Page 14, Early Development, Devel. Physiol., Dr. D. Penney

Cellular Development: (cont.)

The cellular development of the heart can be divided into three stages (Table 2.05): Early on during the primary "hyperplastic" phase, 75-90% of heart cells are myocardial cells (myocytes, muscle cells). These cells are relatively small in size and increase in number by mitotic division as the heart grows.

Fibroblasts (connective tissue), endothelial, smooth muscle (blood vessels) and other non-muscle cells increase in number but not in size.

During late fetal and early neonatal development the mitotic division of myocytes slows and organ growth occurs by both hyperplasia and hypertrophy. Fibroblasts, endothelial cells, etc. continue to increase in number, but not in size. This is known as "transitional" phase.

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