a. Schematic of structural changes observed during WT NMJ development and aging. Note perforation of postsynaptic apparatus between birth and P30 and further postsynaptic fragmentation during aging. b. Postsynaptic structure (α-bungarotoxin only) of NMJs from sternomastoid of P14, P21, P30, and 3-, 14-, and 24- month old WT and ΔIg3-MuSK mice, c. merged images of pre- and post- synaptic apparatus of NMJs in (b). Postsynaptic apparatus (α-bungarotoxin, red); presynaptic (neurofilament with VAChT or synaptophysin; all green). Note increased fragmentation of postsynaptic elements and their persistent co-localization with presynaptic elements. d. Quantification of ΔIg3-MuSK NMJ fragmentation throughout the lifespan. (* p<0.05, ** p<0.01, *** p<0.001, generalized linear models, full statistics presented in Tables 2-1 and 2-2, including median and interquartile range for each measurement, n for each age and sex, tests used, and results of statistical testing.)