Neurogenesis in Adult Hippocampus

Hippocampus as a whole has the shape of a curved tube including CA1-CA4 regions with a single layer of densely packed pyramidal neurons which curl into a tight “U” shape. One edge of the “U”, field CA4, is embedded into a backward facing strongly flexed V-shaped cortex, the dentate gyrus (DG) which comprises molecular, granular, subgranular cell layers and poly-morph layer called hilus (Figure 1). The ability to learn or form a memory requires a neuron to translate a transient signal into gene expression changes that have a long-lasting effect on synapse activity and connectivity. There are many neural circuits formed by multiclass neurons in hippocampus. One of them is the trisynaptic circuit (Figure 1) that is made up of three major cell groups: granule cells, CA3 pyramidal neurons, and CA1 pyramidal cells. The axons of layer II neurons in the entorhinal cortex (EC) project to the dentate gyrus through the perforant pathway. The dentate gyrus sends projections to the pyramidal cells in CA3 through mossy fibres. CA3 pyramidal neurons relay the information to CA1 pyramidal neurons through Schaffer collaterals. CA1 pyramidal neurons send back projections into deep-layer neurons of the EC. This kind of circuit is involved in long term potentiation (LTP) mediating learning and memory. CA3 also directly receives the projections from EC layer II neurons through the perforant pathway. CA1 receives direct input from EC layer III neurons through the temporoammonic pathway. The dentate granule cells also project to the mossy cells in the hilus and hilar interneurons, which send excitatory and inhibitory projections, respectively, back to the granule cells. The complicated neural circuits in hippocampus form the foundation of hippocampal functions.