The allocortex is one of two types in the cerebral cortex, the other being the neocortex. It is characterized by having just three or four cell layers, in contrast with the six layers of the neocortex, and takes up a much smaller area than the neocortex. There are three subtypes of allocortex: the paleocortex, the archicortex, and the periallocortex – a transitional zone between the neocortex and the allocortex. The specific regions of the brain usually described as belonging to the allocortex are the olfactory system, and the hippocampus. Allocortex is termed heterogenetic cortex, because during development it never has the six-layered architecture of homogenetic neocortex. It differs from heterotypic cortex, a type of cerebral cortex, which during prenatal development, passes through a six-layered stage to have fewer layers, such as in Brodmann area 4 that lacks granule cells.
Structure
The allocortex has three or four layers of neuronal cell bodies, in contrast to the six layers of the neocortex. There are three subtypes of allocortex: the paleocortex, archicortex and periallocortex. Paleocortex is a type of thin, primitive corticaltissue that consists of three cortical laminae. The two granular layers II and IV of neocortex are absent in paleocortex. The main areas of paleocortex are the olfactory bulb, olfactory tubercle and piriform cortex. Archicortex is a type of cortical tissue that consists of four laminae . The main areas of archicortex are the hippocampus and dentate gyrus. Periallocortex is a transitional form between neocortex and either paleo- or archi-cortex. It thus can be either peripaleocortex or periarchicortex. 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, and also because olfaction is a major sensory modality in phylogenetically early animals, paleocortex is thought to be the most primitive form of cortex.
