Structural development of myoneural junctions in the human embryo

Summary The structure of myoneural junctions in the tibialis anterior and intercostal muscles was studied after histochemical reaction for myoneural acetylcholinesterase (E.C. 3.1.1.7) in human embryos.Myoneural junctions of a primitive form were first seen in the intercostal muscle at the age of 8.6 weeks (crown-rump length of 3.2 cm), and in the tibialis anterior muscle at the age of 10 weeks (4.3 cm). The postsynaptic membrane was devoid of any junctional folds typical of adult synapses up to the age of about 19 weeks. At first small, the junctional folds gradually became deeper and more prominent during the following weeks, and some ramification of the previously coherent postsynaptic area took place. Myoneural morphogenesis was not completed at birth, although well developed postsynaptic Moldings were present.