Ultrastructure of motor endplates in canine congenital myasthenia gravis.

Morphometric analysis was carried out on electron micrographs of motor endplates from three Jack Russell terriers affected with congenital myasthenia gravis (CMG) aged 10 weeks, 12 weeks and 22 weeks, respectively. Control endplates from age-matched pups and an adult Jack Russell were also examined. The results showed that postsynaptic membrane density was significantly increased in affected animals and secondary fold length was decreased. The ratio of postsynaptic to presynaptic membrane length was normal in the 10 and 12-week-old pups, but reduced in the 22-week-old CMG animal. These changes were unrelated to muscle fibre diameter and there was no evidence of a destructive process. It is suggested that the alteration in membrane folding pattern in this condition may be related to abnormal trophic influences during synaptogenesis.