The basic cytoarchitectonic scheme of the hippocampus was established originally by Ramón Y Cajal (1901) and Lorente de Nó (1934)
. Their pioneering work illustrated the distinct morphological properties of small pyramidal neurons in CA1 (region superior of Cajal), and large pyramidals in CA3 (region inferior of Cajal, with mossy fibers) and CA2 (without mossy fibers). Indeed, Cajal (1901–1902)
was the first to notice differences in the hippocampus across the dorsal-to-ventral axis. He originally distinguished two perforant paths from the entorhinal cortex, “superior” and “inferior,” that target what was later referred to on connectional grounds (Gloor, 1997
; Swanson & Cowan, 1975
) as the “dorsal” and “ventral” hippocampus, respectively. Lorente de Nó (1934)
also divided the “Ammonic system” into three main segments along its longitudinal axis according to their different afferent inputs. He stated that while there is no sharp boundary, each of these segments has special structural features, although he did not give detailed descriptions of their borders.
Two recent reports based on the systematic, high-resolution analysis of a comprehensive, genome-wide digital gene expression library—the Allen Brain Atlas (ABA, www.brain-map.org
) revealed that pyramidal neurons in both CA1 and CA3 display clear regional and laminar specificities in C57Bl/6 mice. Using these robust gene markers, both of these fields were parceled into multiple, spatially distinct molecular domains and subdomains (Dong et al., 2009
; Thompson et al., 2008
). This genomic-anatomic evidence, together with our careful re-evaluation of the hippocampal cytoarchitecture, as well as the literature of numerous neuronal connectivity and functional studies in the last three decades, leads us to provide a testable hippocampal structural-functional model for understanding the heterogeneity of the DH and VH.
Our model suggests that both CA1 and CA3—the Ammon’s horn as a whole—are divided respectively into three major molecular domains: dorsal (CA1d and CA3d), intermediate (CA1i and CA3i), and ventral (CA1v and CA3v) (Dong et al., 2009
). The complex geographic topology of these three domains is better appreciated in the three-dimensional context of the mouse brain (), in which the entire Ammon’s horn appears to be an elongated C-shaped cylinder. Its two free ends compose the major proportions of the dorsal (CA1d and CA3d) or ventral (CA1v and CA3v) domains respectively, arching rostromedially, while the intermediate domains of the CA1 (CA1i) and CA3 (CA3i) defined here occupy the intermediate one-third, primarily the vertical part of the “C”. Our dorsal, ventral, and intermediate domains correspond approximately to the septal, temporal, and caudal poles of Swanson and Cowan (1977)
, although they did not give clear rationale for how these boundaries were drawn. At one sagittal level of the C57/Black/6J mouse brain atlas (~ 2.494 mm lateral to midline) showing the maximal extension of the hippocampus (where the dorsal and ventral parts merge into one unit), the CA3 pyramidal neurons cluster together and appear as one dark “X-shaped pyramidal pool” (, 2nd
row). The geographic scope within the four corners of this “X-shaped-pyramidal pool” (indicated by 1, 2, 3, or 4 in ) corresponds to the CA3i defined here. It is located right in the middle (or intermediate) portion of the hippocampus and appears to be the most obvious landmark between the DH and VH. Starting from this point rostrally and medially, the hippocampus is separated into two individual dorsal and ventral parts. Caudally/laterally, these two parts appear as one entity in which the CA3i, CA2 caudal portion, and CA1i contiguously occupy the vertical portion of the “C” shaped hippocampus progressively towards the more lateral side of the brain on sagittal planes.
Fig. 1 Molecular domains of the hippocampal CA1 and CA3. A shows a three dimensional (3D) model of Ammon's horn, which appears as a “C” shaped cylinder with its dorsal and ventral ends towards rostral and medial directions of brain. CA1 occupies (more ...)
