Synaptic differentiation : the role of agrin in the formation and maintenance of the neuromuscular junction

Upon arrival of a motor axon at the muscle fiber, signals released from its growth cone initiate the formation of a synapse. This process consists of two stages: arrest of axon growth at the target area and differentiation of pre- and postsynaptic cells at the site of nerve-muscle contact. Studies of regenerating neuromuscular junctions in vertebrates have revealed that important signals for the formation of this synapse are located in the synaptic basal lamina, and attempts to identify these signals have led to the isolation of agrin and other components. In this review, we discuss the evidence for the involvement of these molecules and their potential functional role in the formation and maintenance of the neuromuscular junction, with emphasis on agrin.