Open Access
The human periallocortex: layer pattern in presubiculum, parasubiculum and entorhinal cortex. A review
(Frontiers Media, 2017) Insausti, Ricardo; Muñoz López, Mónica; Insausti Serrano, Ana María; Artacho Pérula, Emilio; Ciencias de la Salud; Osasun Zientziak
The cortical mantle is not homogeneous, so that three types of cortex can be distinguished: allocortex, periallocortex and isocortex. The main distinction among those three types is based on morphological differences, in particular the number of layers, overall organization, appearance, etc., as well as its connectivity. Additionally, in the phylogenetic scale, this classification is conserved among different mammals. The most primitive and simple cortex is the allocortex, which is characterized by the presence of three layers, with one cellular main layer; it is continued by the periallocortex, which presents six layers, although with enough differences in the layer pattern to separate three different fields: presubiculum (PrS), parasubiculum (PaS), and entorhinal cortex (EC). The closest part to the allocortex (represented by the subiculum) is the PrS, which shows outer (layers I–III) and inner (V–VI) principal layers (lamina principalis externa and lamina principalis interna), both separated by a cell poor band, parallel to the pial surface (layer IV or lamina dissecans). This layer organization is present throughout the anterior-posterior axis. The PaS continues the PrS, but its rostrocaudal extent is shorter than the PrS. The organization of the PaS shows the layer pattern more clearly than in the PrS. Up to six layers are recognizable in the PaS, with layer IV as lamina dissecans between superficial (layers I–III) and deep (V–VI) layers, as in the PrS. The EC presents even more clearly the layer pattern along both mediolateral and rostrocaudal extent. The layer pattern is a thick layer I, layer II in islands, layer III medium pyramids, layer IV as lamina dissecans (not present throughout the EC extent), layer V with dark and big pyramids and a multiform layer VI. The EC borders laterally the proisocortex (incomplete type of isocortex). Variations in the appearance of its layers justify the distinction of subfields in the EC, in particular in human and nonhuman primates. EC layers are not similar to those in the neocortex. The transition between the periallocortical EC and isocortex is not sharp, so that the proisocortex forms an intervening cortex, which fills the gap between the periallocortex and the isocortex.
Open Access
The CA2 hippocampal subfield in humans: a review
(Wiley, 2023) Insausti, Ricardo; Muñoz López, Mónica; Insausti Serrano, Ana María; Ciencias de la Salud; Osasun Zientziak
CA2 is probably the most enigmatic of the hippocampal fields. It is small in size (in humans about 500 μm across the mediolateral axis), and yet, it is involved in important functions, such as in social memory and anxiety. This study offers a glimpse of several significant aspects of the anatomical organization of CA2. We present an overview of the anatomical structure of CA2, imbued in the general organization of the human hippocampal formation. The location and distinctiveness of CA2 is presented in relation with CA3 and CA1, based in a total of 23 human control cases serially sectioned throughout the whole longitudinal axis of the hippocampus, examined every 500 μm in Nissl-stained sections. The longitudinal extent of CA2 is close to 30 mm, starting in the hippocampal head, 2.5 mm caudal to the DG and 3.5 mm caudal to the start of CA3, approximately 10 mm from the hippocampus rostral end. The connectional information of human CA2 is very scarce, thereby we relied on nonhuman primate tract tracing studies of the hippocampal formation, given its resemblance to the human brain. Human CA2 is subject of neuropathological studies, and we chose to present Alzheimer's disease, schizophrenia, and Mesial Temporal Lobe Epilepsy with hippocampal sclerosis in those aspects that impinge directly into CA2.
