Cholinergic innervation of the ventricular myocardium in the human fetal heart

Specimens of human fetal hearts from the 9th to the 27th week of intrauterine life were processed by the technique of Karnowski and Roots as modified by Lewis for the demonstration of acetylcholinesterase-positive nerve fibers by light microscopy. Cholinergic innervation was present in both atria and ventricles even in the 9th week of intrauterine life. In the ventricles the number of cholinergic fibers augmented with increasing age; they spread in the ventricular wall surrounding the vascular network including capillaries, or following their course in the close proximity of cardiac muscle fibers. Electron microscopy of heart specimens in the 27th week of intrauterine life showed that ventricular cholinergic nerve terminals were present in the subendothelial space of arteries; other cholinergic fibers were in a close apposition with ventricular myocardial cells with an intervening space of about 300 nm. These data provided evidence that the development of cholinergic innervation in both atria and ventricles of human heart began early during the intrauterine life.