Fig. 6.

Gyrification occurs during the period of greatest increase in cerebral cortical volume. (A) The volume of ferret brain structures (cortex, subcortical structures, entire brain) shows the greatest increase between the 2nd and 4th weeks of postnatal development. (B) Magnified view of the inset in (A) shows that the cortical volume actually surpasses the volume of the subcortical structures by the fourth postnatal day (asterisk). This time frame corresponds to the progressive migration of neurons from the ventricular zone to the cortical plate. (C) The cortical growth rate exceeds both the growth rate of the total brain and its underlying structures (remaining volume) in the first postnatal week (asterisk), consistent with expansion of the cortical plate. Thereafter, the cortex undergoes a sustained period of growth between the second and fourth weeks of growth. (D) Gyrification index indicates that gyrification advances most rapidly during the third postnatal week, consistent with the period of most rapid cortical growth. Subsequently, the gyral formation plateaus until adulthood. Image displays a manually traced P28 coronal section used to calculate GI.