Regeneration in Cardiac Muscle.

Proliferation of striated muscle, either in the form of regeneration of damaged tissue or hyperplasia of relatively normal cells, is generally thought not to occur. Such an opinion is an example of the general idea that more " specialized " tissues differ from less " differentiated " tissues in that they are unable to multiply in adult life. This proposition, however, has gradually become more and more untenable during the past fifty years. At an early stage in the development of microscopy the capability of various forms of epithelium-skin, alimentary canal, etc. for proliferation was appreciated. For some time such capacity was denied the more " specialized" epithelia such as the liver and renal cells. When hepatic and renal proliferation was at last recognized, the original view was still applied to other tissues, notably muscular and nervous cells. Whether one regards a mucous cell of the intestine as being more differentiated than a fibrous tissue cell and less differentiated than a liver cell is purely a question of the point of view from which it is considered. The intestinal cell cannot, or in ordinary circumstances does not, produce collagen fibres, nor the liver cell mucin. Apart from this question and the presumed inverse capacity for proliferation, there is no doubt that certain cells may be observed to multiply more often than others. This has been recognized from the earliest days of histological study, but though many cells were thought not to multiply at all, it is now known that, to some extent, they do. Regarding muscle cells, hyperplasia of smooth muscle was the first observed. More recently, regeneration of voluntary muscle, both naturally and experimentally, has been described, but knowledge of such changes in heart muscle has lagged behind.