Abstract
Embryonic and adult muscle express distinct complements of proteins. Transplant experiments have shown that embryonic and denervated muscles can form synapses with foreign neurites, while normal innervated mature muscles cannot, except at the synaptic area. To identify molecular differences between ''innervatable'' embryonic muscle and ''non-innervatable'' mature muscle, we used a differential immunization method to make monoclonal antibodies that recognize antigens present in embryonic muscle rather than normal mature muscle. Four independent monoclonal antibodies that stain embryonic and mature muscle sections differentially have been obtained. One stains the entire embryonic muscle cell surface but only the synaptic area in mature muscle; 3 stain the entire embryonic muscle cell surface but do not stain mature muscle. The antibodies also stain other embryonic tissues at several stages. The antigens are concentrated in basal laminae and in an extracellular-matrix (ECM) fraction, indicating that they are ECM molecules. The temporal expression of all 4 antigens in developing muscle is coincident with muscle innervation. All antigens exhibit temporal and tissue distributions different from those of any reported muscle proteins, suggesting that novel antigens underlie these patterns. These results confirm that the immature muscle ECM has a distinct molecular composition, and suggest that the presence of these antigens may be part of the molecular basis for the innervatability of embryonic muscle.