On coronal planes (), the CA3i, which includes regions 5 (characterized by gene Serpinf1) and 4 (the caudal-dorsal end of the CA3 characterized by gene Col15a1 and Ccdc3) of Thompson et al (2008)
, first appear at the levels where the orientation of the hippocampus sweeps from the transverse (pyramidal neurons are aligned along the medial-to-lateral direction) to vertical (pyramidal neurons are “stacked” along the dorsal-to-ventral direction), and the DH and VH are merging as one unit. The CA3d is defined as the CA3 portion dorsal/rostral to the CA3i towards its septal end. The CA3d can be further subdivided into three subdomains: dorsal-medial (CA3dm, towards the dentate gyrus), dorsal-intermediate (CA3di), and dorsal-lateral (CA3dl; towards the CA2). These three subdomains correspond respectively to regions 1, 2, and 3 of Thompson et al. (2008)
, and at least partially overlap with the CA3c, CA3b, and CA3a of Lorente de Nó (1934)
, which we believe referred mostly to different parts of Ammon’s horn along the horizontal (rostral-to-caudal) and transverse (medial-to-lateral), but not longitudinal (dorsal-to-ventral) axis. The CA3v refers to the portion of CA3 ventral to the CA3i and can also be subdivided into at least two subdomains, CA3 ventral-dorsal (CA3vd) and CA3 ventral-ventral (CA3vv), which correspond respectively to regions 6 (characterized by gene Plagl1
) and 7 (ventral tip of the CA3, characterized by gene Coch
) of Thompson et al (2008)
The CA2 (characterized by Amigo), which is clearly located between the CA1d and CA3d at the rostral one-third of the hippocampus (), should be included in the dorsal domain of the Ammon’s horn. Nevertheless, a number of gene markers in the ABA database, including Map4k3 and Adcy4, reveal that CA2’s caudal portion at the levels where the DH and VH merge, overlap partially with the rostral portion of CA1i that is sandwiched between CA1d and CA1v (depending on the cutting angels of brain sections). Finally, it is worthy noting that gene expression in the dentate gyrus also displays distinct regional specificity. As shown in , Lct is preferentially expressed in the dorsal/septal/rostal part of the dentate gyrus, which runs in parallel with the CA1d and CA3d. In contrast, Trhr is expressed specifically in its ventral/temporal/caudal part, while the intermediate portion contains only sparse signal for these two genes. This suggests that the entire hippocampal region, including both the Ammon’s horn and dentate gyrus, may be composed of three distinct molecular domains, dorsal, intermediate, and ventral.
Fig. 2 Three dimensional model of the dentate gyrus in the context of the whole mouse brain (A, lateral view) and its spatial relationship with Ammon’s horn (dark green in B, medial view). Two genes, Lct (blue) and Trhr (red), are expressed preferentially (more ...)
Of equal importance, gene expression in pyramidal neurons of both CA1 and CA3 also display clear laminar specificities (Dong, et al., 2009
; Thompson et al., 2008
). Accordingly, Dong et al (2009)
subdivided the CA1 pyramidal layer into 2–3 sublayers, which show distinct cytoarchitectonic and gene expression specificities in different domains and subdomains along the longitudinal axis. Domain CA1d pyramidal layer consists of two very distinctive sublayers: the darkly stained, tightly arranged superficial layer (CA1d-sps) and the loosely arranged deep layer (CA1d-spd). These morphological properties become progressively less distinctive towards the ventral (temporal) direction, although the thickness of pyramidal layer (especially the deep layer) increases incrementally. In two dorsally located subdomains of the CA1v (CA1vd and CA1vid), one more sub-layer (the middle sublayer) appears between the superficial and deep layers. Nevertheless, towards the more ventral area, especially in the CA1vv (the most ventral tip of the CA1), all pyramidal neurons appear to have similar morphology and form a uniformed single layer with pyramidal neurons arranged in 7–8 parallel rows. In fact, Lorente de Nó noticed the difference between these types of pyramidal neurons in superficial and deep layers of CA1. According to him, the deep pyramids correspond more or less to what Cajal calls ‘pirámides dislocadas’ (luxated pyramids), which are less numerous in lower mammals (mouse, rabbit, dog, cat) than in the primates (monkey, man). Another important fact is that these two types of pyramidal neurons have a different relation to the basket cells. The superficial pyramids are in contact with the end arborizations of the pyramidal, horizontal and polygonal basket cells, while the deep pyramids are chiefly in contact with the polygonal basket cells, and the deepest have almost no contact with the basket plexus. This distinction is very important considering that basket neurons play a key role in regulating activity of pyramidal neurons.
In summary, although laminar and regional specificities of pyramidal neurons in the isocortex have been studied extensively, surprisingly very little is known about different phenotypes of pyramidal neurons in the hippocampus. Pyramidal neurons within the CA1 or CA3 display both regional and laminar specificities in different molecular domains. Distinctively expressed gene markers will provide an extremely powerful tool for understanding the functional roles of specific neuronal groups in anatomic, physiological, and genetic studies.