Electron microscopic investigations on the differentiation of Purkinje cells in the ontogenetic development of the chicken heart.

The ultrastructural development of subendocardial Purkinje cells of chicken left ventricle was investigated. In 9-day-old chick embryos the cell diameter and the organization of the cell organelles allow a distinction between Purkinje cells and ordinary myocardial cells In 14-day-old chick embryos, Purkinje cells show large accumulations of myosin filaments with interspersed ribosomes in addition to normomeric myofibrils. In these accumulations actin filaments seem to be absent. The deficiency of actin filaments is supposedly the reason for the random distribution of the myosin filaments. Purkinje cells of early chick embryos show areas with densely packed glycogen granules. In older embryos the glycogen concentration declines and only separate glycogen granules are visible. At hatching time the first subsarcolemmal leptomeric fibrils were observed in Purkinje cells. Leptomeric complexes arising in a close spatial relationship to the accumulations of myosin filaments and ribosomes can be seen in 2--4 week-old chickens. With the increasing age of the chickens, the size of these accumulations declines. Adult hens exhibit smaller accumulations, mainly in the neighborhood of leptomeric complexes. Purkinje cells show a distinct ontogenetic development. They are not simple embryonic remnants of ordinary myocardial cells.