Retrosplenial cortex (RSC)
The RSC is a neocortical structure situated in the midline of the cerebrum. It arches around the dorsocaudal half of the corpus callosum in the rat, where it is bordered rostrally by the anterior cingulate cortex, caudoventrally by the PHR and laterally by the parietal and visual cortices. The coordinate system that defines position within the RSC is explained in the image below.
In the connectome, we follow the nomenclature as described by Vogt in 2004, who subdivide the RSC into four areas referred to as A29a, A29b, A29c and A30. Most of the connectional papers do not separate A29a and b and the combined region will be referred to as A29ab in this paper (Figure XXX).
A29a is the most ventral subdivision and it differs from the dorsally adjacent A29b since it lacks a fully differentiated layered structure. Cytoarchitectonically, A29a has a homogenous layer II/III, while in A29b this layer is divided into a thin superficial densely packed zone and a less dense deeper zone.
A29a and A29b are distinguished from A29c most strikingly in layer III, which in A29ab has cells arranged in bands parallel to the pial surface, while in A29c layer III is thinner and the pyramidal cell bodies are randomly spaced. An additional way to compare subregions is by looking at chemoarchitectonic features.
A29ab shows parvalbumin stained cells in layers II, V and VI, which are not as apparent in A29c. In AChE stained sections A29c layer IV shows a widening and increased strength of AChE staining compared to layer IV of A29b.
Cytoarchitectonically, A30 shows an abrupt widening and a less dense packing of layer II/III compared to A29b and A29c. Also, A30 layer IV is wider than in A29b/A29c and A30 layer V neuronal cell bodies tend to be larger. In AChE stained sections, layer I-IV of A30 are evenly and darkly stained, whereas in A29c superficial and deep parts of layer I and layer IV are most densely stained.