Figure 1. Shape and internal structure of the parasubiculum. A, Left, Tangential section stained for acetylcholinesterase activity (dark precipitate). The shape of the parasubiculum is outlined (white dashed line) coinciding with high acetylcholinesterase activity. Right, Tangential section (same section as in A, left) processed for calbindin immunoreactivity (green); the shape of the parasubiculum is negatively outlined by an absence of calbindin immunoreactivity. B, Schematic of the parasubiculum (light blue) and adjacent MEC subdivisions. C, Quantification of parasubiculum size in 10 hemispheres: length, width, and area. D, Tangential sections stained for PV immunoreactivity (green, left) and NeuN immunoreactivity (red, right). The parasubiculum stands out by its intense staining. Three sections are shown: left, most superficial (closest to the pia); middle and right, progressively deeper. Note how the patchy structure of the superficial parasubiculum is replaced by a continuous cell band in deeper sections. E, Tangential sections of the parasubiculum showing the injection site of BDA tracer (red fluorescence) and anterogradely traced circumcurrent axons (according to the terminology of Burgalossi et al., 2011), extending throughout the parasubiculum (see also magnified inset, left). *Injection site. F, Parasagittal sections of the parasubiculum (top) and parasubiculum and MEC (bottom) after the injection of larger amounts of BDA (tracer, dark color). The tracer completely fills the parasubiculum and stains layer 2 of the MEC. *Injection site. A, B, Modified from Ray et al. (2014). D, Dorsal; L, lateral; M, medial; V, ventral; A, anterior; P, posterior.