Calcium-containing vacuolated mitochondria during early heart development in chick embryos as demonstrated by cytochemistry and X-ray microanalysis.

The ultrastructure of mitochondria in the developing heart was examined in chick embryos from 2 to 7 days after fertilization. Vacuolated mitochondria were observed in the heart muscle cells of embryos at all stages examined. The number of vacuolated mitochondria as a percentage of total mitochondria in muscle cells was high in embryos at 3 and 4 days and was much higher in the ventricular cells than in the atrial cells. Examination by cytochemistry and X-ray microanalysis revealed the accumulation of calcium in the vacuoles of mitochondria. These results suggest important roles for vacuolated mitochondria in the regulation of the intracellular concentration of calcium during the early development of the chick heart.