The hippocampal sulcus is a fissure that separates the dentate gyrus from the subiculum and the CA1 field of Ammon's horn in the hippocampus, extending from the splenium of the corpus callosum almost to the tip of the temporal lobe. Enlargement of the hippocampal sulcus has been associated with medial temporal lobe atrophy occurring in Alzheimer's disease. The hippocampal sulcus first appears during the 10th week of gestational age during the embryological development. At this stage it exists as a broad shallow fissure along the surface of the dentate gyrus. Gradually, the fissure deepens and shifts toward the Ammon's horn. After about 18 weeks, the walls of the fissure fold into each other and begin to fuse. By 30 weeks, the hippocampal sulcus is normally obliterated except for its most medial part, leaving a shallow surface indentation.