Postnatal Heart Growth:
Postnatal heart growth to adulthood occurs solely by hypertrophy of myocytes, while the non-muscle cells continue to increase in number but not in size. This is known as the "hypertrophic" phase.
In the rat the transitional phase ends 2-3 weeks after birth, while this same event takes place 2-4 months after birth in humans. Thus, the number of myocytes you have now are the same (maybe less) as you had when you were a tiny baby.
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