THE RELATION BETWEEN Z‐DISK LATTICE SPACING AND SARCOMERE LENGTH IN SARTORIUS MUSCLE FIBRES FROM HYLA CERULEA

Abstract

Sartorius muscles from the green tree frog Hyla cerulea were set at variety of muscle lengths and fixed for electron microscopy using acrolein followed by osmium tetroxide. The sarcomere length,s, was determined in thick sections using laser-diffraction, The Z-disk lattice spacing, z, was measured in electron micrographs of thin sections From the same muscles. The Z-disk lattice was found to expand as sarcomere length decreased such that the quantity sz² was constant at 105 10⁶ nm³ for all sarcomere lengths in the range 19–29 m. Thus, the sarcomere length dependency of the Z-disk lattice is similar to that of the myosin filament lattice. The density of thin filaments per unit cross section of fibril leaving the Z-disk is less than their density in the A band, Thus, fibrils have a smaller cross section in the I band, leaving more inter-fibrillar space there. This may explain why more mitochondria and lipid droplets are located in the f bands than in the A bands. It is suggested that the Z-disk may contribute to the short range elasticity of muscle fibres.