Tag Archives: gyrification

Primate brain folding

A recent work, analyzing the development and evolution of the primate cortical folding, evidences two separate folding processes of the neocortex. Namba and colleagues examine two subtypes of neocortex, the dorsal isocortex, defined as the portion of neocortex limited laterally by the lateral fissure (LF) and medially by the cingulate sulcus (CiS), and the proisocortex within these same boundaries, comprising the insular cortex and the cingulate cortex, respectively. Their sample was composed by three to five specimens from 13 different primate species, including two great apes (humans and chimpanzees), two Old World monkeys, four New World monkeys, one tarsier, one galago, and three lemurs. For each specimen they analyzed five comparable coronal sections. They measured the gyrencephalic index (GI) as the ratio of the length of the inner to outer cortex contours, and calculated the LF score as the ratio of the LF greatest depth to the maximum width of the hemisphere. Then they analyze the relationships between LF score, and the GI values for the dorsal isocortex, the cingulate and insular cortices, and the neocortex, which included the three cortices and the temporal isocortex. To provide an evolutionary perspective, they reconstruct GI and LF score values for the primate ancestors. Furthermore, they examine the timing of folding in a longitudinal sample of humans and long-tailed macaques, and the folding differences in human lissencephalic patients, for a developmental and genetic overview. The proisocortex showed a similar GI across the sample, and the reconstructed ancestor LF scores were in the same magnitude as those of the 13 present-day primates. Conversely, the degree of folding of the dorsal isocortex differs across the species, and between the ancestors and the extant species, increasing with increasing folding of the neocortex. Furthermore, their analyses also revealed that  LF and CiS appear earlier in development, while the dorsal isocortex starts to fold later. This portion of the cortex is also the most affected in human lissencephaly, as the LF, and CiS to a lesser degree, are still detectable in all grades of malformation. Hence, the authors conceptualize the folding of the neocortex as two distinct and sequential processes. The conserved folding occurs earlier in development, at the boundaries between the proisocortex and the dorsal isocortex, and involves the formation of the LF and CiS. The evolved folding occurs within the isocortex, after the onset of the conserved folding. Moreover, the authors suggest these two processes might be influenced by different cellular mechanisms, with neuron production contributing more to the conserved folding, and neuron migration to the evolved folding, a matter deserving further investigation.


Sofia Pedro


Brain gyrification and simulations

The advantages of brain gyrification are well established, but the mechanisms behind this process are yet matter of discussion. However, early this month, a group of researchers published two models for brain gyrification based on the mechanical stress generated by the differential growth of the cortical layer. They created a physical model of a brain in three steps: (1) 3D printing a plastic replica from a MRI of a smooth fetal brain; (2) build a silicon negative mould to cast the core of a gel brain, which would represent the white matter; and after cooled, (3)  deposited the same gel polymer in the surface of the core to form the cortical layer. These polymer layers act as elastic solids. The mimicking of fetal brain growth was accomplished by placing the gel brain in a substrate of hexanes that would cause swelling and differential growth of the outer cortical layer, in respect to the core of the model. Starting from the same MRI, they also built a numerical model based on finite element and parameters like cortical thickness, brain growth and tissue stiffness, creating functions for folding and unfolding simulations. The combined results of the physical and numerical simulations showed that the pattern of gyrification depends on the overall shape of the brain, and the primary sulci are formed perpendicularly to the largest compressive stress. Their models are robust and reproducible, capturing the main gyral scheme and even account for variability and hemispherical differences. Furthermore, when comparing the simulated brain to a real one, they were able to find a correspondence with all the primary folds.

Sofia Pedro

Cortical morphology and ancestry

Fan et al 2015_2A recent study published by a large neuroscience team involved in imaging, cognition, and genetics, showed that cortical geometry is correlated with genetic ancestry. Using a sample of US citizens from the PING data, they reconstructed 3D cortical surfaces to obtain information on the morphological variation of the sulci and gyri. To calculate proportions of genetic ancestry they used as reference populations from west Africa, east Asia, a sample of native Americans and a sample of European descendants. The main finding of the study is that cortical folding patterns are strongly related to the genetic ancestry. According to the authors, African ancestry is associated with more posterior and narrower temporal areas. Frontal and occipital surfaces are more projected in Europeans and flatter in Native Americans. Asians have more variability in the temporal and parietal regions. Their results were similar to  Howells’ craniometric analysis. Moreover, all but Europeans display increased morphological variation in the posterolateral-temporal region. Due to these  morphological differences among populations, the authors warn for a possible methodological bias when mixing sample from different geographical origins in imaging studies.

Sofia Pedro