The development of descending and dorsal root connections in the lumbosacral spinal cord of the postnatal rat.

Light and electron microscopy were used to study degenerating axons and synaptic endings in the lumbosacral spinal cord at different stages of postnatal development in the rat. After dorsal root section (L5-S1) or mid-thoracic hemisection of the spinal cord (T6-T8), the distribution and type of degeneration argyrophilia were studied with the light microscope over varied post-operative survival periods using a modified Fink-Heimer method (Leonard, '74). The light microscopic results were correlated with the appearance of electron dense degeneration seen using the electron microscope. The distribution of degeneration found in the lumbosacral spinal cord after dorsal root section was similar at the neonatal (4d), weanling (21d) and adult stages. At the neonatal stage, the type of degeneration argyrophilia appeared at an earlier postoperative survival period (12 hours), and was maximal at an earlier time (12-18 hours) than in older animals. The appearance of electron dense degeneration using the electron microscope matched the light microscopic findings. Electron dense degenerating synaptic knobs were found at the neonatal stage making asymmetrical contact with small dendrites in areas of the dorsal horn showing degeneration argyrophilia. No degeneration was found in the dorsal funiculus, dorsal horn or intermediate nucleus of Cajal (INC) after mid-thoracic hemisection at the neonatal stage although degeneration was present elsewhere in the intermediate gray, in the ventral gray, and in the ventral and lateral funiculi. At this stage, the type of degeneration argyrophilia and time course of degeneration in the gray and white matter was identical to that found after dorsal root section in neonatal animals. The distribution of degeneration was not mature in the lumbosacral enlargement until 15d of age and the degeneration was still immature in the dorsal horn and INC at 21d of age. Although degeneration argyrophilia and electron dense degeneration was found in the ventromedial gray (lamina VIII) and lateral gray (laminae VI-VII) at the neonatal stage, electron dense degenerating synaptic knobs were not observed in these areas until 12d of age for the ventromedial and 18d of age for the lateral gray matter even using a large number of different survival periods. Degenerating synaptic endings were first observed after mid-thoracic hemisection during the period when behavioral maturation is nearing completion in the hindlimbs, and when response depression (spinal shock) increases and response recovery decreases after mid-thoracic spinal transection (Weber and Stelzner, '77). Our data suggest that either the number of connections from descending sources increases markedly in the gray matter of the lumbosacral spinal cord during this period or connections, already present, reach a state of maturity where they can first be observed using the degeneration method of analysis.