Tubular aggregates observed in spindle muscle fiber of horse lumbrical muscle

Tubular aggregates (TAs) originate from the sarcoplasmic reticulum (SR) and form polymorphic double (or single) -walled structures in cross section. T As are involved in various human skeletal muscle disorders including period ic paralysis, congenital myasthenic syndromes, inflammatory myopathies, and malignant hyperthermias. Horse lumbrical muscle (LM) is a slender fusiform muscle that shows varying degrees of regression due to its limited activit y in the limb. Double-walled TAs were found in degenerating spindle fibers and with a range of 80-116 nm (average 92 nm, n=135) for outer layer and 50 -78 nm (average 59 nm, n=135) for the inner layer. TAs exhibit degradation of myofibrillar proteins, disruption of mitochondria with cristae lost, gly cogen accumulation, electron-dense metabolic products, blebbing appearance of sarcolemma, and presence of various vacuoles. LM fibers also show a simi larly degenerative state. The disassembly of the SR structure probably prod uces a large accumulation of SR proteins which remain as molecules without being further degraded and which could aggregate to form the orderly struct ure of TAs. We believe that TA formation may be an adaptation to store unba lanced extra proteins by forming ordered aggregates in degeneration caused by stress in cells.