CHAPTER III. 



HYPERTROPHY, HYPERPLASIA, METAPLASIA, 

 REGENERATION. 



Hypertrophy and Hyperplasia. 



UXDER a variety of conditions, parts of the body or organs become 

 larger than normal. The structural change to which this enlargement is 

 due, may be a simple increase in size of the elementary structures of the 

 part-cells, and other tissue elements. This is called simple hypertrophy. 

 It is usually associated with some increased functional demand upon the 

 cells ; as, for example, in the hypertrophy of the heart with lesions of 

 the valves, or in the hypertrophy of one kidney, which in case of dimi- 

 nution or suspension of function in the other assumes the work of both 

 compensatory hypertrophy. ' 



On the other hand, in many cases the increase in size of a part or 

 organ is due not only, or not at all, to the increase in size of its elemen- 

 tary structures, buj^ to an increase in their number. This increase in 

 number of the structural elements of a tissue or organ is called numer- 

 ical hypertrophy, or hyperplasia.' 2 



Simple hypertrophy and hyperplasia are frequently associated. 



** Metaplasia. 



The members of the connective-tissue group fibrous tissue, mucous 

 and fat tissue, cartilage, bone, etc. are so closely related in nature and 

 structure that not infrequently and under a variety of conditions one 

 form of tissue assumes the characters of another. This change of one 

 form of closely related tissue into another is called metaplasia. 



Thus, by a gradual change in the cells and stroma of fibrous tissue, 

 this may be converted into bone, as mucous tissue may become fat tis- 

 sue, and hyaliii cartilage become fibrous. Metaplasia is a process in- 

 volving active changes on the part of the living cells of the tissue, and 

 should be clearly distinguished from certain degenerative processes, in 

 the course of which one form of connective tissue may assume super- 

 ficial resemblances to others of the group, as in calcareous and .mucoid 

 degeneration. 



While metaplasia is most common among the members of the con- 

 nective-tissue group, it sometimes occurs in other tissues. Thus, for 



1 For bibliography of compensatory hypertrophy see reference to Asc7ioff, p. 106. 

 * Consult for examples of hyperplasia and hyp'ertrophy the following sections on 

 Regeneration and Inflammation. 



