Gyrus formation in the cerebral cortex of the ferret. II. Description of the internal histological changes.

The internal changes within a developing gyrus of the ferret cerebral cortex were studied by recording (i) the changing length and direction of the radial tissue lines and (ii) the emergence of the tangential banding of the classical six cortical layers. Together these lines provided a coordinate net whose deformations during development gave an indication of the differential growth occurring within a gyrus. The changes in these features suggested that a gyrus was initiated by an area of local growth appearing in the subplate and then in the suprajacent segment of cortical plate. During subsequent growth there was tangential spreading of the more mature tissue at the gyral crown while at the site of the future sulci the cortical plate remained immature and growth was retarded. During later stages the majority of tangential growth occurred in the parasulcal area. At this site a very much thinner cortex was generated from a segment of cortical plate of the same depth and degree of nuclear crowding as elsewhere, implying that growth here was resolved into tangential spreading. The cells and fibres of the deeper cortical layers of the sulcal cortex eventually became tangentially orientated suggesting that they subserved a commissural function between the columnar systems of adjacent gyri. At the scale prevailing in the ferret, gyrus formation was seen as a configuration which tended to conserve both the total length of the cortical columns and the depth of the individual cortical layers.

Gyrus formation in the cerebral cortex in the ferret. I. Description of the external changes.

The external features of gyrus formation in the postnatal ferret cerebral cortex are described and correlated with certain internal changes. The observations indicate that gyri are formed by longitudinal and radial expansion of the cortical compartment occurring between relatively fixed areas which form the sulcal floors. The gyri were initially rounded with open sulci and the cerebrum had a rectangular outline when seen in lateral and dorsal view. By adult life the hemisphere had been subjected to considerable moulding by the growing skull, so that the frontal pole of the cerebrum became pointed while the sulcal walls became closely opposed and the gyral crowns flattened.

Cell production gradients in the developing ferret isocortex.

The accumulation of cells within the cortical plate was studied in ferrets at two developmental ages. A survey method based on the presumed radial organisation of cortical neuron production was used to sample variations in cell production along the rostrocaudal and laterodorsal axes of the brain. The resulting cell counts confirmed the presence of a gradient of cortical plate formation, with a rostrolateral focus. These findings were discussed in relation to some recent teratological studies on brain development in ferrets, where there has been a lack of normative data.

Mapping of cortical histogenesis in the ferret.

The columnar organization of forebrain cortical neuron production allows the neuroepithelium to be analysed in terms of a simple growth model. The pattern of neuron release within each column is repeated with great regularity across the surface of the developing cerebral hemisphere, resulting in the accumulation of cortical neurons in radial stacks above the proliferative ventricular epithelium. The time course of recruitment of adjacent tissue into neuron production accounts for the observed rostrocaudal and laterodorsal gradients of cortical neuron release. The present study plots the history of this front of neuron production across the surface of the developing cerebral vesicle in the ferret and interprets the resulting maps of cortical development stages in terms of a unified growth model that links the radial and tangential aspects of neuron production. These observations are discussed in relation to other studies of gradients in developing nervous tissue and their implications for the study of experimentally induced abnormalities of brain development.

Growth patterns in the lateral wall of the mouse telencephalon. II. Histological changes during and subsequent to the period of isocortical neuron production.

The histogenesis of the isocortical segment of the lateral telencephalic wall at the coronal level of the interventricular foramen was studied in mice between the ages of E10 and the adult. The proliferative activity of the periventricular germinal layers was correlated with changes in cell distributions in the intermediate layer. The appearances were consistent with a wave of differentiation moving across the ventricular layer from lateral to medial and a peak of neuron production occurring about E13. The sequence of changes was analysed using the concept of a radial unit composed of ventricular cells and their related progeny of neurons. The observed histological changes were interpreted as the result of radial units of similar productive history entering and completing the histogenetic sequence at successively later times along a lateromedial gradient. Some of the implications of this approach were examined and discussed in relation to the general evolutionary properties of such a system of histogenesis.