Ultrastructural study of developing rabbit diaphragm.

Diaphragms obtained from forty rabbit fetuses during the last third of gestation and from five rabbits one week old were studied ultrastructurally. Four gestational ages were chosen for the investigation. The diaphragm at 20 days gestation revealed early myogenesis which was characterised by many myogenic cells in the process of mitosis and by fusion of myoblasts to form myotubes. A large number of lipid droplets and a moderate quantity of glycogen were observed in the myotubes. These myotubes contained a few myofibrils peripherally. At 22 days of gestation, the myotubes exhibited a relatively large number of myofibrils and large amounts of glycogen. They also showed sarcolemmal modifications which appeared to be developing motor end plates. Mitosis of the myogenic cells persisted even after fusion. At 25 days of gestation, connective tissue sheaths appeared around the muscle fibres as well as further differentiation of the muscle fibres. At 30 days of gestation, the muscle fibres exhibited nearly complete differentiation, with the formation of myotendinous junctions. At one week post partum, the diaphragm showed full development of its muscle fibres. These morphological observations suggest a very rapid functional differentiation of the rabbit diaphragm during the last third of gestation.