Development of rat neuromuscular junctions in organ culture

Pieces of thoracic body wall, including intercostal muscles, ribs, and the spinal cord were explanted from 15 to 18-day embryonic rats and maintained in organ culture for up to 6 days. During the time in culture muscle fibers continued to increase in size, and nerve sprouts extended along the center of the muscle. When muscle-spinal cord explants were cultured at 15 days gestation, the number of synaptic inputs per fiber increased with time in culture. Subsequently synapse elimination began with a time course similar to that recorded in vivo. In 15-day explants acetylcholine receptors were uniformly distributed along the fibers and focal cholinesterase (ChE) was not detected. The cholinergic receptors started to cluster at the midregion of the fibers after 1 day explantation, and ChE was detected in the fibers after 2 days in culture. The central receptor clusters were associated with ChE and their formation was dependent on the presence of nerve terminals. We conclude that neuromuscular contacts develop in organ culture with a pattern and time course similar to that of synapes developing in utero.