Ultrastructure of developing substantia nigra in humans

Abstract

Electron microscopy of the maturing neurons and developing and maturing synapses in the substantia nigra of 14 human embryos/foetuses of 8–24weeks of gestation are reported. At 8weeks, cells were immature with very little cytoplasm and cellular organelles. Contact sites of processes appeared more electron dense than the other areas. At 12weeks, many of the cells had acquired more cytoplasm and cellular organelles and could be identified as neurons. Asymmetric synapses with clear, round synaptic vesicles also were identifiable at this age. Such synapses, first to appear in the developing substantia nigra, are reported to be formed by recurrent collateral nigro-striatal fibres. Substance P fibres from the striatum also are contributing to this type of synapse. At 15–16weeks, not only was the number of such synapses increased, but many appeared morphologically mature. Symmetric synapses having clear round vesicles along with a few dense core vesicles also appeared at this stage, suggesting striatal input. By 24weeks of gestation, most of the neurons had cytological features comparable to that of the mature neurons. There was an increase in the total number of synapses and the individual variety from 15 to 24weeks of gestation. The present study indicates that synaptogenesis starts at 8weeks and continues beyond 24weeks of gestation.