THE SUBPLATE AS AN EARLY LANDMARK OF REGIONAL DIFFERENTIATION IN THE FRONTAL AND OCCIPITAL CORTEX OF HUMAN AND RHESUS MACAQUE BRAINS

During mid-gestation, the subplate (SP) is a transient, voluminous, and synapse-rich compartment of the developing primate cerebral cortex, critical for connectivity establishment and regional cortical differentiation. SP formation between 12–13 postconception weeks (PCW) arises from the spread-down of the deep cortical plate (CP) and exhibits early regional differences in size, extracellular matrix composition, fibre and projection neuron content, and contribution of late-arriving cells (Duque, et al, 2016). We hypothesised that the mode of SP formation reflects these regional differences, with frontal and occipital cortices diverging earlier than previously recognised.
Postmortem human fetal brains were sampled across key SP developmental stages and analysed using histology, immunohistochemistry, and immunofluorescence. Comparable prenatal Macaca mulatta brains were examined using similar molecular markers, with developmental stages and cortical regions matched for cross-species comparison.
We observed region-specific SP patterns: in the frontal cortex, a prominent double cortical plate (Kopić et al, 2023), incomplete delamination of the anterior cingulate cortex; in the occipital cortex, a sharply delineated SP–intermediate zone boundary and more advanced laminar organisation. Molecular markers further revealed regional differentiation of SP sublaminar composition, including extracellular matrix and fibrillar structures. These features are pronounced in associative cortices and areas receiving extensive thalamic, basal forebrain, and associative axons.
Our results demonstrate that SP regional differences serve as a robust landmark of early regional differentiation in primate cortex. Comparative analysis indicates both conserved and species-specific features, highlighting the SP’s central role in guiding cortical connectivity and establishing the foundations of higher-order associative networks in humans.