Abstract
The human hippocampal formation is relevant for various aspects of memory and learning, and the different hippocampal regions are differentially affected by neuropsychiatric disorders. Therefore, the hippocampal formation has been subject of numerous cytoarchitectonic and other mapping studies, which resulted in divergent parcellation schemes. To understand the principles of hippocampal architecture, it is necessary to integrate different levels of hippocampal organisation, going beyond one modality. We here applied a multimodal mapping approach combining cyto- and multi-receptorarchitectonic analyses, and generated probabilistic maps in stereotaxic space of the identified regions. Cytoarchitecture in combination with the regional and laminar distribution of 15 neurotransmitter receptors visualized by in vitro receptor autoradiography were analysed in seven hemispheres from 6 unfixed shock frozen and serially sectioned brains. Cytoarchitectonic delineations for generation of probabilistic maps were carried out on histological sections from ten fixed, paraffin embedded and serially sectioned brains. Nine cyto- and receptorarchitectonically distinct regions were identified within the hippocampal formation (i.e., fascia dentata, cornu Ammonis (CA) regions 1–4, prosubiculum, subiculum proper, presubiculum and parasubiculum), as well as the hippocampal-amygdaloid transition area and the periallocortical transsubiculum. Subsequently generated probabilistic maps quantify intersubject variability in the size and extent of these cyto- and receptorarchitectonically distinct regions. The regions did not differ in their volume between the hemispheres and gender. Receptor mapping revealed additional subdivisions which could not be detected by cytoarchitectonic analysis alone. They correspond to parcellations previously found in immunohistochemical and connectivity studies. The multimodal approach enabled the definition of regions not consistently reported, e.g., CA4 region or prosubiculum. The ensuing detailed probabilistic maps of the hippocampal formation constitute the basis for future architectonically informed analyses of in vivo neuroimaging studies.
Highlights
The hippocampal formation, which comprises the hippocampus proper and the subicular complex, plays a crucial role in the formation, organization, and retrieval of memories, and is involved in the control of mood, alertness and attention (e.g., Sweatt 2010, McDonald and Hong 2013).3 C. & O
Whereas Amunts et al (2005) provided delineations for the hippocampal-amygdaloid transition area (HATA), dentate gyrus (DG), cornu Ammonis (CA) and the subicular complex, but not for subdivisions of the latter two regions, the updated version resulting from the present study provides information concerning intersubject variability of the fascia dentata (FD), CA4, CA3, CA2, CA1, ProS, subiculum proper (Sub), PreS and PaS regions, and will enable future comparisons of cytoarchitectonically informed parcellation schemes in stereotaxic space with high resolution structural and/or functional imaging data
FD, located on the DG together with the CA4 region, is the most medial of the regions defined within the hippocampus proper, CA1 is the first one to appear when moving from rostral to caudal (Supplementary Table 1; Fig. 3, sections 4486 and 4081)
Summary
The hippocampal formation, which comprises the hippocampus proper and the subicular complex, plays a crucial role in the formation, organization, and retrieval of memories, and is involved in the control of mood, alertness and attention (e.g., Sweatt 2010, McDonald and Hong 2013).3 C. & O. Vogt Institute for Brain Research, Heinrich-Heine-University, 40225 Düsseldorf, Germany. In humans different aspects of memory and learning have been associated with distinct regions of the hippocampus and/or subicular complex (Zeineh et al 2003; Eldridge et al 2005; Bakker et al 2008; Suthana et al 2009). There are differences in the criteria applied for the definition of both the outer and inner hippocampal borders (Konrad et al 2009). To overcome such problems, the Hippocampal Subfields Group was formed as an international initiative to develop a harmonized protocol for segmentation of the hippocampal formation on high-resolution MRI (Wisse et al 2017). Currently available maps provide separate delineations for the dentate gyrus (DG), the cornu Ammonis (CA) region and the subicular complex, but not subdivisions of the latter two (Amunts et al 2005)
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