CHAPTER XXII. 

 HYPERTROPHY AND HYPERPLASIA. 



BY hypertrophy is meant an increase in the size of the elements 

 composing a tissue ; by hyperplasia, an increase in their number. 

 Both conditions usually lead to an enlargement of the organ in' 

 which they are found, but this is not necessarily the case, for all the 

 elements in the organ need not participate in the increase ; some 

 may diminish in bulk. 



1. Functional Hypertrophy. This process, like that of functional 

 atrophy, depends upon the activity of the part undergoing the 

 change. In this case the parenchyma of the part is increased to 

 meet a gradually increasing demand for the work it is fitted to 

 perform. This increase may take the form of hypertrophy or that 

 of hyperplasia. The muscular tissues meet the demand by an 

 increase in the size of the muscle-cells. This is illustrated in the 

 hypertrophy of the heart in valvular lesions, which throw extra 

 work upon the muscle; in the enlargement of the uterus during 

 gestation, fitting it for the strong contractions during labor ; and 

 in the enlargement of the voluntary muscles by exercise. 



In glandular organs an additional demand for work results in 

 hyperplasia, in which the epithelial cells of the parenchyma multi- 

 ply (Fig. 259). 



Functional hypertrophy, or hyperplasia, takes place only under 

 certain favorable conditions. The demand for extra functional 

 activity must not be too great, otherwise degenerative changes 

 ensue. The same result would follow were the nutritive supply 

 insufficient to meet the loss of material and force sustained by 

 the cells in doing the increased work. It is evident, then, that 

 the condition occasioning the hypertrophy or hyperplasia must 

 develop gradually, and not interfere with the supply of nutrition. 

 The nature of the tissue also influences the result. In general, it 

 may be stated that tissues of high specialization are less capable of 

 either hypertrophy or hyperplasia than those less specialized, and 

 that hypertrophy is the rule in tissues of higher function, while 



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