Gyrencephalic describes a brain whose outer surface, the cerebral cortex, is folded into ridges and grooves rather than left smooth. Those ridges are called gyri and the grooves between them are sulci. The condition is found across nearly every order of mammals and is considered a hallmark of the group rather than a quirk of just a few large-brained species.1Frontiers in Neuroanatomy. The secondary loss of gyrencephaly as an example of evolutionary phenotypical reversal The opposite condition, a smooth cortex, is called lissencephalic. Understanding what makes a brain gyrencephalic touches on everything from cell biology and evolution to clinical neurology, because the same process that builds a healthy folded brain can misfire in ways that cause serious developmental disorders.
Why Brains Fold in the First Place
The simplest way to think about cortical folding is as a packing solution. The cerebral cortex is a thin sheet of neural tissue, roughly two to four millimeters thick in humans, but its total surface area is enormous. Folding lets a large cortical sheet fit inside a skull that is constrained by the size of the birth canal and the structural limits of bone. But how the folds physically form has been debated for over a century, and researchers are still sorting out the contributions of different forces.
The leading ideas center on three interacting factors: the cortex expanding faster than the tissue beneath it, radial growth pushing outward, and tension along the nerve fibers (axons) that connect distant cortical regions. Mathematical models and experimental measurements over the past two decades have helped clarify how these forces interact during development.2PubMed Central. Mechanical forces in cerebral cortical folding: a review of measurements and models The current consensus treats differential growth as the primary driver: when the cortical layer expands more rapidly than the underlying white matter, the surface buckles, much like skin wrinkling on a drying fruit.
Axonal fibers play a secondary but important role. Rather than causing the folds outright, the stiff bundles of axons beneath the cortex help determine where gyri and sulci land. Computational modeling paired with anatomical data shows that regions with dense, stiff fiber bundles tend to get pulled toward the crests of gyri, while sulci form in areas with lower fiber density. Quantitative work finds that roughly three-quarters of fiber bundles end up in gyri and their walls, with fiber density on gyral crests about three times higher than in the depths of sulci.3Proceedings of the National Academy of Sciences. Role of axonal fibers in the cortical folding patterns: A tale of variability and regularity – Section: 3. Results and discussions So differential growth creates the instability that makes folding happen, and the spatial layout of axon bundles steers the pattern, determining which spots become ridges and which become grooves.
The Cells That Drive Cortical Expansion
At the cellular level, gyrencephaly depends heavily on a population of neural progenitor cells called outer radial glia. These cells live in a region of the developing brain known as the outer subventricular zone. In species with highly folded brains, this zone is large and teeming with actively dividing progenitors; in lissencephalic species like mice, it is sparse. Experiments in mouse models have shown that when you expand the population of outer radial glia artificially, the normally smooth mouse brain begins to develop folds.4Proceedings of the National Academy of Sciences. A kinase-independent function of cyclin-dependent kinase 6 promotes outer radial glia expansion and neocortical folding Conversely, when a genetic manipulation depletes outer radial glia in a model that normally produces folds, folding disappears. This back-and-forth evidence makes a strong case that outer radial glia are not just correlated with gyrencephaly but are necessary for it.
Several genes that boost outer radial glia numbers have been identified, and some are exclusive to primates or even to the great apes and humans. One, called TBC1D3, is found only in hominoids. When researchers introduced it into the developing mouse brain, ventricular progenitor cells detached and transformed into cells resembling outer radial glia, and the cortex began to fold in the areas where those cells proliferated.5eLife. The hominoid-specific gene TBC1D3 promotes generation of basal neural progenitors and induces cortical folding in mice Blocking TBC1D3 in cultured human brain tissue reduced outer radial glia production, confirming the gene works in both directions. Another primate-specific gene, TMEM14B, produces a similar effect: expressing it in mouse embryos triggers cortical thickening and gyrification, with proportional increases across all cortical layers and normal layering preserved.6PubMed. The Primate-Specific Gene TMEM14B Marks Outer Radial Glia Cells and Promotes Cortical Expansion and Folding
Beyond individual genes, broader genomic analysis has revealed that DNA sequences that evolved specifically on the human lineage tend to regulate genes active in the cell types and cortical layers responsible for expansion and gyrification.7Nature Communications. Human evolved regulatory elements modulate genes involved in cortical expansion and neurodevelopmental disease susceptibility This suggests that gyrencephaly in humans was not built by one or two dramatic mutations but by a web of regulatory tweaks that converge on a shared outcome: more progenitor cells, more cortical surface, more folds.
