Corticogenesis and folding process of the neopallium in the South American plains vizcacha, Lagostomus maximus

Abstract

The plains vizcacha, Lagostomus maximus, is a precocial hystricomorph rodent with agyrencephalic brain. This work aimed to perform a time-lapse analysis of the embry-onic brain cortical development in the plains vizcacha to establish a species-specifictemporal window for corticogenesis and the gyrencephaly onset. Additionally, a com-parative examination with evolutionarily related rodents was conducted. Embryosfrom 40 embryonic days (ED) until the end of pregnancy (∼154 ED) were evalu-ated. The neuroanatomical examination determined transverse sulci at 80 ED androstral lateral and caudal intraparietal sulci around 95 ED. Histological examinationof corticogenesis showed emergence of the subplate at 43 ED and expansion of thesubventricular zone (SVZ) and its division into inner and outer SVZs around 54 ED.The neocortical layers formation followed an inside-to-outside spatiotemporal gradi-ent beginning with the emergence of layers VI and V at 68 ED and establishing thefinal six neocortical layers around 100 ED. A progressive increment of gyrencephal-ization index (GI) from 1.005 ± 0.003 around 70 ED, which reflects a smooth cortex,up to 1.07 ± 0.009 at the end of gestation, reflecting a gyrencephalic neuroanatomy,was determined. Contrarily, the minimum cortical thickness (MCT) progressivelydecreased from 61 ED up to the end of gestation. These results show that the decreasein the cortical thickness, which enables the onset of neocortical invaginations, occurstogether with the expansion and subdivision of the SVZ. The temporal comparison ofcorticogenesis in plains vizcacha with that in relative species reflects a prenatal longprocess compared with other rodents that may give an evolutionary advantage to L.maximus as a precocial species.