Formation and growth of the cerebral convolutions. I. Postnatal development of the median-suprasylvian gyrus and adjoining sulci in the cat.

The postnatal development of these median-suprasylvian gyrus and adjoining sulci was studied in cats 1, 5, 15, 25 days and six months old. The median-suprasylvian gyrus (G.S.-Syl.) grows according to a lateral to medial intracortical gradient in which the adjoining sulci, sulcus lateralis (S.L.) and median-suprasylvian sulcus (S.S-Syl.), are considered to be fixed zones because of their relatively constant distance from the ventricular wall throughout the development. Thus the formation of the S.L. is a consequence of the increase in volume of the gyral region adjacent to this developing sulcus, whereas there is a smaller increase in volume of the gyral region adjacent to the almost fully formed, at birth, S.S-Syl. This increase in volume is associated with a regional increase in the number of nerve cells and with an increase in the density of neurons in the region adjacent to the S.L. as it fades in the region adjacent to the S.S-Syl. This process takes place from Day 1 until about Day 25 of postnatal life. An intralaminar displacement of nerve cells also occurs during the process of cortical folding:nerve cell columns converge towards the hilum in the gyral region, but the columns progressively curve following the concavity of the infolding in the sulcal zones; as a result, although the length of nerve cell columns tends to be preserved to some extent along the gyrus, the cerebral cortex is progressively thinner in the sulci than in the gyri and the molecular layer is progressively thicker in the former than in the latter. This process also occurs following a lateral to medial gradient in the G.S.-Syl. The present observations may suggest that cortical folding is largely dependent on intracortical mechanical forces but the regular distribution of the sulci, together with the orderly spatio-temporal pattern of gyral growth, points to the conclusion that this process may be controlled by extracortical signals.