A morphological investigation of an early reflex pathway in developing rat spinal cord

Abstract The early development of the disynaptic cutaneous reflex pathway in the brachial spinal cord of rat fetuses was investigated both light and electron microscopically. The spinal cord areas which contain the neurons (association interneurons and lateral motor nucleus neurons) involved in this reflex pathway were identified in both neurofibrillar (silver‐stained) and semithin plastic embedded specimens. Equivalent areas were identified in adjacent ultrathin preparations for electron microscopy and micrographs of these sample areas were taken of specimens at embryonic days 13.5–19.5 as well as for postnatal day 4 and adult specimens. The relative volume of association and dorsolateral motor neuropils occupied by synaptic boutons in these micrographs was determined using stereological methods. The mean number of synaptic junctions and boutons per 100 μ 2 was also determined for the two neuropil areas at each developmental stage. These analyses show that synaptic junctions and boutons occur in the dorsolateral motor neuropil before they can be found in association neuropil. Synapses within motor neuropil are first seen at embryonic day 13.5. Synaptogenesis in the dorsolateral motor neuropil is temporally correlated with the presence of numerous fibers which, in silver‐stained specimens, can be seen to arise from the association interneurons and to course into the dorsolateral aspects of the motor nucleus. Synaptic junctions and boutons are not observed in the association neuropil until embryonic day 14.5 and this correlates well with the time when collaterals of the primary afferent fibers can first be observed penetrating into the dorsal half of the intermediate zone from the anlage of the dorsal funiculus. The precocious development of synaptic junctions in dorsolateral motor neuropil in comparison to the association neuropil continues throughout the developmental period examined. These data support a retrograde pattern of synaptogenesis in the early development of the spinal cutaneous reflex pathway. Association interneurons make synaptic contacts with motor neurons before synapses are formed between primary sensory neurons and association cells. This sequence is the reverse of the normal flow of nerve impulses through spinal reflex pathways.