Ultrastructural study of muscle fibers and neuromuscular junctions in the opossum thyroarytenoid muscle

ABSTRACT

Some muscle groups of mammals, such as the laryngeal muscle, present tonic muscle fibers among fast twitch muscle fibers. The latter are supplied by single en plaque neuromuscular junctions (NMJs) and the former by multiple NMJs. The aim of the present study was to characterize the muscle fiber types and their NMJs in the thyroarytenoid (TA) muscle of the opossum. Five adult opossums (Didelphis albiventris) were anesthetized and perfused with Karnovsky solution. The TA muscles of the right side were processed for TEM. The contralateral TA muscles were submitted to connective tissue digestion with HCl before scanning electron microscopy processing. Based on myofibril morphology, the number and arrangement of mitochondria, sarcoplasmic reticulum and T tubule profiles and Z-line width, three fast twitch muscle fiber types were identified. Tonic fibers characterized by small and compact myofibrils were also found. Although tonic muscle fibers were present, only single NMJs were observed. In these NMJs the axon terminals occupy the synaptic clefts, which have variable depths. The sarcolemmal folds were not homogeneously arranged along the NMJ cleft. The Schwann cell bodies and their cytoplasmic projections were covering the axon terminals. Scanning electron microscopy analysis revealed empty synaptic clefts with irregular distribution of junctional folds. At some NMJs, the axon terminals were not removed and were present, filling up the synaptic cleft. The presence of only the en plaque NMJ type is discussed in view of the functions performed by the opossum TA muscle. Moreover, we demonstrate the similarity in NMJ distribution between the opossum TA muscle and those of rats and humans, with the opossum thus representing another useful experimental animal model for studies regarding intrinsic laryngeal muscles.