Prenatal and postnatal development of lamina IX neurons in the rat thoracic spinal cord.

The maturation of lamina IX neurons in the thoracic spinal cord of the rat was studied from day 13 of gestation until adulthood using a modified Golgi-Cox stain. The large multipolar neurons developed in an orderly and progressive manner. At day G-13, the round or stellate perikarya were smooth. Primary and secondary dendrites were adult in number with numerous irregular expansions and growth cones. Tertiary dendrites were beginning to form. Between day G-18 and postnatal day 5 the irregular surface of the perikarya and all portions of the dendrites had numerous long spines and filopodia. Growth cones were present at the end of terminal dendrites and also along the shafts of secondary and tertiary dendrites. By postnatal day 11 there was a decrease in the number of spines on the perikarya and proximal dendrites but spines remained prominent on secondary and tertiary dendrites. Between postnatal days 18 and 31 the adult morphology was attained. The perikarya and dendrites of adult neurons were devoid of spines except for occasional spines on the distal half of the dendritic tree. Elongated smooth varcosities were present except for the distal half of terminal dendrites where these expansions were more beaded. Although these morphologic changes were, in certain respects, the reciprocal of the regressive changes seen in these neurons after spinal hemisection in the adult rat (3), a major difference was the apparent absence of a "spiny stage" in the denervated lamina IX neurons of the adult.