Abstract
Both acetyl- and pseudocholinesterases have been detected in the atrioventricular specialized tissue of the human fetus. The upper part of the node, derived from sinus horn musculature, is cholinesterase-negative but contains acetylcholinesterase-positive nerves. The lower portion of the node and the common trunk of the bundle differentiate from atrioventricular canal muscle, are intrinsically pseudocholinesterase-positive and do not contain nerves up to the 190-mm crown-rump length stage. These observations are compared with those of previous workers who believed the specialized tissue of adult specimens of several species to contain only acetylcholinesterase. The possibility of a developmental change in the type of cholinesterase in the specialized tissue is discussed in relation to a recent study of the atrioventricular node of the neonatal rat.