In muscle fibres of the rat diaphragm, there is widespread accumulation of free ribosomes and rough-surfaced endoplasmic reticulum in the subsarcolemmal sarcoplasm at 28 days following section of the phrenic nerve. A corresponding accumulation of basophilic material is visible at the periphery of the fibres when examined with the light microscope. Both the ribosomes and the basophilic material are removed by preliminary treatment with ribonuclease. Fibres comprising the diaphragm of the newborn rat display similar aggregations of subsarcolemmal ribosomes and peripheral basophilia. These ultrastructural and cytochemical manifestations of protein synthesis correspond closely in time to the well documented ‘supersensitivity’ to acetylcholine, which occurs along the entire length of muscle fibres in the newborn and denervated rat diaphragm. It is suggested, therefore, that this protein-synthetic machinery, which is concentrated at the cell surface, is involved in the formation of new receptors.

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