Which Animals Have the Most Folded Brains
Gyrencephaly is widespread among mammals, but the degree of folding varies enormously. Researchers measure it with a gyrification index, which compares the total cortical surface area (including the buried walls and floors of sulci) to the smooth outer contour of the brain. A perfectly smooth brain scores close to 1.0; the more folded the cortex, the higher the number.
By this metric, the most gyrencephalic mammals studied so far are cetaceans: dolphins, porpoises, and whales. A quantitative comparison across multiple cetacean species found an average gyrification index of about 5.4, which is higher than what would be predicted from their brain mass when compared to other mammals. Strikingly, the gyrification index stayed nearly constant across cetacean species even though brain mass ranged from roughly 577 grams to over 5,600 grams.8Journal of Comparative Neurology. Quantitative analysis of neocortical gyrencephaly in African elephants (Loxodonta africana) and six species of cetaceans: Comparison with other mammals In most mammalian lineages, bigger brains tend to be more folded. Cetaceans break that pattern, maintaining extreme gyrification regardless of size, which makes them genuine neuroanatomical outliers.
Cetacean brains are also organized differently from primate brains. In primates, the sensory and motor cortices are widely separated by broad regions of association cortex, the areas involved in higher-order processing. In cetaceans, the primary sensory cortices stay packed closer together along the inner wall of the hemisphere, and do not appear to spread across the outer surface the way they do in primates.9Brain Research Bulletin. The anatomy of the brain of the bottlenose dolphin (Tursiops truncatus). Surface configurations of the telencephalon of the bottlenose dolphin with comparative anatomical observations in four other cetacean species This means the folding pattern in a dolphin brain is not simply a scaled-up version of the folding pattern in a monkey or human brain. The underlying cortical map is arranged differently, so the folds serve a somewhat different organizational logic.
On the other end of the spectrum, some mammals have secondarily lost their cortical folds. Small rodents like mice and rats are lissencephalic, but comparative analysis suggests that their smooth cortices are a derived condition, meaning their ancestors were gyrencephalic and the folds were lost over evolutionary time as brain size shrank.10Frontiers in Neuroanatomy. The secondary loss of gyrencephaly as an example of evolutionary phenotypical reversal This makes gyrencephaly the ancestral state for mammals as a whole, not an innovation that appeared independently in only the biggest-brained lineages.
How Folds Develop in the Human Fetus
The human brain starts out smooth. At around 22 weeks of gestation, the cortical surface is essentially lissencephalic. Over the next 16 weeks, folds appear in a remarkably predictable sequence. Prenatal MRI studies have mapped this timetable, identifying when each primary and secondary sulcus becomes visible between 22 and 38 weeks of gestational age.11American Journal of Neuroradiology. Fetal Cerebral Cortex: Normal Gestational Landmarks Identified Using Prenatal MR Imaging The earliest folds, like the Sylvian fissure, appear first. Secondary and tertiary folds fill in later, producing the increasingly crumpled appearance familiar from any photograph of an adult brain.
The consistency of this timetable matters clinically. When a prenatal scan shows that folds are delayed or missing at a gestational age when they should be present, it can be an early sign of a cortical malformation. This brings us to what happens when the machinery of gyrification goes wrong.
When Folding Fails
Lissencephaly, the pathological absence of normal cortical folds, is the most dramatic failure of gyrification. The name literally means “smooth brain.” In type I lissencephaly, the cortex is abnormally thick and either completely smooth (agyria) or has only a few broad, shallow folds (pachygyria). The underlying problem is a failure of neuronal migration: young neurons that should travel from the deep zones where they are born to their correct positions in the cortical layers get stuck partway through the journey.12PubMed Central. Role of cytoskeletal abnormalities in the neuropathology and pathophysiology of type I lissencephaly Mutations in at least seven genes, including LIS1, DCX, ARX, TUBA1A, VLDLR, RELN, and WDR62, have been linked to the condition, each disrupting different aspects of the cytoskeletal machinery that neurons use to move.13PubMed. Neuronal migration disorders A more recently identified variant in the gene BAIAP2 also disrupts neuronal migration and causes lissencephaly.14Development. A lissencephaly-associated BAIAP2 variant causes defects in neuronal migration during brain development Children with lissencephaly typically have severe neurological impairment, including intellectual disability, motor deficits, and difficult-to-control seizures.
