The Radial Unit Hypothesis (RUH) is a conceptual theory of

cerebral cortex The cerebral cortex, also known as the cerebral mantle, is the outer layer of neural tissue of the cerebrum of the brain in humans and other mammals. The cerebral cortex mostly consists of the six-layered neocortex, with just 10% consisting o ...

development, first described by Pasko Rakic. The RUH states that the

develops during

embryogenesis An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sperm ...

as an array of interacting

cortical column A cortical column is a group of neurons forming a cylindrical structure through the cerebral cortex of the brain perpendicular to the cortical surface. The structure was first identified by Mountcastle in 1957. He later identified minicolumns as t ...

s, or 'radial units', each of which originates from a transient stem cell layer called the ventricular zone, which contains

neural stem cell Neural stem cells (NSCs) are self-renewing, multipotent cells that firstly generate the radial glial progenitor cells that generate the neurons and glia of the nervous system of all animals during embryonic development. Some neural progenitor ste ...

s known as

radial glial cell Radial glial cells, or radial glial progenitor cells (RGPs), are bipolar-shaped progenitor cells that are responsible for producing all of the neurons in the cerebral cortex. RGPs also produce certain lineages of glia, including astrocytes and ol ...

Cortical evolution

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 The ventricular system is a set of four interconnected cavities known as cerebral ventricles in the brain. Within each ventricle is a region of choroid plexus which produces the circulating cerebrospinal fluid (CSF). The ventricular system is c ...

in the ventricular zone and

subventricular zone The subventricular zone (SVZ) is a region situated on the outside wall of each lateral ventricle of the vertebrate brain. It is present in both the embryonic and adult brain. In embryonic life, the SVZ refers to a secondary proliferative zone ...

Protomap

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. Within each developing radial unit, the process of

neurogenesis Neurogenesis is the process by which nervous system cells, the neurons, are produced by neural stem cells (NSCs). It occurs in all species of animals except the porifera (sponges) and placozoans. Types of NSCs include neuroepithelial cells (NECs ...

gives rise to post-mitotic (non-dividing) cortical

neuron A neuron, neurone, or nerve cell is an membrane potential#Cell excitability, electrically excitable cell (biology), cell that communicates with other cells via specialized connections called synapses. The neuron is the main component of nervous ...

s, which begin the process of radial neuronal migration from the ventricular zone and adjacent

to form the cortical plate in the classic 'inside-out' manner beginning with the deep cortical layers. Once their final destination is achieved, cortical neurons begin to form circuits with other cortical and subcortical neurons, often taking on a columnar shape following the radial migration route. Some localized lateral dispersion takes place during cortical column development in the mouse, but the degree of dispersion is molecularly regulated and indeed could vary across species.

Principles of development

Together, the RUH and protomap hypothesis represent two core principles of early cerebral cortex development. After neurons arrive in the cortical plate, other processes—especially activity-dependent processes—govern the maturation of cortical circuitry.

References

{{Reflist Developmental neuroscience Cerebral cortex

Cortical evolution

Protomap

Principles of development

See also

References