Synaptogenesis in the cervical cord of the human embryo: sequence of synapse formation in a spinal reflex pathway

Synaptogenesis in the cervical cord was studied by light and electron microscopy in human embryos ranging from four to seven weeks of ovulation age. The stage of embryonic development was estimated on the basis of external morphology of embryos and histology of the eye ball with reference to Streeter's horizon. No synapses were found in the cervical cord of the embryo at Streeter's horizon XIV (8 mm; estimated ovulation age, 28-30 days). A small number of axodendritic synapses appear in the motor neuropil of the cervical cord at Streeter's horizon XVII (14 mm; estimated ovulation age, 34-36 days). Since no primary afferents are demonstrated to reach the motor neuropil at this stage (the premotile period), these synapses are considered to be formed between interneurons and motor neurons. On the other hand, the formation of synapses outside the motor neuropil of the cervical cord was recognized at Streeter's horizon XX (22 mm; estimated ovulation age, 40-42 days), which corresponded to the period of onset of the precocious reflex, but not by horizon XIX (18 mm; estimated ovulation age, 38-40 days). The first axosomatic synapses were found in the motor neuropil at Streeter's horizon XVII (estimated ovulation age, 34-36 days). The present study suggests that the formation of synapses between interneurons and dendrites of spinal motor neurons precedes that of synapses between interneurons and collaterals of primary afferents. This sequence of synaptogenesis is in agreement with that reported in earlier studies with silver stain methods.