Hippocampal Complex (Allocortex)
Neocortex differs from allocortex primarily in the number of layers present. Go to Link 5. Reduce the magnification until you can see the entire image. Move the image so that you can view the medial part of the temporal lobe (move down along the pial surface then medially), past the inferior temporal sulcus (identified in this image as OTS, for occipitotemporal sulcus), and rhinal sulcus (RS) to the parahippocampal gyrus. Increase the magnification, see below. Notice that within the parahippocampal gyrus the cortex changes in lamination. Proceeding dorsolaterally around the parahippocampal gyrus you will encounter several allocortical structures, the subiculum (SB), hippocampal cortex (CAM, for cornu ammonis), and dentate gyrus (DG). These cortices are examples of cortex that has fewer than six layers.
To view the above listed structures at high magnification go to
Link 6. This is a section through the monkey telencephalon that, with adequate magnification, allows one to observe the general morphology of the neurons that make up the hippocampus, dentate gyrus and parahippocampal gyrus. Increase the magnification as much as possible until you can identify individual neurons (see below). Note that the dark staining neurons in the central, prominent layer of the hippocampus are pyramidal in shape. Since this is a Nissl stain you cannot readily see the full extent of their dendritic arbors, which extend in both directions from their cell bodies. The apical dendrites are pointing toward the dentate gyrus. The pial surface was obliterated when the two surfaces of the hippocampal fissure fused. The remnant of the hippocampal fissure can be seen as a thin slightly darker line of cells between the dentate gyrus and the most external portion of the hippocampus and proximal portion of the subiculum.
The granule cells (neurons) of the dentate gyrus form a compact intensely stained C-shaped lamina. Larger polymorphic cells located in the hilus (the region inside the “C”) of the dentate gyrus are isolated and easy to see.
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