Application of morphometric analysis to the study of neuromuscular junction is helpful to the quantitative approach of ultrastructural changes. In this study, twelve autopsied subjects of different ages were examined on the ultra‐structure of neuromuscular junctions using morphometric analysis. The postsynaptic area and postsynaptic membrane length were significantly greater in adults than in infants. The results indicate that the number and/or depth of the secondary synaptic clefts of adults are greater than those of infants. In the aged subjects, presynaptic membrane length and postsynaptic membrane density showed a significant decrease. The latter observations were thought to be the results of regressive changes of pre‐ and postsynaptic structure with aging. The variations of each structural element were also studied. The large variations were found in the postsynaptic area, postsynaptic membrane length, and membrane length ratio of adults and aged group. On the other hand, variations in postsynaptic membrane density were small in all age groups. The importance of the knowledge of these age related changes and variations in the studies of neuromuscular junction was discussed. ACTA PATHOL. JPN. 34: 1243–1249.
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