A+Embryology

Development The cerebral cortex develops from the most anterior part of the [|neural plate], a specialized part of the embryonic [|ectoderm]. The neural plate folds and closes to form the [|neural tube]. From the cavity inside the neural tube develops the [|ventricular system], and, from the epithelial cells of its walls, the [|neurons] and [|glia] of the nervous system. The most anterior (frontal) part of the neural tube, the [|telencephalon], gives rise to the cerebral hemispheres and cortex. Cortical neurons are generated within the ventricular zone, next to the ventricles. At first, this zone contains "progenitor" cells, which divide to produce glial and neuronal cells.[|[1]] The glial fibers produced in the first divisions of the progenitor cells are radially oriented, spanning the thickness of the cortex from the ventricular zone to the outer, [|pial] surface, and provide scaffolding for the migration of neurons outwards from the ventricular zone. The first divisions of the progenitor cells are symmetric, which duplicates the total number of progenitor cells at each [|mitotic cycle]. Then, some progenitor cells begin to divide asymmetrically, producing one postmitotic cell that migrates along the radial glial fibers, leaving the ventricular zone, and one progenitor cell, which continues to divide until the end of development, when it differentiates into a [|glial cell] or an [|ependymal cell]. The migrating daughter cells become the [|pyramidal neurons] of the cerebral cortex.[|[2]] The layered structure of the mature cerebral cortex is formed during development. The first [|pyramidal neurons] generated migrate out of the ventricular zone and subventricular zone, together with [|Cajal-Retzius cells] from the [|preplate]. Next, a cohort of neurons migrating into the middle of the preplate divides this transient layer into the superficial [|marginal zone], which will become layer one of the mature neocortex, and the [|subplate], forming a middle layer called the [|cortical plate]. These cells will form the deep layers of the mature cortex, layers five and six. Later born neurons migrate radially into the cortical plate past the deep layer neurons, and become the upper layers (two to four). Thus, the layers of the cortex are created in an inside-out order. The only exception to this inside-out sequence of neurogenesis occurs in the layer I of primates, in which, contrary to rodents, neurogenesis continues throughout the entire period of corticogenesis.[|[3]]

[[|edit]] Evolution
The cerebral cortex is derived from the [|pallium], a layered structure found in the forebrains of all vertebrates. The basic form of the pallium is a cylindrical layer enclosing fluid-filled ventricles. Around the circumference of the cylinder are four zones, the dorsal pallium, medial pallium, ventral pallium, and lateral pallium, which are thought respectively to give rise to the neocortex, hippocampus, amygdala, and olfactory cortex. Until recently, no counterpart to the cerebral cortex had been recognized in invertebrates. However, a study published in the journal //Cell// in 2010, based on gene expression profiles, reported strong affinities between the cerebral cortex and the [|mushroom bodies] of [|ragworms].[|[4]] Mushroom bodies are structures in the brains of many types of worms and arthropods that are known to play important roles in learning and memory; the genetic evidence indicates a common evolutionary origin, and therefore indicates that the origins of the earliest precursors of the cerebral cortex date back to the early [|Precambrian] era.