Activities and Seminars

Ricardo Insausti. Anatomical boundaries in the human hippocampal and parahippocampal regions applied to Neuroimage.
Date: Feb 27, 2014

What: Anatomical boundaries in the human hippocampal and parahippocampal regions applied to Neuroimage.

Where: BCBL auditorium

When: 12 noon

The human hippocampus is a key player in memory processing and other neuropsychological functions. The hippocampus and surrounding cortices (parahippocampal region, PHR) are reciprocally interconnected, and both are necessary for processing and consolidation of memories. The hippocampus and the parahippocampal region are located in the ventromedial temporal lobe, stretching from the greater wings of the sphenoid (temporal pole cortex) as far as the tentorium of the cerebellum, and the beginning of the calcarine fissure (tail of the hippocampus and posterior parahippocampal cortex. PHC). Although the nomenclature applied to this brain region has somewhat changed over the time, a consensus of terminology can be achieved to provide a common nomination for different gross anatomical landmarks, which point to the relative extent of more elusive histological fields. The hippocampal formation is made up of several fields and types of cortices, in which the flow of information is characteristically unidirectional, and, ultimately, distributes in the cerebral cortex for storage and further use. Histologically, the hippocampal region can be classified as a “primitive” type of cortex named “archicortex” (largely made up of three layers, which corresponds to the Hippocampus proper (dentate gyrus and fields CA3, CA2 and CA1, as well as the subiculum). The presubiculum, parasubiculum (sometimes referred to as the subicular complex) and the entorhinal cortex are “periarchicortex”, and make up the rostral part of the parahippocampal gyrus. Experimental studies in nonhuman primates, as well as functional studies in humans, indicate that the entorhinal cortex receives cortical input from different polysensory association cortices, most notably the components of the PHR (temporopolar, TPC, perirhinal PRC, and posterior parahippocampal gyrus, PHC, which provide more than two-thirds of the cortical input to the entorhinal cortex. Layer II neurons of the entorhinal cortex project to the molecular layer of the dentate gyrus, which starts a cascade of unidirectional projections that progress from the dentate gyrus to CA1 and subiculum, but not the other way around. Ultimately, CA1 and the subiculum project to the deep layers of the entorhinal cortex. The projections from the entorhinal cortex to the dentate gyrus follow a phylogenetically preserved pattern of projection, and the rostral and medial entorhinal cortex projects to the uncal part of the hippocampus; increasingly more lateral parts of the entorhinal cortex project to the body (midportion) and tail (lateral part) of the hippocampus. The PHR in nonhuman primates and very likely in humans, provides highly processed polysensory input to the entorhinal cortex, the gateway of polysensory, highly processed information to the hippocampus, The PHR itself is one site where unimodal and polymodal association cortical areas converge. The hippocampal formation and the PHR, for the most part, lack consistent criteria for its delimitation and segmentation in structural and functional MRI studies in humans and nonhuman primates. Traditional cytoarchitectonic parcellations cannot be directly applied to MRI imaging because of resolution constraints. Therefore, and given the variability of the sulcal and gyral patterns among individuals, an indirect approach must be taken based on the correlation of gross anatomy points and their corresponding histological analysis to place more precise limits.

The medial temporal lobe presents a structure that forms small prominences or bumps, which correspond to defined neuroanatomical structures that help in the delimitation of the cortical areas in the medial temporal lobe, in particular in the coronal plane of section.

In a rostrocaudal sequence of steps, the following is likely to happen:

The rostralmost part of the temporal lobe is TPC. Although some variability exists, the TPC in coronal sections extends for the first 6-8 mm, being more restricted to the dorsomedial part of the temporal pole as the neocortex of the superior and inferior temporal gyri appear laterally.

