Page 16, Early Development, Devel. Physiol., Dr. D. Penney

Heart Desoxyribonucleic Acid:

Heart DNA concentration is highest at birth, and decreases with normal growth as myocytes increase in volume during the transitional and hypertrophic phases of growth. The nuclear content of DNA is, so to speak, diluted out by the cytoplasmic components.

At birth about one-half of the DNA is in myocytes; by childhood this values reaches 40%, and by adulthood only 25-30%. This is because the myocytes cease dividing, whereas the non-muscle cells continue to do so.

Thus, while myocytes eventually come to make up only 25-30% of the cells in the heart by number, they continue to make up 75-78% of the heart by volume. This is because a myocyte on average is are many times larger than a non-muscle cell.

The number of myocyte nuclei (and DNA) double in the left ventricle after birth, due to the more rapid growth there (Table 2.06). Postnatal growth of the right ventricle is slower.

At birth the human heart contains approximately 900,000,000 myocytes, and this number increases to 1 - 2,000,000,000 in childhood, and approximately 2,000,000,000 in adulthood. There are a number of fibroblasts at birth equal to the myocytes, but this number later swells to 4 - 6,000,000,000.

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