302 existence of functionally different subﬁelds although they may not strictly correspond to the a-c subﬁelds recognized in earlier Golgi studies. It appears that ‘‘… information processing from one ﬁeld to the next in the hippocampal formation follows channels formed by subsets of neurons in different proximodistal portions of each ﬁeld’’ (Amaral 1993). Connectional evidence for a distinction of infor- mation channels has been elaborated upon by gene expression studies of hippocampal pyramidal cells. Distinct gene expression patterns can be found along the septo- temporal axis of the hippocampal ﬁelds (Thompson et al. 2008; Fanselow and Dong 2010; Dong et al. 2009) and, at a particular septotemporal level, in different segments of the CA3 pyramidal cell (Thompson et al. 2008). Thus, the concept of a functional and morphological division along the transverse axis (or in the tangential plane), which we know so well from the neocortex, seems to also apply to the ﬁelds of the archicortical hippocampus. A second concept of neocortical organization—that of a radial subdivision in cell layers tiered from the white matter at depth to the pia—is not recognized in the hip- pocampus. While layers harboring polymorphic (inter-) neurons are sometimes understood as separate cell layers, the bulk of the cells, hippocampal pyramidal cells, are largely thought of as a homogeneous population, which forms one lamina, the pyramidal cell layer, which is not further divided along its radial axis. Based on a brief review of the histology, development, neurochemistry, and connectivity of the pyramidal cell layer, with some emphasis on CA1, we will discuss (1) if this view is still tenable, (2) which subdivision may be suggested, and (3) what implications such a subdivision may have for our understanding of hippocampal function. Nomenclature We follow Ishizuka et al. (1990) and use the terms proxi- mal (nearer the dentate gyrus) and distal (further away from the dentate gyrus) to designate positions along the transverse axis of the CA1 and CA3. The terms septal (closer, by way of the ﬁmbria, to the septum) and temporal (further away from the septum) are used to designate positions along the longitudinal axis of the hippocampus. This nomenclature is independent of differences in position that accompany phylogenetic changes in the size and shape of the adjacent cortical areas. Molecular and connectional criteria have replaced the original deﬁnition of CA2 based on Golgi stains (Lein et al. 2005). With one exception (see Neurochemistry), we did not ﬁnd evidence that would distinguish this ﬁeld from CA1 as far as lamination is concerned, and it is here treated as the part of CA1. For the sake of simplicity, we use ‘‘CA1’’ throughout instead of the 123 Brain Struct Funct (2011) 216:301–317 Fig. 1 Illustration of the nomenclature used in this review. The top left inset shows the hippocampus of one hemisphere from which non- hippocampal tissue had been removed and illustrates the terms ‘‘septal’’ and ‘‘temporal’’. The approximately mid-septotemporal location of the main image of a horizontal section is indicated in the top left inset by a line. The main image illustrates the terms ‘‘proximal’’ and ‘‘distal’’. The proximal border of CA1 (more correctly CA1/2) towards CA3 and between distal CA1 and the subiculum is marked by arrowheads. The top right inset shows the pyramidal cells marked in the main image and illustrates the use of the terms ‘‘superﬁcial’’ and ‘‘deep’’. Scalebar 100 lm more correct ‘‘CA1/2’’. Terms and deﬁnitions are illus- trated in Fig. 1. Histology Cortical layers can be deﬁned on the basis of the types of cells that are found in a particular layer and/or the spatial distribution and density of the types within a layer. For both of these criteria early descriptions suggest subdivi- sions of the CA1 pyramidal cell layer. Looking at cell types, Schaffer (1892) understood the pyramidal cell layer