be by a proliferation of the epithelium lining 

 the smaller excretory ducts that the restitution is accomplished, rather 

 than by the more highly differentiated secreting cells. This is of espe- 

 cial interest because it affords an excellent example of the rehearsing under 

 abnormal conditions and for reparative ends of a developmental phase 

 of cell life. 



In the livjer very extensive new formation of liver cells may occur 

 after experimental removal of a portion of the organ. jTj_the thyroid 

 ^^ eland also new gland tissue maybe formed after injury. In the kid- 

 ney the regenerative capacity of the epithelium apj>ears to be less 

 marked, though proliferation^ of the epithelium of the collecting tubes 

 may lake place. T>u1 in all these cases it is often difficult to determine 

 h"6w much of the increase in parenchyma, which undoubtedly may occur, 

 is due to a formation of new gland tissue and how much to a compensa- 

 tory hypertrophy of the old. 



While, therefore, it is true thai after in juries to t he glands a consider- 

 able regeneration of epithelium may occur, when the loss of substance 

 is extensive it is usually rather by a fibrous-tissue repair or by a com- 

 pensatory hypertrophy or hyperplasia in the uninjured portions of the 

 organ than by the new formation of true gland tissue that restitution 

 occurs. 



Regeneration of Connective Tissue, We have seen again and again in 

 reviewing pathological regeneration that, in local restoration after in- 

 jury, fibrous tissue plays an important part, either by itself or in asso- 

 ciation with various forms of parenchyma. New fibrous tissue may be 

 formed in the adult to replace tissue which has been destroyed. It re- 

 sults from many kinds of prolonged chemical and mechanical irritants. 

 Thus atrophy of the parenchyma may be followed by interstitial fibrous- 

 tissue growth, or new fibrous tissue may develop under the influence of 

 bacterial and other toxic substances, and it frequently forms dense cap- 

 sules about the seat of old lesions or around foreign bodies. 



Many forms of compensatory fibrous-tissue development will be de- 

 scribed in the second section of this book, such as thickening of the 

 walls of blood-vessels, inflammatory adhesions, replacement hyperplasia, 

 etc. 



We have now to consider briefly the changes involved in the new 

 formation of this fibrous tissue. Here, as in all other tissues of the body, 



1 For a study of the regeneration of mucous membranes see Cor nil and Carnot, Arch. 

 de Med. Exp., tome xi., p. 413, 1899. 



