Motor Endplates in Developing Chick Embryo Skeletal Muscle

Abstract

A developmental history of the motor endplate in the chick embryo skeletal muscle has been studied with SUZUKI's silver impregnation method and the thiolacetic acid method for specific cholinesterase activity. The neuro-muscular junction occurs initially in a 12-13 day development; evidence is presented that SCHWANN cells which encapsulate the axon, contact with the sarcoplasm, leaving the axonal end distant from the junctional surface where intense cholinesterase activity is demonstrated. In a 15 day development the subjunctional sarcoplasm shows a gradual eminence and fundamental nuclei start to divide amitotically. In a 17-18 day development the axonal terminal swelling shows a marked increase in size. The subneural apparatus is depressed by an exerting pressure of the terminal swelling. From 19 days in ovo to 1 day ex ovo unstained contents of the terminal swelling disappears, leaving the capsular neurofibrillar network; the network forms a large loop on the muscle fiber. The axon continues to grow and turns along the loop. In a course of ex ovo development subterminal axonal sproutings frequently occur. The subneural apparatus also develops in association with a growing pattern of nervous elements.