Development of the human cervical spinal cord with reference to synapse formation in the motor nucleus.

Synaptic development in the motor neuropil of the cervical spinal cord was quantitatively studied by light and electron microscopy in human embryos and fetuses ranging from five to 19 weeks of ovulation age. The numbers of axodendritic synapses increase substantially at the end of the eighth week. However, axosomatic cynapses rapidly proliferate between 10.5 and 13 weeks of ovulation age. The increases in both types of synapses generally coincide with the behavioral changes in human fetuses that have been reported by other investigators. Only synaptic boutons containing spherical vesicles (S-type synapses) were found in the motor neuropil throughout the stages examined; no synapses with flattened vesicles (F-type synapses) were encountered. The majority of these synaptic boutons contain only a small number of synaptic vesicles (fewer than 20), although the number tends to increase with maturation. There is no significant maturational change in the size of synaptic vesicles. The present study suggests that synapse formation in the motor neuropil of the human fetus cervical spinal cord may continue up to 19 weeks of ovulation age because immature types of synapse are found in all fetuses. Myelin formation probably begins by the 11th week.