Nucleic acid studies in experimental cardiomegaly.

• The nature of the cellular changes that occur in an enlarging heart are unclear. Whether cardiac enlargement, even in the young, occurs by increase in size of the cell (hypertrophy) or cell division (hyperplasia) or both remains an unsettled question. Mitotic figures, an indication of hyperplasia, have not been convincingly demonstrated by microscopic study of the adult heart even when enlarged. They are rarely found in the hearts of normal infants one to two months of age. In contrast, mitosis has been demonstrated in enlarged hearts of children up to the age of 20 months. An increased number of fibers in the heart of a two-month-old child with "idiopathic hypertrophy" was reported by Dammin and Moore. The number of nuclei in the fibers, however, was not increased. These workers concluded that increased syncytial development had taken place. Such techniques, designed to determine the presence or absence of hyperplasia, are tedious and subject to considerable difference in interpretation. Likewise, the presence of cellular hypertrophy under the microscope is determined by laborious and time-consuming measurements of the diameters of myriads of fibers using an ocular micrometer. The difficulties of making reproducible measurements of such diameters are well-known to pathologists. There is no uniformity of opinion whether fiber diameter should be measured in transverse or in longi-