Morphological aspects of formation of neuronal pathways in the chick spinal cord--Golgi and electron microscopic studies.

The early formation of neuronal connections in the cervical cord of chick embryos at stages 17 to 31 was studied by observing axonal courses with Golgi preparations and the distribution of synapses with electron microscopy. The results are summarized as follow: 1. Synaptic contacts between spinal interneurons and ipsilateral as well as contralateral motor neurons first develop at stage 22. The early central pathways from the dorsal root to the ventral root may be formed at stage 25 with intervention of interneurons of the primordial dorsal horn, which is composed of neurons of the zona spongiosa and the nucleus proprius of the dorsal horn, and at stage 27 with the intervention of interneurons of the zona intermedia, the nucleus proprius of the ventral horn and the primordial dorsal horn as well. 2. These polysynaptic, bilateral central pathways appear to be established just before the arrival of supraspinal descending fibres at the cervical cord, and one or two days before the formation of ipsilateral monosynaptic spinal reflex arch. 3. These early spinal central pathways are connected by synapses with spherical synaptic vesicles, and almost all of these synapses are of the axo-dendritic type and located in the spinal white matter.