Immunohistochemical studies on the spinal dorsal horn of the turtle Chrysemys d'orbigny

Immunohistochemical methods were used to characterize some of the systems of nerve fibers occurring in the spinal dorsal horns of the turtle Chrysemys d'orbigny. Substance P (SP), calcitonin gene-related peptide (CGRP) and leuenkephalin (Enk) immunoreactive fibers were found concentrated in the superficial horn region, termed here synaptic field Ia. From this zone the immunoreactive fibers project to deeper dorsal horn regions. Comparison with histological images obtained after HRP labeling of dorsal root axons indicates that SP-, CGRP- and Enk-immunoreactive fibers are small-diameter primary sensory fibers entering the cord via synaptic field Ia. It is posulated here that these three substances may coexist in the same fibers. Enk-positive fibers also occur randomly scattered in the lateral funiculi, showing a conspicuous increase in density at the perimedullary plexus level. Tyrosine hydroxylase (TH) immunoreactive fibers were found in the more compact dorsal horn neuropil (synaptic field II) and also forming bilateral conspicuous bundles in the lateral funiculi. TH-immunoreactive cell bodies were found in the epithelium lining the central canal. Taking into account data derived from Golgi impregnated material it is proposed that they represent epithelial cells undergoing neural differentiation.