The fine structure of the spinal cord in human embryos and early fetuses.

The developing human spinal cord was studied by electron microscopy in 7 embryos (18--28 mm (C.-R.) and 5 fetuses (35--45 mm). It was found that, during the last week of the embryonic period, the spinal cord undergoes distinct maturation, as evidenced by cytoplasmic organelles, glial differentiation, and the formation of synaptic junctions. The ventricular (ependymal) cells are ciliated; densely grouped cells with scanty processes occupy the early intermediate (mantle) zone; and glioblasts are present in the marginal zone. Degenerating cells are evident in the spinal cord. The first axondendritic synapses appear in the marginal layer of the basal plate in the cervical part of the spinal cord in a human embryo of 28 mm, i.e., during the embryonic period proper. In the fetal period, synapses are found also in the alar plate, and axosomatic synapses are present. The formation of synapses and the general maturation of the spinal cord seem to proceed craniocaudally. The appearance of synapses is in keeping with the finding of reflex movements in the human during the late embryonic and early fetal periods.