Radial unit hypothesis

The Radial Unit Hypothesis (RUH) is a conceptual theory of cerebral cortex development, first described by Pasko Rakic.

[1][2] The reiterative nature of the cerebral cortex, in the sense that it is a vast array of repeating functional circuits, led to the idea that cortical evolution is governed by mechanisms regulating the addition of cortical columns, enabling additional functional areas to become specialized and incorporated into the brain.

The addition of new radial units is thought to depend on control of the cell cycle (proliferation) of cortical stem cells lining the ventricular system in the ventricular zone and subventricular zone.

[3] Intimately related to the RUH is the 'protomap' hypothesis, which states that the primordial identity of each functional area of the cerebral cortex is encoded within the cortical stem cells prior to the formation of the cortical layers.

[7] Together, the RUH and protomap hypothesis represent two core principles of early cerebral cortex development.

Schematic of the Radial Unit Hypothesis as related to the Protomap Hypothesis (colored regions) of cortical development and evolution. Adapted from Rakic, 1995, 2009. VZ, ventricular zone; SVZ, subventricular zone; IZ, intermediate zone; SP, subplate; CP, cortical plate; MZ, marginal zone; RG, radial glia; MN, migrating neuron; TR, thalamic radiation; CC, cortico-cortical axons. E## represents post-conceptional age of macaque monkey.