Abstract
Acetylcholinesterase (AChE) activity has been studied in the myotube and in extrajunctional skeletal muscle of the rabbit fetus. Some observations on the developing motor end plate are included. Both insoluble and soluble AChE's are present at all stages, but butyrylcholinesterase seems to be absent. Enzymic activity is high during myotube formation. The insoluble AChE probably corresponds to the cytochemical end product which is bound to the elements of the reticulum in the myotube, i.e., the nuclear envelope, the sarcoplasmic reticulum and some elements of the Golgi complex. The possibility that the enzyme-containing reticulum is involved in the spontaneous contraction of early myotubes is discussed. The number of active sites of reticulum-bound enzyme decreases markedly as the myotubes mature into early muscle. The soluble AChE is probably derived from the AChE-containing mononuclear cell and the Schwann cell which accompanies the spinal nerve to the motor end plate. These cells have a random distribution of end product and are probably the source of a similar end product found at the surfaces of these cells and at the sarcolemma of adjacent myotubes. The following possibilities that are discussed are that soluble AChE may (a) play a role in fusion of myotubes, (b) be involved in widespread sarcolemmal acetylcholine sensitivity and (c) contribute to the junctional enzyme. The randomly distributed end product in the AChE-containing mononuclear cell permits the tracing of its further differentiation. This cell appears to be a stem cell, which can fuse directly with the myotube; it can give rise to a more differentiated type of myoblast, as well as to the muscle satellite cell. AChE-containing mononuclear cells may also contribute to the cells associated with the neuromuscular junction. The soluble AChE in the muscle satellite cell seems to be associated with an "active" phase of this cell, since enzymic activity is not seen in the adult stage in which it is presumed to be dormant.