Development of the rat superior cervical ganglion: initial stages of synapse formation.

Synapse formation in the rat superior cervical ganglion has been investigated electrophysiologically and at the ultrastructural level. Preganglionic axons first enter the superior cervical ganglion between days 12 and 13 of gestation (E12 to E13), and on E13 a postganglionic response can be evoked by preganglionic stimulation. The susceptibility of this response to fatigue and to blocking agents indicates that it is mediated by cholinergic synapses. On E14, the overall strength of ganglionic innervation arising from different spinal segments already varies in a pattern resembling that found in maturity. However, the distribution of synapses on individual target cells gradually changes in the prenatal period. Transmission begins prior to the elaboration of ganglion cell dendrites; therefore, the first contacts to form are largely axosomatic. As dendrites appear (beginning on E14), ultrastructural evidence shows that synaptogenesis becomes focused upon these processes. The significance of these findings is discussed in relation to possible mechanisms for the formation of appropriate synaptic connections.