 For further information on the regions, see Hippocampus anatomy.It is composed of a densely packed bundle of mossy fibers (unmyelinated) and spiny and aspiny interneurons that lie immediately above the CA3 pyramidal cell layer in the hippocampus, and immediately below the stratum radiatum.Most mossy fiber axons are perpendicular to the CA3 pyramidal region where they project and synapse to either the CA3 pyramidal cells or the stratum oriens below the pyramidal region.