Archipallium

In anatomy of animals, the archicortex or archipallium is the phylogenetically oldest region of the brain's pallium or cortex. In anatomy of animals, the archicortex or archipallium is the phylogenetically oldest region of the brain's pallium or cortex. The archicortex or archipallium is often considered contiguous with the olfactory cortex, but the extent of the archipallium varies among species. In older species, such as fish, the archipallium makes up most of the cerebrum. Amphibians develop an archipallium and paleopallium, whereas reptiles develop an archipallium, paleopallium and a primitive neopallium. In humans, the archipallium makes up the hippocampus. Archicortex is also defined as a type of cortical tissue that consists of three cortical laminae (layers of neuronal cell bodies). It has fewer laminae than both neocortex, which has six, and paleocortex, which has either four or five. Archicortex, along with paleocortex and periallocortex, is a type of allocortex. Because the number of laminae that compose a type of cortical tissue seems to be directly proportional to both the information-processing capabilities of that tissue and its phylogenetic age, archicortex is thought to be the oldest and most basic type of cortical tissue. Archicortex is most prevalent in the olfactory cortex and the hippocampus, which are responsible for processing smells and forming memories, respectively. Because olfaction is considered to be the phylogenetically oldest sensory modality, and the limbic system, of which the hippocampus is a part, is one of the oldest systems in the brain, it is likely that archicortex was one of the first types of tissue to develop in primitive nervous systems. Archicortical precursor cells are also present in the dentate gyrus of the developing mammalian embryo.