Overview (green). The hippocampus arises from the medial telencephalon. In lower mammals, the hippocampus is located dorsally. Considerable expansion of the cerebral cortex in higher mammals (notably in humans) displaces the hippocampus ventrally where it protrudes inferiorly into the lateral ventricles.
Principal cells Neural progenitors that become hippocampal principal neurons (
pyramidal and
granular cells) arise from the ventricular zone of the lateral ventricle. In contrast to neural proliferation that leads to cortical formation, hippocampal precursors are produced directly in the ventricular zone because there is no
subventricular zone or outer subventricular zone adjacent to the hippocampus. Pyramidal CA1 and CA3 precursor cells, therefore, do not have to migrate far to reach their final destination. The figure to the right indicates migration of
pyramidal neurons forming the CA3 (orange) and CA1 (red) cell body layers. These cells populate the hippocampus early in development and can be morphologically distinguished from one another in the embryo by 4 months. Granular cells populate the hippocampus slightly after pyramidal cell migration. These cells have farther distance to travel and follow along the pyramidal cells before entering the hilus; this is represented in the figure as the continuation of migration with the green arrows. Granular cell precursors that will populate the dentate gyrus proliferate locally in the hilus. This area, also known as the subgranular zone, retains a portion of neurogenic precursors in the adult.
Role of reelin As in the cortex, it is believed that
reelin plays an important role in layering of hippocampal neurons through inhibition of migration. Reelin knockout mice lack a single, distinct pyramidal cell body layer due to excess migration. Unexpectedly, these mice have reduced migration into dentate gyrus. The mechanism of this involves disruption in radial glial scaffolding. ==Glucocorticoid signaling==