Polymicrogyria represents a different kind of folding error: too many folds, not too few, and the folds are abnormally small. It is one of the most common malformations of cortical development, and its appearance on brain imaging can vary considerably, suggesting that multiple different developmental insults can produce it.15PubMed Central. Current concepts of polymicrogyria The pathology typically involves disruption of the brain surface during development, with breaks in the pial membrane (the thin tissue covering the cortex), over-migration of cells beyond their intended destinations, and thickening of the collagen layers surrounding the brain.16PubMed Central. Polymicrogyria: pathology, fetal origins and mechanisms
Population-level data on polymicrogyria give a sense of its clinical weight. In a cohort study, genetic testing identified a disease-causing variant in about a third of patients tested, while another 12 percent had variants of uncertain significance. Roughly half of affected individuals developed epilepsy, with seizure onset during the first year of life in nearly half of those. Over half of the patients with epilepsy required more than two anti-seizure medications, meaning their seizures were resistant to standard treatment. Neurodevelopmental symptoms were present in 94 percent of individuals.17Brain Communications. Polymicrogyria: epidemiology, imaging, and clinical aspects in a population-based cohort – Section: Results These numbers underscore that both too little and too much folding are associated with serious consequences.
Brain Size, Sex, and the Scaling of Folds
Among healthy humans, the degree of cortical folding does not stay constant across different brain sizes. As total brain volume increases, the folding index rises disproportionately, driven by a dramatic expansion of sulcal area. Sulcal area grows faster than brain size would predict, while the outer hull area of the brain grows almost proportionally. Digging deeper into the geometry, the extra sulcal area comes mainly from sulci getting longer rather than deeper: sulcal length scales faster than expected with brain volume, while sulcal depth actually grows a bit slower than expected.18Cerebral Cortex. Influences of Brain Size, Sex, and Sex Chromosome Complement on the Architecture of Human Cortical Folding – Section: Results
This has a practical implication for comparing brains across individuals: you cannot simply compare raw folding measurements between a smaller and a larger brain without accounting for the scaling relationship. A larger brain is expected to be more folded, so finding more gyrification in a bigger brain does not by itself mean anything unusual is going on. Researchers need to separate the effect of size from whatever biological signal they are interested in, whether that is a disease marker, a sex difference, or an age-related change.
Preterm Birth and Disrupted Folding
Babies born prematurely miss the final weeks of gestation when cortical folding is progressing most rapidly. The consequences are visible on brain scans. At what would have been full term, preterm infants show lower overall gyrification than their full-term peers, along with reduced cortical surface area. In one study, the gyrification index in preterm infants averaged about 1.80 compared to 2.06 in controls, and the regions hit hardest were the insula, the superior temporal sulcus, and the ventral portions of the motor and sensory cortex.19PubMed Central. Regional impairments of cortical folding in premature infants
These differences are not just a temporary delay. Follow-up imaging into early childhood shows that preterm children still have reduced local gyrification and shallower sulci in specific regions, particularly the bilateral superior temporal gyrus and the left superior frontal gyrus, compared with children born at term.20Brain Communications. Long-term effects of preterm birth on cortical folding trajectories in early childhood – Section: Results Even premature infants without any obvious neurological injury can show subtle deviations in folding patterns when analyzed with sensitive spatial and spectral methods.21PubMed. The dynamics of cortical folding waves and prematurity-related deviations revealed by spatial and spectral analysis of gyrification Researchers are interested in whether these persistent folding differences contribute to the cognitive and behavioral difficulties that many preterm children experience later in life, though establishing a direct causal link is challenging.
