The ZIO-reactive sites in the extraocular muscles of the rat

The extraocular muscles of the rat were investigated by means of the zinc-iodide-osmium (ZIO) impregnation technique. It has been disclosed that the ZIO reaction product has the ability to impregnate synaptic vesicles of the motor axons. Within the muscle fibres ZIO impregnation has concerned mitochondria, multivesicular bodies, small sarcoplasmic vesicles, some dense bodies, and, sporadically, terminal cisterns (diads) of the sarcoplasmic reticulum. Although it would be difficult to indicate any chemical substances in the muscles which would be directly involved in the ZIO reaction, the problem of specificity of the method is discussed.