Open Access
Ex vivo, in situ perfusion protocol for human brain fixation compatible with microscopy, MRI techniques, and anatomical studies
(Frontiers Media, 2023) Insausti, Ricardo; Insausti Serrano, Ana María; Muñoz López, Mónica; Medina Lorenzo, Isidro; Arroyo-Jiménez, María del Mar; Marcos Rabal, Pilar; Rosa-Prieto, Carlos de la; Delgado González, José Carlos; Montón Etxeberria, Javier; Cebada Sánchez, Sandra; Raspeño-García, Juan Francisco; Iñiguez de Onzoño, María Mercedes; Molina Romero, Francisco Javier; Benavides-Piccione, Ruth; Tapia-González, Silvia; Wisse, Laura E. M.; Ravikumar, Sadhana; Wolk, David A.; DeFelipe, Javier; Yushkevich, Paul; Artacho Pérula, Emilio; Ciencias de la Salud; Osasun Zientziak
We present a method for human brain fixation based on simultaneous perfusion of 4% paraformaldehyde through carotids after a flush with saline. The left carotid cannula is used to perfuse the body with 10% formalin, to allow further use of the body for anatomical research or teaching. The aim of our method is to develop a vascular fixation protocol for the human brain, by adapting protocols that are commonly used in experimental animal studies. We show that a variety of histological procedures can be carried out (cyto- and myeloarchitectonics, histochemistry, immunohistochemistry, intracellular cell injection, and electron microscopy). In addition, ex vivo, ex situ high-resolution MRI (9.4T) can be obtained in the same specimens. This procedure resulted in similar morphological features to those obtained by intravascular perfusion in experimental animals, provided that the postmortem interval was under 10 h for several of the techniques used and under 4 h in the case of intracellular injections and electron microscopy. The use of intravascular fixation of the brain inside the skull provides a fixed whole human brain, perfectly fitted to the skull, with negligible deformation compared to conventional techniques. Given this characteristic of ex vivo, in situ fixation, this procedure can probably be considered the most suitable one available for ex vivo MRI scans of the brain. We describe the compatibility of the method proposed for intravascular fixation of the human brain and fixation of the donor’s body for anatomical purposes. Thus, body donor programs can provide human brain tissue, while the remainder of the body can also be fixed for anatomical studies. Therefore, this method of human brain fixation through the carotid system optimizes the procurement of human brain tissue, allowing a greater understanding of human neurological diseases, while benefiting anatomy departments by making the remainder of the body available for teaching purposes.
Open Access
Cytoarchitectonic areas of the gyrus ambiens in the human brain
(Frontiers Media, 2019) Insausti, Ricardo; Córcoles Parada, Marta; Ubero Martínez, María del Mar; Rodado, Adriana; Insausti Serrano, Ana María; Muñoz López, Mónica; Ciencias de la Salud; Osasun Zientziak
The Gyrus ambiens is a gross anatomical prominence in the medial temporal lobe (MTL), associated closely with Brodmann area 34 (BA34). It is formed largely by the medial intermediate subfield of the entorhinal cortex (EC) [Brodmann area 28 (BA28)]. Although the MTL has been widely studied due to its well-known role on memory and spatial information, the anatomical relationship between G. ambiens, BA34, and medial intermediate EC subfield has not been completely defined, in particular whether BA34 is part of the EC or a different type of cortex. In order to clarify this issue, we carried out a detailed analysis of 37 human MTLs, determining the exact location of medial intermediate EC subfield and its extent within the G. ambiens, its cortical thickness, and the histological-MRI correspondence of the G. ambiens with the medial intermediate EC subfield in 10 ex vivo MRI. Our results show that the G. ambiens is limited between two small sulci in the medial aspect of the MTL, which correspond almost perfectly to the extent of the medial intermediate EC subfield, although the rostral and caudal extensions of the G. ambiens may extend to the olfactory (rostrally) and intermediate (caudally) entorhinal subfields. Moreover, the cortical thickness averaged 2.5 mm (1.3 mm for layers I-III and 1 mm for layers V-VI). Moreover, distance among different landmarks visible in the MRI scans which are relevant to the identification of the G. ambiens in MRI are provided. These results suggest that BA34 is a part of the EC that fits best with the medial intermediate subfield. The histological data, together with the ex vivo MRI identification and thickness of these structures may be of use when assessing changes in MRI scans in clinical settings, such as Alzheimer disease.

Insausti Serrano, Ana María

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