HYPERTROPHY, HYPERPLASIA, METAPLASIA, REGENERATION. 101 



It is especially in certain forms of degeneration of the contractile 

 substance, after typhoid fever for example, in which the nuclei, the sar- 

 colemma, and the general framework of the tissue are . uninjured, that 

 regeneration of striated-muscle fibres is most complete. After injuries 

 with considerable destruction of the muscle tissue, regeneration is apt to 

 be irregular and incomplete. While there is often much nuclear divi- 

 sion and often the formation of large numbers of more or less striated 

 and variously shaped cells, these are apt not to develop into useful 

 muscle fibres and may disappear by degeneration and absorption or by 

 pressure atrophy. Here, as elsewhere in highly organized tissues, such 

 restitution as is possible after considerable injury is achieved by fibrous 

 tissue. 



It is interesting to note that such regeneration as does occur in 

 striated muscle is not initiated by the highly differentiated contractile 

 substance, but by the nuclei and small residual amount of undifferen- 

 tiated protoplasm, and that such more or less definitely striated cells as 

 arc formed, are in many respects similar to certain forms of developing 

 muscle cells in the embryo. 



Although mitosis and nuclear division have been seen in the muscle 

 fibres of the heart after injury, there is no evidence that new muscle can 

 be formed. Kepair, which is not infrequent, is secured by fibrous tissue. 



Regeneration of Epithelium. Owing to continuous shedding or to 

 functional destruction of epithelium of the skin and mucous membranes 

 and certain of their aduexa, physiological regeneration by mitosis is 

 common. 







FIG. 33. REGENERATION OF EPITHELIUM. 



From a wound of the tongue. At the left is normal epithelium ; at the right the thin pellicle of new formed 

 cells is extending over the surface of the wound. 



After injuries also regeneration of epithelium in these situations oc- 

 curs by mitosis and may be extensive and complete. The new epithelium 

 always forms from the old, and, when surface losses are to be made good, 

 extends inward from the edges across the injured area after a suitable 

 substratum has been formed by fibrous tissue and blood-vessels (Fig. 33;. 

 The new epithelium is at first atypical in form and arrangement owing 

 to the necessity for a gradual adaptation to the sustaining and associated 

 tissues. Thus the epithelium, which at first presses forward over a heal- 

 ing wound of the skin, may be in the form of a single layer or a thin 

 smooth pellicle without the usual variations in size and shape by which 



