308 The distribution of calbindin in the primate hippocam- pus, taking into account differences in histoarchitecture, corresponds to the pattern observed in the rat (Seress et al. 1991). The number of CaBP-ir CA1 pyramidal cells and the intensity of the reaction observed in different primate species is, however, variable. In African green monkeys (Chlorocebus aethiops), only a few, weakly reacting cells are seen, which show a prevalence for the upper half of the pyramidal cell layer (Leranth and Ribak 1991). In Rhesus monkeys (Macaca mulatta) many CA1 pyramidal cells are immunoreactive (Seress et al. 1991). In this species, a preference for a speciﬁc (superﬁcial) location within the pyramidal cell layer is only shown by strongly CaBP-ir cells (Seress et al. 1991, their Figure 2). In the baboon (Papio papio), virtually all CA1 pyramidal cells have been reported to contain calbindin (Sloviter et al. 1991). Cal- bindin also marks a subpopulation of human CA1 pyra- midal cells (Seress et al. 1992, 1993), which are distributed similarly to those in other primates. Possibly extending the sparse presence of very deep CaBP-ir pyramidal cells in mice, a distinct third band of CaBP-ir pyramidal cells is present in dogs at the deep border of the cell layer (Hof et al. 1996) in addition to the superﬁcial tier. We observed a similar pattern in the fox, in which the number of deep CaBP-ir pyramidal cells increased towards the temporal pole of the hippocampus while immunoreac- tivity in superﬁcial cells gradually decreased (Fig. 4e). Of those species in which there are no appreciable radial differences in cell-density in the CA1 pyramidal cell layer, data on the distribution of CaBP-ir is only available for the guinea pig and rabbit (Rami et al. 1987b; de Jong et al. 1996). CaBP-ir pyramidal cells were not reported in these species. CaBP-ir pyramidal cells are also absent from much of CA1 of elephant shrews (Fig. 4f). However, a change is seen in approximately the temporal one quarter of CA1, in which CaBP-negative superﬁcial pyramidal cells are supplemented by a band of strongly CaBP-ir deep pyramids (Fig. 4g). Interestingly, green ﬂuorescent protein (GFP) in thy1- GFP (M line) transgenic mice is expressed in a calbindin- negative subset of CA1 pyramidal cells both in septal (Fig. 4a) and temporal (Fig. 4b) CA1. Although the reason for this selective expression is not clear, the possibilities that have been raised suggest a relation between the labeled cells in terms of either normal thy1 expression or a dif- ferential cell type- or layer-speciﬁc recognition by tran- scription factors at the site of transgene integration (Feng et al. 2000) and imply a functional differentiation between expressing and non-expressing cells. Zinc. Zinc, located in the synaptic vesicles of a subset of telencephalic boutons, was identiﬁed as the endogenous substrate of Timm’s sulphide silver methods, which has frequently been used to illustrate hippocampal layers. Like CaBP-ir cells, zinc-containing neurons exhibit a striking 123 Brain Struct Funct (2011) 216:301–317 preference for sublayers along the radial axis of the pyra- midal cell layer in the rat and mouse (Slomianka and Geneser 1991, 1997; Slomianka 1992). They form a den- sely packed superﬁcial band within the pyramidal cell layer of CA1 throughout the septotemporal extent of the hippo- campus in both species (rat: Figs 4c and 5a). Zinc-con- taining neurons rendered visible in macaque monkey CA1 by retrograde selenite tracing from the inferotemporal cortex are also located in the superﬁcial part of the pyra- midal cell layer (Ichinohe and Rockland 2005). The localization of zinc-containing cells in CA1 suggests a colocalization of calbindin and zinc in these cells. CaBP-ir CA1 pyramidal cells were indeed found to be zinc-con- taining neurons and vice versa (compare Fig. 4c and d, described in detail in Slomianka et al. 1997). Also, the developmental expression of calbindin-immunoreactivity and the ability to label CA1 pyramidal cells (or dentate granule cells) for zinc appear to coincide temporally (Rami et al. 1987a; Baimbridge 1992; Slomianka and Geneser 1997), which may indicate that the speciﬁc cellular prop- erties that depend on the presence of calbindin and zinc become functionally important within the same period of postnatal development. In CA3 of the rat, zinc-containing neurons are scattered along the route of migration of lately formed pyramidal cells, deep to a compact layer of superﬁcial CA3 pyramids (Fig. 5b, c). Their distribution is somewhat more extensive in CA3 of mice, comprising a larger portion of lately formed CA3 pyramids (Slomianka and Geneser 1997). Calbindin is not found in CA3 pyramidal cells of any commonly used laboratory species or in those of the majority of species we have examined. However, in naked mole rats (Heterocephalus glaber) a striking band of strongly calbindin-immunoreactive pyramidal cells is found at the deep limit of the CA3 pyramidal cell layer (Fig. 4h). Although the localization of calbindin shows bewilder- ing variations, it remains, when expressed, a marker with laminar speciﬁcity. Phylogenetic variability and laminar speciﬁcity also characterize this protein in the neocortex (Hof et al. 1999). Possible physiological correlates of dif- ferences in the expression of calbindin and zinc are dis- cussed below. Connectivity Efferent connections The differential expression of Satb2 and Sox5 in CA1 pyramidal cells, which specify efferent connectivity by their interactions with other transcription factors in the neocortex (Britanova et al. 2008; Leone et al. 2008), implies differences in the connectivity of deep and