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

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