Determination of myofibrillar diameter by light diffractometry.

Distinctive oscillations in the diffraction line intensity were observed when a laser beam was directed at selected spots on single skeletal muscle fibres of frog and normally to the fibres. These intensity oscillations were interpreted as the diffraction from cylindrical myofibrils because they followed the first-order Bessel function. This interpretation allowed a direct determination of the myofibrillar diameter from the first intensity minimum of the zerothorder diffraction line. The hypothesis that the intensity oscillations were related to the myofibrillar diameter was substantiated by further experiments. At fixed sarcomere length the measured myofibrillar diameter increased when the fibre was immersed in hypotonic solution and decreased in hypertonic solution. In another experiment the diameter decreased and the sarcomere volume remained constant when the fibre was stretched passively. Furthermore, there was excellent agreement between the myofibrillar diameters measured by light diffractometry and electron microscopy.