Quantitative and neurogenic analysis of neurons with supraspinal projections in the superficial dorsal horn of the rat lumbar spinal cord.

Dual retrograde axonal tracers, Fluoro-Gold (FG) and true blue (TB), were used in conjunction with [3H]thymidine autoradiography to determine the number and neurogenic pattern of neurons with supraspinal projections in the superficial dorsal horn (SDH), i.e., laminae I and II, in spinal segment L1 of the rat. FG was injected into rostral brain centers (dorsal thalamus and midbrain), and TB was injected into the caudal brainstem (medulla) in young adult rats previously administered [3H]thymidine in utero. Following stereological correction, each dorsal horn had an average of 1.22 neurons in lamina I and 0.24 neurons in lamina II that had supraspinal projections per 10-microm transverse section. In the SDH, 52% of the neurons with supraspinal projections were found to project to rostral brain centers alone, 3.0% only to the caudal brainstem, and 45% to both areas. There was no significant difference in the percentage distribution of each of the three groups of neurons between lamina I and lamina II. Cell counts in the present study, in conjunction with previous observations in the literature, suggest that the majority of supraspinal projection neurons in the SDH fall into two groups: 1) spinomesencephalic neurons with collaterals to the medulla and 2) spinothalamic neurons with collaterals to the midbrain. The neurogenesis of supraspinal projection neurons in the SDH proceeded along an axon-length gradient, whereby neurons with the longest axons, those with projections to rostral brain centers, completed neurogenesis prior to neurons with shorter axons, those with projections only to the caudal brainstem. The generation of all SDH neurons with supraspinal projections was completed on embryonic day 14 (E14), 2 days prior to the completion of neurogenesis for SDH neurons with intraspinal projections.