Extensive nerve overgrowth and paucity of the tailed asymmetric form (16 S) of acetylcholinesterase in the developing skeletal neuromuscular system of the dysgenic (mdg/mdg) mouse.

In muscular dysgenesis (mdg) both skeletal muscle and motor innervation present an abnormal differentiation at birth (Rieger and Pinçon-Raymond. 1981). We establish in this report that the abnormalities of the motor innervation, extension of the innervation territory and nerve branching, and the low level of tailed asymmetric (16 S) acetylcholinesterase (AChE) are observed in the diaphragm as soon as the mutation is phenotypically expressed in the mdg/mdg embryo, at embryonic Day 13 1/2 or 14. The nerve-muscle interaction is thus abnormal at the very beginning of the formation of the neuromuscular system. The comparative electron microscopic study of normal and mutant muscle basal lamina shows that the newly formed mdg/mdg myotube lacks a basal lamina at Day 14. However, the biosynthesis of the basal lamina is not totally defective as, at Day 18, mdg/mdg myotubes possess basal lamina sheaths. Such delayed formation of extracellular material in the mutant may partly explain the abnormally low induction of 16 S AChE which is thought to be localized in the extracellular matrix and has potential important implications in nerve-muscle recognition and synapse stabilization.