Sequential pattern of sublayer formation in the paleocortex and neocortex.

The piriform cortex (paleocortex) is the olfactory cortex or the primary cortex for the sense of smell. It receives the olfactory input from the mitral and tufted cells of the olfactory bulb and is involved in the processing of information pertaining to odors. The piriform cortex and the adjoining neocortex have different cytoarchitectures; while the former has a three-layered structure, the latter has a six-layered structure. The regulatory mechanisms underlying the building of the six-layered neocortex are well established; in contrast, less is known about of the regulatory mechanisms responsible for structure formation of the piriform cortex. The differences as well as similarities in the regulatory mechanisms between the neocortex and the piriform cortex remain unclear. Here, the expression of neocortical layer-specific genes in the piriform cortex was examined. Two sublayers were found to be distinguished in layer II of the piriform cortex using Ctip2/Bcl11b and Brn1/Pou3f3. The sequential expression pattern of Ctip2 and Brn1 in the piriform cortex was similar to that detected in the neocortex, although the laminar arrangement in the piriform cortex exhibited an outside-in arrangement, unlike that observed in the neocortex.