Onset of synapse formation in the human spinal cord

The time of onset of synapse formation was examined in the cervical spinal cord of eight human embryos ranging from 4 (8 mm in crownrump length) to 6 (22 mm) weeks of ovulation age. The first synapses were found in the motor nucleus of a 10‐mm embryo (Streeter's horizon XV). They contained only a small number of synaptic vesicles and had asymmetrical membrane thickenings, with thicker postsynaptic membranes than presynaptic ones. With the exception of an occasional axosomatic synapse, nearly all synapses found in a 10‐mm embryo were axodendritic. In an 11‐mm embryo (Streeter's horizon XVI), synapselike contacts (SLC) were found in the dorsal marginal layer. They contained a small number of synaptic vesicles and had “reversed membrane specalization” with prominent presynaptic membrane densities. The number of SLC decreased in older embryos. Judging from the morphological characteristics of membrane specializations and the temporal decrease in number, SLC are considered to be either transient synapses on the borderline cells or axoglial synapses. Synapses in the regions outside the motor nucleus first appeared in the dorsal marginal layer of a 14‐mm embryo (Streeter's horizon XVII).