Fine structural details of human muscle fibres after fibre type specific glycogen depletion.

Type 1 and Type 2 fibres of skeletal muscle (human m. vastus lateralis), selectively depleted of glycogen by sustained submaximal muscular exercise (running 30 km), were identified at light and electron microscopical level by examination of thin and ultra-thin serial sections treated particularly for visualization of glycogen. Averaged images, obtained by lateral smearing of depleted fibres (Type 1) exhibited five clearly visible cross-bridges in the M-band and had broad Z-bands. Non-depleted fibres (Type 2) showed either three central strong and two weak outer lines in the M-band and intermediate Z-bands (Type 2A), or only three central strong lines in the M-band and narrow Z-bands (Type 2B). The depleted fibres had no subsarcolemmal accumulation of glycogen particles and practically no intermyofibrillar particles. The remaining particles were small in size and seemed almost rudimentary. In non-exercised individuals, a peculiar distribution of individual glycogen particles in the I-band and A-band was found. This distribution was accounted by the structural arrangement of the myofibrillar material.