Acetylcholine receptors at the rat neuromuscular junction as revealed by deep etching.

Collagenase treatment of rat intercostal muscles yielded single muscle fibres in which the nerve terminals and basal lamina were removed allowing an unimpeded view of the external surface of the postsynaptic membrane. This was revealed by deep etching of freeze-fractured preparations and appeared as a maze of folds separated by deep troughs, showing on the crests of the folds a densely packed population of protrusions about 8.5 nm in diameter. These densely packed protrusions (ca. 9000 microns-2) are mainly confined to the postsynaptic regions of the sarcolemma and presumably represent the acetylcholine receptor molecules, which are highly concentrated in these areas. The protrusions are generally tightly packed without obvious regular arrangement, but in some areas, usually on the tops of the crests, they are arranged into irregular rows normal to the long axis of the folds.