Morphological comparison by scanning electron microscopy of transient retina-muscle synapses and long-lived spinal cord-muscle synapses.

Retinal neurons will form cholinergic synapses on muscle cells in tissue culture, however, these inappropriate synapses are transient. Although within 24 hrs. myotubes are readily innervated, by 7 days of coculture no physiological response can be detected. In contrast, spinal cord neurons form long-lived synapses on muscle cells which can be detected for several weeks. In this study, we examined retina-muscle and spinal cord-muscle cocultures to determine whether retinal neurons withdrew their processes from muscle cells, and preferentially maintained contacts on other retinal neurons. Using scanning electron microscopy, we observed that during the first 24 hrs. of coculture both types of neurons extend processes along myotubes. The neurite outgrowth occurs concomitantly with the observed increase in physiologically detected synaptic muscle responses. Over the next 6 days, retinal neurite contacts on the muscle cells are withdrawn, while spinal cord contacts remain. Retinal neurons remain in contact with other retinal neurons at day 7 of coculture. Thus, retinal neurons form and maintain synapses on their appropriate target (other retinal neurons); whereas, they withdraw contacts from inappropriate targets (muscle cells). By contrasting retinal and spinal cord synapses on muscle cells, we hope to elucidate the mechanisms of synaptic recognition and maintenance.