Distribution of GAP-43-immunoreactive structures in the human fetal amygdala.

The growth-associated protein GAP-43 is a developmentally regulated protein which is involved in the formation of neuronal contacts. In immunohistochemical studies, GAP-43 is detected within axons during their elongation; thus a fibrous immunoreactivity is visible. After axonal growth is completed there is a shift from a fibrous to a punctate immunoreactivity. The latter has been shown to correlate with synaptogenesis. In the amygdala of the 5th gestational month, a fibrous GAP-43-immunoreactivity is seen in the basolateral nuclei, whereas the corticomedial nuclei exclusively show a punctate immunoreactivity. In the 7th month, all amygdaloid nuclei display immunoreactive puncta, but no fibers. In the 9th month GAP-43-immunoreactivity is no longer visible within the amygdala. The results demonstrate the differential distribution of GAP-43-immunoreactive structures in the amygdaloid nuclei. The nuclear specific immunostaining and its changes may indicate the sequential appearance of the monoaminergic innervation of the amygdala, as GAP-43 is known to occur in monoaminergic systems. Nuclei involved in high levels of the cortical processing hierarchy such as the lateral or basal nucleus display a late occurrence of GAP-43-immunoreactivity. In general, anti-GAP-43 has been shown to be an appropriate tool to investigate axonal growth and synaptogenesis in the developing human brain.