An ultrastructural study of the segmental uptake of horseradish peroxidase in the endplate region of denervated skeletal muscle fibres

Abstract

The segmental uptake of horseradish peroxidase (HRP) in the endplate region of denervated skeletal muscle fibres has been studied ultrastructurally using a method for selecting single muscle fibres with high segmental peroxidase staining from denervated mouse tibialis anterior muscle. Segments containing large peroxidase positive phagosomes could already be seen 10–15 min after i.v. injection of HRP. Such segments were still present 24 h after HRP injection. The localization of phagosomes, deep in the fibres rather than immediately under the sarcolemma, suggests that the uptake occurs from t-tubuli. Vivid proliferation of t-tubuli, consisting of vesiculation, enlargement and encircling of cytoplasmic components, was also observed. The HRP accumulates in phagosomes of varying size and shape. Similar membrane-limited bodies without or with very weak peroxidase staining were also observed. The peroxidase-positive phagosomes participate in autophagic processes as suggested by their content of undegraded cellular material. Golgi profiles, which occurred deep in the muscle fibres, and enlarged components of the sarcoplasmic reticulum were frequently encountered in the segments. Myofibrillar degeneration occurs in the segments and progresses with time after denervation. The described segments may be related to the increased membrane turnover in denervated muscle fibres and/or they may be related to processes aimed at establishing new synaptic contacts.