Development of glomerular synaptic complexes and immunohistochemical differentiation in the superficial dorsal horn of the embryonic primate spinal cord

Development of glomerular synapses in the superficial dorsal horn has been studied in the embryonic macaque spinal cord using light and electron micro scopic techniques including Golgi impregnation, H-3-thymidine radioautograp hy and pre-embedding immunohistochemistry of substance P (SP), calcitonin g ene related peptide (CGRP), calbindin D-28 K (CB) and parvalbumin (PV). The study revealed that substantia gelatinosa cells of the primate dorsal horn are generated last, but unlike in rodents, synaptogenesis in this region s tarts at early embryonic (E) stages of the 165-day long gestation. Already by E30, both Type 1 (light) and 2 (dark) dorsal root axons and their growth cones are identifiable within the oval bundle of His, before they form syn aptic contact with their final target cells. Subsequently they invade the d orsal horn and enter the bisecting interfaces formed by orderly programmed cell death. Each type of scalloped (sinusoid) central primary afferent term inal (i.e. DSA, RSV and LDCV) have well defined pre- and post-synaptic spec ializations already by E40. Among the neuropeptides studied, SP appears fir st at E67 and CGRP at E70 in the lateral position but within a few days bot h of them are spread to the entire superficial dorsal horn. Both SP and CGR P are present in the thin dorsal root axons and their growth cones, giving rise to scalloped and simple axon terminals. PV is transiently present in t he entire length of the thick dorsal root afferents before becoming concent rated in the synaptic boutons. CB is displayed mainly in neurons of the lam ina I and III. Dendrites of CB-immunoreactive cells establish synaptic conn ection with each type of dorsal root afferents, including glomerular synapt ic complexes. These data reveal that the superficial dorsal horn in the pri mate spinal cord develops its characteristic synaptic complexes much earlie r in gestation than in any other mammalian species studied. Furthermore, ch aracteristic cytological features of the prospective glomerular complex eme rge before establishment of the final synaptic contacts.