The following landmark is the limen insulae at the frontotemporal junction, which points to the rostral limit of the insula, and is unmistakable on coronal sections. Approximately 2 mm behind the limen insulae, the entorhinal cortex starts. Around this point, the rostralmost extent of the collateral sulcus is usually visible, which points out the commencement of the PRC. The start of the collateral sulcus is highly variable, and it leaves a dorsal portion of the perirhinal cortex, variable in extent, in front of the rostramost portion of the entorhinal cortex, separated by the rhinal sulcus. The amygdaloid complex begins about 5 mm behind the start of the entorhinal cortex. The anterior limit of the temporal horn of the lateral ventricle is variable, but usually it starts a few mm caudal to the amygdala. Shortly after the hippocampus appears as an ellipsoid structure that histologically corresponds to the subiculum, at the rostralmost point where the hippocampus forms the bend to the uncal portion of the hippocampus. The appearance of the white matter surrounding the cell layer of the subiculum would correspond to the molecular layer of the subiculum. From that point, the length of the hippocampus may vary between 3 and 5 cm, (and about 7-8 cm from the tip of the temporal pole). The hippocampus folds itself in a medial direction, forming the hippocampal head. The separation of fields in the hippocampal head acquires the highest complexity at any plane of section. From the beginning of the hippocampus, the complete set of hippocampal fields follows a rostrocaudal sequence: first, CA1 field joins the subiculum laterally; then, CA3 field arises dorsally, and separates a medial and lateral portions of CA1 (dorsal subiculum and CA1). The dentate gyrus becomes evident as a rounded, closed structure with its three sublayers (molecular, granular and polymorph). The molecular layer is separated from the underlying hippocampus by the fused hippocampal fissure, noted by the presence of small blood vessels. The hippocampal fissure opens up at a variable distance. Here, a number of digitations (between 3-5) indicate flexures that present a mixture of hippocampal fields, thereby several sections through the dentate gyrus may take place. The typical, C shape of the hippocampus becomes progressively evident, and the presubiculum appears medially. The subiculum is also continues medially into the amygdalo-hippocampal transitional area. On the other hand, a similar sequence takes place at the medial portion of the hippocampal head, thus forming the dorsal presubiculum, subiculum and hippocampal fields as far as the dentate gyrus at the band of Giacomini. However, it is worth noting that, in a caudal direction, the sequence of fields follows the opposite, thereby the presubiculum, subiculum, CA1 and CA3 progressively disappear in each of the digitations. The gyrus intralimbicus, a small, rounded structure at the caudalmost limit of the hippocampal head, containing only CA3 and some dentate gyrus. The hippocampal head span along a rostral-to-caudal direction is about 12 mm. The gyrus intralimbicus is a landmark easy to pick and heralds the caudal limit of the entorhinal cortex, as well as the caudal limit of the PRC, at the commencement of the lateral geniculate nucleus.

At this point, the body of the hippocampus starts with all the hippocampal fields. The choroidal fissure points to the transition between the presubiculum and the parasubiculum, which continues with the entorhinal cortex, at the ventromedial aspect of the temporal lobe.

The lateral geniculate nucleus extends for about seven mm, and it forms a distinct prominence in the basal diencephalon, easy to detect. The limit between body and tail of the hippocampus is indistinct. As an approximation, and consistent with nonhuman primate literature, the limit can be placed at the midpoint of the distance between the end of the gyrus intralimbicus and the end of the hippocampus. The basis for this distinction would be the separation provided by the entorhinal cortex projections to the molecular layer of the dentate gyrus thorough the angular bundle. In this way, the intermediate portion of the entorhinal cortex would be linked to the body of the hippocampus, while the lateral portion would be related to the tail of the hippocampus.

The calcarine fissure, or parieto-occipital junction is present at the caudal part of the ventromedial surface. It is usually is coincident with the crura of the fornix, point at which the PHC endsn and thorough the istmus of the parahippocampsl gyrus, The caudal PHR is continuous around the splenium of the corpus callosum with the cortex at the cuneus, mostly formed by retrosplenial corex.

While the criteria for the segmentation of the rostral PHR has been reported (Insausti et al., 1998), the PHC caudal limit on MRI images can be placed approximately by the caudal part of the collateral sulcus (lateral limit), and the crura of the fornix at the approximate level of the end of pulvinar in the thalamus.

In conclusion, the hippocampal formation the temporopolar, perirhinal and posterior parahippocampal cortices that constitute the parahippocampal region, benefit of the close relationship with gross anatomical landmarks, which can be used to place natural boundaries among those important regions of the human brain.