Gyrification is the process of forming the characteristic folds of the cerebral cortex. The peak of such a fold is called a gyrus (pl. gyri), and its trough is called a sulcus (pl. sulci). The neurons of the cerebral cortex reside in a thin layer of gray matter, only 2–4 mm thick, at the surface of the brain. Much of the interior volume is occupied by white matter, which consists of long axonal projections to and from the cortical neurons residing near the surface. Gyrification allows a larger cortical surface area, and hence greater cognitive functionality to fit inside a smaller cranium. In most mammals, gyrification begins during fetal development. Primates, cetaceans, and ungulates have extensive cortical gyri, with a few species exceptions, while small rodents such as the rat, and mouse have none. Gyrification in some animals, for example the ferret, continues well into postnatal life.
Human brain development
As fetal development proceeds, gyri and sulci begin to take shape with the emergence of deepening indentations on the surface of the cortex. Not all gyri begin to develop at the same time. Instead, the primary cortical gyri form first (beginning as early as gestational week 10 in humans), followed by secondary and tertiary gyri later in development. One of the first and most prominent sulci is the lateral sulcus (also known as the lateral fissure or Sylvian fissure), followed by others such as the central sulcus, which separates the motor cortex (precentral gyrus) from somatosensory cortex (postcentral gyrus). Most cortical gyri and sulci begin to take shape between weeks 24 and 38 of gestation, and continue to enlarge and mature after birth.
Evolutionary advantages One advantage of gyrification is thought to be increased speed of brain cell communication, since cortical folds allow for cells to be closer to one other, requiring less time and energy to transmit neuronal electrical impulses, termed action potentials. There is evidence to suggest a positive relationship between gyrification and cognitive information processing speed, as well as better verbal working memory. Additionally, because a large cranium requires a larger pelvis during childbirth, with implied difficulty in bipedalism, a smaller cranium is more easily delivered.
Theories on causality
Mechanical buckling The mechanisms of cortical gyrification are not well understood, and several hypotheses are debated in the scientific literature. A popular hypothesis dating back to the time of Retzius in the late 19th century asserts that mechanical buckling forces due to the expanding brain tissue cause the cortical surface to fold. Many theories since have been loosely tied to this hypothesis. An external growth constraint of the cranium is not thought to cause gyrification. This is primarily because the primordium of the cranium during the period of fetal brain development is not yet ossified (hardened into the bone through calcification). The tissue covering the embryonic cerebral cortex is several thin layers of ectoderm (future skin) and mesenchyme (future muscle and connective tissue, including the future cranium). These thin layers grow easily along with cortical expansion but eventually, the cranial mesenchyme differentiates into cartilage; ossification of the cranial plates does not occur until later in development. The human cranium continues to grow substantially along with the brain after birth until the cranial plates finally fuse after several years. Experimental studies in animals have furthermore shown that cortical folding can occur without external constraints. Cranial growth is thus thought to be driven by brain growth; mechanical and genetic factors intrinsic to the brain are now thought to be the primary drivers of gyrification. The only observed role that the cranium may play in gyrification is in flattening of gyri as the brain matures after the cranial plates fuse.
Axonal tension An alternative theory suggests that axonal tension forces between highly interconnected cortical areas pull local cortical areas towards each other, inducing folds. This model has been criticised: A numerical computer simulation could not produce a biologically realistic folding pattern. One study showed that gyrification can be experimentally induced in the embryonic mouse, but at early stages in the absence of axonal connections.
Differential tangential expansion A later theory of differential tangential expansion has been proposed, stating that folding patterns of the brain are a result of different tangential expansion rates between different cortical areas. This is proposed to be due to areal differences in early progenitor division rates.
Mechanical factors
Cortical thickness Early conditions of the brain have a strong influence on its final level of gyrification. In particular, there is an inverse relationship between cortical thickness and gyrification. Areas of the brain with low values of thickness are found to have higher levels of gyrification. The reverse is also true, that areas of the brain with high values of thickness are found to have lower levels of gyrification.
Growth speed There is some dispute over the growth rates through which cortical and subcortical layers of the brain develop. Purely isotropic growth suggests that the grey (outer shell) and white matter (inner core) layers each grow at separate rates, that are uniform in all dimensions. Tangential growth suggests that the grey matter grows at a faster rate than the inner white matter and that the growth rate of the grey matter determines the growth rate of the white matter. Though both methods are differential, with the cortex growing more rapidly than the subcortex, tangential growth has been suggested as a more plausible model. Creases on the brain's surface are formed as a result of instability, and tangential growth models reach levels of instability that cause creasing more frequently than isotropic models. This level is called a critical point, at which, the models prefer to release potential energy by destabilizing and forming creases to become more stable.
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