Evolution, development, and initial function of the mammalian neocortex: response of the germinal zones to endothermy.

In the mouse the release of neocortical neurons from the periventricular germinal layers of the forebrain commences towards the ventral margin of the lateral pallium at the level of the interventricular foramen and is propagated from there across the lateral wall of the hemisphere. In the adult cortex the origin of the gradient corresponded to the ventral portion of the somatotopic map of the body, that is, to the area representating structures derived from the embryonic branchial arches, namely, the peri-oral region and laryngo-pharyngeal masticatory apparatus. Branchial arch nerves also innervate the fore- and mid-gut and all the related exocrine and endocrine glands. This suggests that the mammalian neocortex evolved from a visceral integration area in a positionally equivalent area in the pallium of a reptilian ancestor which expanded in relation to extensive changes taking place in the visceral and branchial systems of the body during the transition from reptilian ectothermy to mammalian endothermy. The practical problem facing early mammals was to acquire and process the extra energy required to sustain a continuously high metabolic rate. Improvements to the food processing capabilities of the visceral and branchial systems and the expansion of their neural control were important components in the conglomerate of changes required to sustain the increased energy demands of endothermic tissues. Endothermy also bestowed the ability to sustain greater numbers of metabolically expensive neurons and this, in turn, required an appropriate response from the cell production mechanisms in the periventricular germinal layers.

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.

Differential growth of the cell production systems in the lateral wall of the developing mouse telencephalon.

Three major cell production systems were identified in the lateral wall of the mouse telencephalon. These were morphologically evident as the medial and lateral elevations and the pallial crescent. Each was originally derived from a small, circumscribed, subset of ventricular cells. These formed the parent populations of large and proliferatively complex precursor pools which gave rise to the large numbers and considerable variety of neuron populations of the telencephalon. An attempt was made to identify the major neuron groups derived from each system by using the ventricular cell processes as a guide to the site of neuron origin. The proliferative changes occurring in the two elevations, the variety of their neuronal output, the early loss of a radial structure and the diversity of the final adult configurations were considered to represent a more complex series of changes than the corresponding events in the cortical tissue, which was generated from the pallial crescent where neurons accumulate within an adaptation of the original radial structure. It was considered that the number and complexity of the changes in the genome required to produce and organise such subcortical diversity was of a greater order of magnitude than those evident in the cortical areas. It was suggested that the genesis of the mammalian basal telencephalon should be considered as one of the major evolutionary achievements in the conglomerate of changes which occurred during the transition from the reptilian to the mammalian grade of organisation in the forebrain.

A localised growth zone in the wall of the developing mouse telencephalon.

In sections of the prenatal mouse brain, sites of maximum area increase of the lateral ventricle were mapped onto reconstructions of the ventricular surface. This was done by identifying areas of ventricular layer where mitotic density was high and the adjacent intermediate layer either absent or thinly populated with neurons. It was assumed that in these areas, cell division was producing ventricular cells rather than neurons and that they were therefore gaining in area, whereas sites against which neurons were accumulating were either ceasing to increase in area or at least were increasing more slowly. Such an area occupied a zone at the junction between the medial and lateral telencephalic walls. The zone was eliminated during development in a rostrocaudal direction. It is suggested that modulation of growth along this zone may be an important factor in fashioning the form of the ventricular cavity.

Histogenesis of the mesocortical area of the mouse telencephalon.

The histogenesis of the mesocortex of the mouse telencephalon was studied by plotting the progress of neuron release on reconstructions of the medial pallial wall. Histological changes were correlated with cell birth date using data obtained from autoradiographs of mice pulse-labelled with tritiated thymidine during the prenatal period of neuron birth. It was found that neuron release for this area began rostrally at about E12-E13 and spread rapidly from this origin in a caudal direction across the medial wall. Neurons accumulating in the intermediate layer were at first more or less equally spaced. About E14, neurons in the outer intermediate layer began to line up to form a 'mesocortical plate'. This plate was formed from older nuclei and therefore overlay a deeper intermediate layer composed of younger cells. The mesocortex continued to develop by progressive withdrawal of younger cells from the deep intermediate layer into more superficial layers of the definitive cortex. In most of the mesocortical area, however, this original pattern was superseded by release of neuron generations which migrated directly to the outer intermediate layer to form a plate of densely packed immature neurons. This population was continuous with a similar population forming the isocortical plate of the lateral telencephalic wall. It was postulated that the wave of neuron birth and release which gave rise to the isocortical plate was propagated beyond the isocortical boundary into mesocortical territory as far as the boundaries of the subiculum, indusium griseum and anterior hippocampal rudiment.

Three dimensional growth of the mouse isocortex.

The three dimensional growth of the mouse isocortex was examined by plotting the variations in intermediate layer depth on orthogonal projections of the telencephalic surface at successive periods of development; a histological status was assigned to each depth. Thus portrayed, the development of the isocortex was seen as a propagated sequence of histological change, commencing at a rostral focus coextensive with the caudatopallial angle and thence spreading across the telencephalic wall. Growth was asymmetric about the focus of origin and terminated in a rostrocaudal direction as the spread of neuron production reached and extinguished a growth zone along the sagittal perimeter of the hemisphere. The possibility of mouse isocortical histogenesis representing a variation of a general mammalian pattern was noted, as was the evolutionary and methodological significance of the apparent coincidence of the origin of the gradient of isocortical neuron release with the region of cortex representing oropharyngeal structures. An alternative form of representation of the isocortical gradient, as the summation of a number of radial strips of tissue each with a similar history of neuron release and migration, was used to lay a foundation for a three dimensional model.

