Appearance and distribution "in situ" of nicotinic acetylcholine receptors in cervical myotomes of young chick embryos. Radioautographic studies by light and electron microscopy.

Localization of the acetylcholine (nicotinic) receptor sites was investigated in the developing cervical myotomes of the early chick embryo by radioautography at the light and electron microscope level, using 125I-alpha-bungarotoxin. The presence of cholinergic receptor sites was detected in situ as early as 60 hours of incubation (stage 17); their relative density increased in the myotome during the differentiation of the somite. Specific labeling of these receptor sites was detected in the myotomal tissue but not in the notochord, spinal cord or periaxial mesenchyme. The distribution of the receptor sites was uniform in the myotome at 3 days in ovo. An anterior-posterior asymmetry of the density appeared at 4 days in ovo and developed up to the 6th day. The highest density of these toxin-binding receptor sites was observed near the spinal motor nerve bundle as revealed by silver staining. These observations, made in situ, are discussed with respect to the possible neurotrophic or physical effects of the early motor innervation.