The medial temporal lobe (MTL) is composed of the hippocampus (HPC) and

Evidence from clinical studies attribute to the MTL an essential role for correct memory function, at least for episodic memories. Defining its anatomy and connections is thus essential. Because the present work will mainly focus on mice’s memory I will here develop the anatomical profile of the rodent’s hippocampal formation. Most of the anatomical descriptions that will follow in the text were elaborated and extracted from “The Hippocampus book” (Per Andersen, 2007) as it is one of the most detailed and complete anatomical work on the hippocampus (human and rodent). The MTL includes the hippocampus (HPC) and Brodmann areas 35 (A35) and 36 (A36). The HPC is divided into the hippocampal formation (HF) and the parahippocampal region (PR). The HF is composed of subregions: dentate gyrus (DG), Cornu Ammonis (CA) and subiculum. The PR consists of: medial and lateral entorhinal cortex (MEC and LEC), presubiculum (PrS) and parasubiculum (PaS) (Figure 3).

In rodents, the HPC is located in the caudal part of the brain and has a c-shaped structure (Figure 3 A). The HPC is composed of different relay-areas that specifically encode spatial information, emotions, olfaction and memory (Figure 3 B). These areas are: DG, the CA areas (CA3, CA2 and CA1 ordered by information flow) and the subiculum. The HPC has well defined cortical layers. The HF appears in a three-layered structure for each of its regions while the PR has a six-layered structure (Figure 3 B).

Figure 3. Anatomy of the rodent’s hippocampus, adapted from Moser et al. (2014) and Van Strien et al., (2009).

A. Mid-sagittal view of the rodent hemisphere showing hippocampal and parahippocampal positions. The dashed line indicates the dorsoventral position of the section. B. Horizontal cross section (Nissl-stained) of medial temporal lobe (DG, CA3, CA1, Subiculum, PrS, PaS, MEC, LEC, A35, A36). Cortical layers are shown by Roman numerals and the position of each layer is indicated. Figure legend: DG, dentate gyrus;

CA, cornu ammonis; PrS, presubiculum; PaS, parasubiculum; MEC, medial entorhinal cortex; LEC, lateral entorhinal cortex; A35, Brodmann area 35; A36, Brodmann area 36; Dist, distal; encl, enclosed blade of the DG; exp, exposed blade of the DG; gl, granule cell layer; luc, stratum lucidum; ml, molecular layer; or, stratum oriens; prox, proximal; pyr, pyramidal cell layer; rad, stratum radiatum; slm, stratum lacunosum-moleculare.

Dentate Gyrus

In the DG, the first layer is called hilus (often referred as polymorphic cell layer) containing both afferent and efferent fibers as well as glutamatergic neurons (mossy cells) and ɣ-aminobutyric acid expressing cells (GABAergic). On top of this layer we find the granule cell layer, which is mostly composed of granule cells (GCs) as the name states it. Granule cells have an elliptical cell body with a diameter of around 10 μm (Claiborne et al., 1990).

The rodent granule cell layer of the DG contains approximately 1.2x10⁶ GCs that are densely compacted (Rapp and Gallagher, 1996). This layer is one of the few brain region where we have neurogenesis continuously producing new granule cells throughout adulthood (Cameron and McKay, 2001). Superficially to this layer, we have the molecular layer, stratum moleculare that is a mainly free-cell body layer, principally composed of dendrites from granule cells and interneurons (INs). Together, the granule cell layer and the molecular layer form a u-shaped structure (Figure 3 B).

Cornu Ammoni Areas

The CA areas are divided into three main subregions: CA3, CA2 and CA1 as firstly described by Lorente de Nó in 1934. These regions connect the DG with the subiculum and are highly interconnected through densely packed pyramidal neurons. The deeper CA

layer, stratum oriens, contains the basal dendritic trees of pyramidal cells and different interneurons. Superficial to the stratum oriens layer, lays the stratum pyramidale, which correspond to the granule cell layer in the DG. It is principally composed of pyramidal cells (the bodies), which are the main excitatory cells of the hippocampal formation, as well as different types of cell body interneurons. The stratum radiatum is composed of apical dendrites and some interneurons. The most superficial layer of the CA regions is the stratum lacunosum-moleculare containing the very apical tufts of the apical dendrites from pyramidal cells. Although this three-layered structure is found in all the CA regions of the hippocampal formation, the CA3 region has one additional layer compared to the other CA areas, called stratum lucidum where DG cells send their fibers (mossy fiber projections).

Subiculum

The subiculum, located immediately at CA1 border, has a similar cytoarchitecture as the CA areas (O'Mara et al., 2001). The main difference in the subiculum is that the stratum radiatum of CA1 is replaced by the stratum moleculare that physically merge in a superficial and deeper manner the two layers of CA1, the stratum radiatum and the stratum lacunosum-moleculare of CA1. Another difference is that in the subiculum we do not find the stratum oriens. The subiculum is principally composed of large pyramidal neurons but are less tightly packed as in CA1 (Funahashi and Stewart, 1997; Harris et al., 2001). Particularly, two different pyramidal cells have been described based on their physiological profile: intrinsically bursting cells and regular spiking cells (Greene and Totterdell, 1997). Regular spiking cells are found mostly in the superficial pyramidal cell layer while bursting cells are found more in the deeper part of the same layer. Together, large pyramidal neurons are projecting neurons but it seems that only bursting cells project to the EC. The subiculum is populated by several types of INs similarly to the CA areas.

Entorhinal cortex

The entorhinal cortex (EC) is the major input and output structure of the hippocampal formation. The EC is generally divided into two main regions: the medial entorhinal cortex (MEC) and the lateral entorhinal cortex (LEC). MEC and LEC provide different information to the HF: the MEC is commonly considered to provide spatial input to the HF while the LEC provides non-spatial information.

The architecture of the EC is generally defined in reciprocally connected six layers.

All the entorhinal layers are reciprocally connected. Layer II contains pyramidal and stellate cells that project to the DG and CA3, giving birth to the perforant pathway. LEC

23 layer II pyramidal cells group into islands and directly project to CA1 (Kitamura et al., 2014). Layer III contains manly pyramidal cells that also project to CA1 and subiculum.