[Changes in the ultrastructure and electrolyte composition of the muscles of rats during embryogenesis. I. Electron microscopic study of the structure of skeletal and heart muscle].

The ultrastructure of rat skeletal and cardiac muscles during embryogenesis was studied. Uninucleated skeletal muscle myoblasts of 13 day old embryos contain no myofibrils or discrete sarcomers, although intermediate filaments (8-13 nm) are observed. In cardiomyocytes of this stage, a formation of myofibrils from thick (15-16 nm) and thin (5-7 nm) filaments takes place. In 18-19 day old embryos, the ultrastructure of myofibrillar material is similar in skeletal and cardiac muscle cells. In cardiac muscle of 19 day old embryos, mitochondria with unusual structure were discovered: their membranes form invaginations filled up with glycogen.