Growth and refinement of excitatory synapses in the human auditory cortex

Abstract

We had earlier demonstrated a neurofilament-rich plexus of axons in the presumptive human auditory cortex during fetal development which became adult-like during infancy. To elucidate the origin of these axons, we studied the expression of the vesicular glutamate transporters (VGLUT) 1 and 2 in the human auditory cortex at different stages of development. While VGLUT-1 expression predominates in intrinsic and cortico-cortical synapses, VGLUT-2 expression predominates in thalamocortical synapses. Levels of VGLUT-2 mRNA were higher in the auditory cortex before birth compared to postnatal development. In contrast, levels of VGLUT-1 mRNA were low before birth and increased during postnatal development to peak during childhood and then began to decrease in adolescence. Both VGLUT-1 and VGLUT-2 proteins were present in the human auditory cortex as early as 15GW. Further, immunohistochemistry revealed that the supra- and infragranular layers were more immunoreactive for VGLUT-1 compared to that in Layer IV at 34GW and this pattern was maintained until adulthood. As for VGLUT-1 mRNA, VGLUT-1 synapses increased in density between prenatal development and childhood in the human auditory cortex after which they appeared to undergo attrition or pruning. The adult pattern of VGLUT-2 immunoreactivity (a dense band of VGLUT-2-positive terminals in Layer IV) also began to appear in the presumptive Heschl's gyrus at 34GW. The density of VGLUT-2-positive puncta in Layer IV increased between prenatal development and adolescence, followed by a decrease in adulthood, suggesting that thalamic axons which innervate the human auditory cortex undergo pruning comparatively late in development.

Structural Covariance of the Duplicated Heschl’s Gyrus: A Sulcal/Gyral Template Morphology Approach

“…This qualitatively unique variability includes a completely duplicated posterior gyrus that emerges from the circular sulcus immediately behind the anterior HG and runs parallel in an anterolateral direction with the anterior HG. This morphology, which is present in gestation (Pundir et al, 2016), has generated interest because it is more commonly observed among people with right-handedness (Marie et al, 2015), musical training (Benner et al, 2017), dyslexia (Leonard et al, 2001), and schizophrenia (Takahashi et al, 2021). Thus, there are questions about the structural and functional significance of duplicated HG.…”