Ultrastructural sublaminar-specific diversity of excitatory synaptic boutons in layer 1 of the adult human temporal lobe neocortex

Layer (L)1, beside receiving massive cortico-cortical, commissural and associational projections, is the termination zone of tufted dendrites of pyramidal neurons and the area of Ca²⁺spike initiation. However, its synaptic organization in humans is not known. Quantitative 3D-models of synaptic boutons (SBs) in L1 of the human temporal lobe neocortex were generated from non-epileptic neocortical biopsy tissue using transmission electron microscopy, 3D-volume reconstructions and EM tomography. Particularly, the size of active zones (AZs) and the readily releasable, recycling and resting pool of synaptic vesicles (SVs) were quantified.

SBs had a single large AZ (∼0.20 µm²), a total pool of ∼3500 SVs, a large readily releasable (∼4 SVs), recycling (∼470 SVs) and resting (∼2900 SVs) pool. Astrocytic coverage suggests cross talk at synaptic complexes.

Thus, L1 SBs mediate, integrate and synchronize contextual and cross-modal information, enabling flexible and state-dependent processing of feedforward sensory inputs from other layers of the cortical column.