Scanning electron microscopy of the development of layer I of the human visual cortex (area 17)

The development of layer I of the human visual cortex was studied. Tissues were obtained from 14 aborted and stillborn human fetuses, ranging in age from 13 to 32 fetal weeks. The middle third of the rostrocaudal area 17 above the left calcarine sulcus was selected for observation. The sections were cut perpendicular to the pial surface of the cortex. The specimens were processed for scanning electron microscopy. At 13-16 weeks the Cajal-Retzius neurons were the predominant neuronal elements of layer I, with a small amount of fibers present. In addition, it was found that at 13 weeks the folding plasma membranes of the glial cells formed primary myelin sheaths which wrapped around the nerve cell bodies and processes. Axosomatic and axodendritic contacts were present in the cortices of the fetuses. The nerve fibers of layer I increased progressively with age. However, at 17 weeks, some degenerating apical dendrites were occasionally found. These degenerating dendrites appeared fragmented and beaded. At 21-23 weeks, layer I was well developed with all its essential components present. However, the upper half of the cortical plate was still undifferentiated at this age. At 26-32 weeks, layer I had a prominent plexiform lamina. This lamina was poor in neurons and rich in fibers. In summary, during layer I ontogenesis, there were the following major changes: (1) a change from higher to lower neuronal density: (2) changes in morphology of neurons; (3) initial appearance of synapses early in development; (4) early onset of glial wrapping; (5) presence of degenerating fibers during ontogenesis, and (6) increase in fiber proportion as fetus aged.