Dissociation of the ‘Surface Membrane Complex’ in Atrophic Muscle Fibres

Abstract

WHEN one examines the cross-section of a muscle fibre with the electron microscope, a ‘membrane-complex’¹ of the type shown in Fig. 1 is observed, consisting of two dense layers separated by a light zone of about 200–300A. width. The sections shown in Figs. 1–3 has been fixed in osmium tetraoxide and stained with phosphotungstic acid in alcohol. (All scales equal 1µ.) There are reasons for regarding only the inner layer as a ‘physiological’ cell membrane and the outer layer as an extra-cellular component, frequently described as a ‘basement membrane’². Some authors even regard the outer layer as a fixation artefact. The two layers can be distinguished, not only by their different appearance (the outer layer is more ‘fuzzy’, tending to fray out and connect with fine connective tissue fibrils), but also by their different ‘electron-staining’ properties. Thus, in permanganate preparations in which cellular and mitochondrial membranes show up in good contrast, the outer layer is usually barely visible, and in general, its relative density under different conditions of fixation and staining corresponds more closely to that of the protein filaments in the muscle fibre than to the cellular or intra-cellular membrane structures in muscle, nerve or Schwann cells (Birks, Huxley & Katz, unpublished work).