Subplate neuron dendritic morphology during development in gyrencephalic and lissencephalic brains.

Subplate neurons (SPNs) are among the earliest-born and most mature neurons in the developing cortex and are critical for thalamocortical axon pathfinding, radial migration, and early thalamocortical circuit formation. SPNs have been proposed to drive cortical folding, but their specific morphological features are not fully understood. This study examined SPN dendritic arborisation in two gyrencephalic species (sheep and ferret) before, during, and after the onset of primary folding, as well as at developmentally equivalent stages in the lissencephalic spiny mouse. Golgi-stained brains were analysed using Sholl analysis, along with additional metrics including number of primary branches, maximum intersections, path length, and skewness (branching asymmetry). Qualitative observations revealed region-specific differences in dendritic orientation and cell-type distribution between gyri and sulci in the gyrencephalic species, which were not found in the lissencephalic spiny mouse. MANOVA and two-way ANOVA with Bonferroni post-hoc tests showed no overall difference in SPNs arborisation throughout development in any species. Similarly, no differences were identified between gyral and sulcal SPNs in the ferret throughout gyrification, except for skewness in the sheep following the emergence of primary folds, where sulcal SPNs showed less branching away from the soma. These findings provide a valuable reference for future studies investigating SPNs and early cortical circuit formation in both gyrencephalic and lissencephalic brains. Further studies using advanced imaging and analytical methods may uncover more subtle region-specific differences in dendritic architecture and neuronal orientation.