Some observations on the filaments of frozen‐etched striated muscle

SUMMARY Frozen‐etched rabbit psoas muscle and locust flight muscle are illustrated. The myofibril structure of striated muscle as seen by freeze‐etching is wholly compatible with the well‐established interdigitating sliding filament model. A major difficulty in interpretation arises from the detailed appearance of projecting myofilaments which arise when myofibrils lie obliquely to the fracture plane. They appear to be surrounded by a grey halo, with a light central spot, but without sufficient projection to account for the length of the shadow. The grey halo consists of undissolved myosin released from the replicated projecting myofilament during the drying of the replica following cleaning. It may be removed by recleaning. The surface tension forces involved in removing the myosin from its casing, however, cause the case itself to be destroyed, leaving a phantom shadow. The light, central spot is a resultant hole in the replica. Thick and thin filaments are found in longitudinally fractured myofibrils. There is a close correspondence with the images of frozen‐etched muscle from previous work.