On the origin of Z-band material and myofilaments in myoblasts from the human atrial wall.

The origin of cardiac myofibrils in cells from the atrial wall in human embryos was studied, Z-band substance appears throughout the cytoplasm as irregular electron dense patches in a network of thin filaments. The thin and thick filaments are synthesized as separate units in the sarcoplasm and are later aggregated into myofibrils. Complexes of Z substance and thin filaments occur numerously at different stages of myofibrillar organisation. Thick filaments are formed in close proximity to free ribosomes and are later incorporated in an hexagonal pattern into the Z-band/thin filament complex.