What Happens to Folds as You Age
Cortical folding does not remain stable across the lifespan. Starting in early adulthood, gyrification gradually declines, on the order of about 0.04 gyrification index points per decade. The change is global but most pronounced in the parietal lobe. The mechanism behind it is largely the shallowing of sulci, which accounts for the majority of the variability in age-related gyrification changes, with sulcal widening playing a smaller but measurable role.22bioRxiv. Age-related decrements in cortical gyrification: Evidence from an accelerated longitudinal dataset – Section: Results
In Alzheimer’s disease, this normal age-related decline appears to accelerate. Patients with even mild Alzheimer’s show lower global gyrification and wider sulci than age-matched controls, and the degree of gyrification loss tracks with cognitive decline as measured by standard clinical assessments.23PubMed Central. Cortical gyrification and sulcal spans in early stage Alzheimer’s disease Separate work using fractal-based measures of cortical folding complexity has confirmed that the pattern of folding decline in Alzheimer’s disease resembles an exaggerated version of healthy aging, with reductions in cortical thickness and surface area compounding the loss of folding.24Cerebral Cortex. Cortical folding correlates to aging and Alzheimer’s Disease’s cognitive and CSF biomarkers – Section: Results This has raised interest in gyrification metrics as potential early biomarkers for neurodegeneration, since the changes appear to be detectable even when cognitive symptoms are still subtle.
Gyrification and Neurodevelopmental Conditions
Beyond the severe malformations of lissencephaly and polymicrogyria, subtler variations in cortical folding have been linked to neurodevelopmental conditions. In autism spectrum disorder, for instance, a study of monozygotic twin pairs found that children with autism and their co-twins showed increased cortical folding in the right parietal lobe compared with typically developing children. Greater folding in that region correlated with more pronounced symptoms of autism among the co-twins.25Autism Research. Gyrification patterns in monozygotic twin pairs varying in discordance for autism The finding also suggested that the normal relationship between cortical folding and intelligence might be disrupted in autism, potentially contributing to the deficits in visual-spatial attention and social cognition that characterize the condition.
Findings like these are preliminary and based on small samples, so they should be taken as signposts rather than settled science. But they illustrate a broader point: gyrification is not just a structural curiosity. It reflects the developmental history of the cortex, and deviations from normal folding patterns can serve as windows into what went differently during brain development. Researchers increasingly treat gyrification measures as biomarkers that complement traditional measures like cortical thickness and brain volume.
The Microstructure Inside Folds
The distinction between a gyral crest and a sulcal depth is not just geometric. The tissue inside folds has a different microstructural character depending on its position. In the rhesus monkey, a detailed survey found roughly 1.5 times more white matter neurons at gyral crowns than at sulcal depths, with gyral density averaging about 75 neurons per counting frame compared to about 47 in sulci. This difference held across multiple brain regions, though absolute densities varied by area, being lowest in the superior frontal gyrus and highest in temporal cortex.26Frontiers in Neuroanatomy. A Survey of White Matter Neurons at the Gyral Crowns and Sulcal Depths in the Rhesus Monkey – Section: Results
Combined with the fiber-density differences mentioned earlier, this means that the ridges and grooves of a gyrencephalic brain are not interchangeable real estate. Gyri are structurally richer in both connective fibers and certain neuron populations. This asymmetry likely matters for how signals travel through the cortex, though exactly how it shapes function at the level of circuits and behavior is an active area of investigation.
The Ferret as a Folding Model
Studying cortical folding in humans is limited by what you can do with imaging alone. Mice are easy to experiment on but have smooth brains, so they are a poor model for gyrification unless genetically modified. The ferret fills this gap. Ferrets are naturally gyrencephalic, with a pattern of cortical folds that develops postnatally, making it accessible for experimental manipulation. Many of the genes associated with human cortical malformations produce comparable folding defects in ferrets, which makes the species especially useful for testing hypotheses about what goes wrong in disorders like lissencephaly and polymicrogyria.27PubMed Central. Biophysical basis for brain folding and misfolding patterns in ferrets and humans
Researchers have built both physical gel models and computer simulations of ferret brain gyrification, using MRI data to set the starting geometry and then letting growth mechanics play out. These models can reproduce normal folding patterns and, when parameters are altered to mimic genetic mutations, they generate the kinds of abnormal folding seen in human patients. This cross-species, cross-method approach is gradually turning cortical folding from a descriptive phenomenon into something that can be modeled, predicted, and potentially intervened upon during development.