Radial unit analysis of hippocampal histogenesis in the mouse.

The technique of radial unit analysis was applied to the development of the hippocampal region in the mouse. A radial unit was defined as a transect through the neural wall of sufficient size to contain a representative sample of ventricular cells together with their product of neurons. This convention allowed growth to be resolved into two components: a radial component which was a function of the productivity of an individual unit and a tangential component which was a function of the number of participating units. The sequences of cell production, accumulation and dispersal in an average radial unit of the regio superior, regio inferior and dentate gyrus were worked out, and for each area a pattern of summation of such average units into the adult structure was suggested. The approach seemed to offer a useful method for the study of ontogenetic and phylogenetic change in this part of the medical telencephalic wall.

Growth patterns in the lateral wall of the mouse telencephalon: III. Studies of the chronologically ordered column hypothesis of isocortical histogenesis.

The vertical distribution of neurons of different birth dates in the mouse isocortex was measured and compared with the theoretical distributions of neurons accumulation in chronologically ordered columns. The agreement between observed and predicted results was close, so that, in spite of considerable scatter and overlap in the observed distribution of successive generations, the hypothesis that the isocortex is formed of columns of cells arranged from deep to superficial in chronological order of birth is still tenable. A computer model of a differentiation wave traveling across a proliferative system generating chronologically ordered columns was constructed and used to simulate the results of tritiated thymidine labelling experiments. Predicted and observed distributions were again close. The approach has been of value in pointing the way for future qualitative and quantitative studies.

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.

Growth patterns in the lateral wall of the mouse telencephalon: I. Autoradiographic studies of the histogenesis of the isocortex and adjacent areas.

The distribution of cells of different labelling intensities in the anterior forebrain of adult mice injected with tritiated thymidine at daily intervals during prenatal life was determined by mapping the location of labelled cells on enlarged photographs of autoradiographed sections. The isocortical arc was subdivided into an arbitrary number of radially orientated units. Each radial unit was found to have a similar sequence of arrival and distribution of labelled cells; the ventrolateral units, however, entered and completed the sequence ahead of dorsomedial units indicating the presence of a wave of differentiation spreading in this direction across the generative layers giving rise to cortical neurons. An attempt was made to identify (from differential grain counts) comparable samples of first and second generation cells produced after each pulse of labelled thymidine. The changing ratio between the two generations suggested that there may be two peaks in neuron birth during the generative period.

A morphological study of the mouse subependymal layer from embryonic life to old age.

The natural history of the subependymal layer around the lateral ventricle of the mouse brain was studied from its appearance at E11 up to 22 months postnatum. In the young embryo four regions of the ventricle can be recognized by their histological characteristics: (1) the ventricular roof, (2) the medial roof, (3) the ventricular elevations and (4) the medial wall. The characteristics of the ventricular roof and ventricular elevations were examined in detail. The ventricular roof appears to be the main site of production of cortical neurons while the subependymal layer of the ventricular elevations seems to be the main site of origin of forebrain glia. The age of differentiation of the ependyma differs for each region, with the medial roof differentiating first, followed by the ventricular roof and medial wall, and ventricular elevations or lateral wall last. Differentiation begins with a change from pseudostratified columnar epithelium to simple columnar epithelium and the appearance of cilia in large numbers.

The location of nuclei of different labelling intensities in autoradiographs of the anterior forebrain of postnatial mice injected with [3H]thymidine on the eleventh and twelfth days post-conception.

The location of neuron nuclei of different labelling intensities in autoradiographs of the anterior forebrain of two 22 day old mice which had been injected with [3H]thymidine at 11 and 12 days post-conception respectively was charted on photocollages of sections enlarges 175 times. The pattern of distribution of the heavily labelled nuclei, i.e. those nuclei belonging to cells most likely to have been born shortly after the time of [3H]thymidine injection, indicated that the inner two thirds of the neocortex is laid down along a ventro-dorsal gradient, i.e. the lateral neocortex starts to form before the dorsal; and that cells born at a particular time lie in cortical layer VI at the dorsal edge of the gradient is traced ventrally. Progressively more weakly labelled cells formed intermediate steps in this migration. A model or cortical growth fitting these findings is presented. Some inferences are also made about the possible role of the ganglionic eminences in providing cortical cells, at least during the initial stages of cortical histogenesis.

A pilot study of cell production by the ganglionic eminences of the developing mouse brain.

Cell production in the medial and lateral ganglionic eminences in pre- and post-natal mouse brains was studied by recording the number, location and plane of cleavage of mitotic figures. The site of maximum cell production shifted progressively from the ependymal layer to an adjacent sub-ependymal proliferative compartment. In the latter, mitosis occurred without the nuclei participating in the migratory movement to the ventricular surface which is characteristic of the nuclei of the ependymal compartment. The sub-ependymal compartment persisted vestigially into post-natal life as the well known sub-ependymal layer.