The proliferation of myofibrils during muscle fibre growth

Myofibrils in muscle fibres of different sizes and different ages were examined and measured using phase-contrast and electron microscopy. During the post-natal growth of the mouse biceps brachii muscle the number of myofibnls in some fibres increases from about 75 to 1200 The range of myofibnl size was from 0·4–12 μm. The distribution of myofibnl sizes in muscles of all ages studied was bimodal A high incidence of longitudinal splitting of myofibrils was observed with the electron micro scope in differentiating muscle fibres and in some medium and large muscle fibres. Size measurements with the electron microscope showed that the splitting myofibrils were about twice as large as non-splitting myofibrils and that the myofibrils split more or less down the middle. A possible explanation for the splitting is that the peripheral I filaments are pulled at an angle slightly oblique to the myofibnl axis, because of the discrepancy in the A and I-filament lattice spacings. When the myofibnl reaches a certain size the oblique pull of the peripheral I filaments is strong enough to cause the Z disks to rip. From data on the size, shape and number of myofibnls at different stages of growth it was concluded that longitudinal splitting is the means by which the number of myofibrils increases during post-natal growth.