Development of the rat superior cervical ganglion: ingrowth of preganglionic axons.

The growth of sympathetic preganglionic axons has been studied in the fetal rat. Preganglionic axons first emerge from the spinal cord and enter the paravertebral chain of ganglia between days 12 and 13 of gestation (E12 to E13). By E14, hundreds of preganglionic neurons have sent axons into the sympathetic chain. Even at this age, the arrangement of the preganglionic axons conforms to the adult organization in a number of ways. As in maturity, these axons leave the spinal cord only through the ventral root of the segment in which their cell bodies reside. Furthermore, axons grow into the superior cervical ganglion only from the same set of spinal segments as supply this ganglion in maturity, and in correct segmental proportions, although there is a slight rostral-to-caudal lag along the spinal cord in the full establishment of the projection. The development of appropriate projections in this system owes little to the growth of axons along stereotyped pathways. Rather, preganglionic organization apparently reflects a more general instruction that governs the relative number of axons from each spinal segment which course rostrally or caudally within the sympathetic chain. The appropriate distribution of preganglionic axons early in development suggests that a fiber's segmental level of origin biases the axon to grow in a particular direction.