Precision in the development of neocortical architecture: From progenitors to cortical networks

Abstract

Of all brain regions, the 6-layered neocortex has undergone the most dramatic changes in size and complexity during mammalian brain evolution. These changes, occurring in the context of a conserved set of organizational features that emerge through stereotypical developmental processes, are considered responsible for the cognitive capacities and sensory specializations represented within the mammalian clade. The modern experimental era of developmental neurobiology, spanning 6 decades, has deciphered a number of mechanisms responsible for producing the diversity of cortical neuron types, their precise connectivity and the role of gene by environment interactions. Here, experiments providing insight into the development of cortical projection neuron differentiation and connectivity are reviewed. This current perspective integrates discussion of classic studies and new findings, based on recent technical advances, to highlight an improved understanding of the neuronal complexity and precise connectivity of cortical circuitry. These descriptive advances bring new opportunities for studies related to the developmental origins of cortical circuits that will, in turn, improve the prospects of identifying pathogenic targets of neurodevelopmental disorders.

Keywords: Axon guidance; Cell type specification; Cerebral cortex; Circuits; Connectivity; Excitatory neurons; Experience; Genetics; Histogenesis; Human; Microcircuit; Neural network; Refinement; Reprogramming; Rodent; Specification; Synaptic specificity; Synaptogenesis; Thalamus.

Figure 1.. Patterning the cortical area map.

Illustrations of the dorsal surface of the developing mouse brain at different pre and postnatal stages. The first stage depicts the morphogen signals arising from the rostral patterning center (purple, Fgf8 and Fgf17) and cortical hem (green, Bmp and Wnt) beginning around embryonic day 9 in mice (corresponds approximately to human gestational week 6 or 7). These morphogens induce gradients of transcription factor (TF) expression, including Sp8, Pax6, Emx2 and Couptf1. The transcriptional programs regulated by these transcription factors establish cortical fields and the guidance cues that attract area-specific thalamic innervation. The innervation of cortex by thalamic axons, which happens postnatally in rodents (this occurs in the second and third trimester of human pregnancy), drives the sharpening of molecular and cytoarchitectural boundaries between primary sensory cortex (e.g. V1, dark blue) and adjacent higher order cortical areas (e.g. VHO, light blue).

Figure 2.. Cortical projection neuron diversity.

There are three primary classes of glutamatergic projection neurons in the cerebral cortex. Each class has a unique laminar distribution pattern. The corticothalamic neurons (CT, magenta) are located mostly within layer 6 and send axons to the thalamus and a narrow radial domain of the cortical column proximal to their cell bodies. The pyramidal tract neurons (PT, yellow) are positioned almost exclusively within layer 5B. These neurons project to the brainstem and spinal cord, and many issue colateral axons to other subcortical targets such as the thalamus. In contrast to the restricted laminar distribution of the first two classes, the intratelencephalic neurons (IT, green), which project axons only within the telecephalon, are distributed throughout all six layers. As noted in the text, these primary classes are divisible into secondary taxa, but consensus regarding more refined cell classes awaits further multi-dimensional, integrative analysis.

Figure 3.. Transcriptional control of cortical projection neuron specification and wiring.

Several transcription factors that contribute to the differentiation of the three first-order classes of cortical projection neurons (CT, PT, and IT neurons) have been identified. Connections made by these cortical neurons are depicted in the context of the wild-type (WT) mouse cortex, along with the associated connectivity changes caused by the mutation (or ectopic overexpression) of these developmentally important transcription factors (please see text for references; knockout mutations are denoted by the gene symbol followed by −/−, e.g. Fezf2−/−). Fezf2 is a PT neuron selector gene that regulates the expression of many functionally important genes. When Fezf2 is deleted, the cortex no longer sends projections to the spinal cord, but, instead, the PT neurons upregulate genes that promote CT and IT neuron phenotypes. Accordingly, these mutant PT neurons send ectopic projections to the thalamus or across the corpus callosum. Ctip2 also contributes to the development of PT-type neurons, as projections from the cortex to the spinal cord are disrupted in Ctip2−/− mice. Tbr1 promotes the development of CT neurons, as evidenced by the fate-conversion of CT neurons into PT neurons in Tbr1−/− mice. In Tbr1−/− mutant mice, CT neurons upregulate Fezf2 and project toward the brainstem and spinal cord. Satb2 is a critical regulator of IT neurons, as Satb2−/− mice do not send axons through the corpus callosum to the contralateral hemisphere. Instead, upper layer neurons upregulate Ctip2 and project subcortically. When Fezf2 is ectopically expressed in layer IV IT neurons, these neurons are reprogrammed into PT neurons; they adopt several molecular and connectivity phenotypes that are characteristic of PT neurons, but normally excluded from layer IV IT neurons. Red, strikethrough font indicates the loss of projections from the cortex to the indicated structure (e.g. Fezf2−/− mice lose projects from cortex to spinal cord).

Figure 4.. The temporal development of local coritcal microcircuity.

In mice, thalamocortical (TC) axons (orange) begin to innervate the developing cortex around birth (P0) before superficial layer neurons (blue and green) have finished migrating (this begins around the 12^th week of gestation in humans). Radial migration concludes around P4, in mice (in humans the six layers of cortex are fully distinguishable by the 28^th week of gestation), a timepoint at which immature synapses between TC axons and layer IV neurons are present. These TC to layer IV synapses mature through AMPA-receptor insertion between P4 and P8 (denoted by thickening of orange lines, and appearance of arrowheads at P8; increases in TC innervation of the human cortical plate continues between the 24^th and 30^th week of gestation). Meanwhile immature connections between layer IV and layer II/III, and between layer II/III and layer V begin to develop. These later developing synapsese mature between P8 and P16 (denoted by thickening of green and blue lines, and appearance of arrowheads; in humans, these later processes occur from approximately the 32^nd week of gestation through several months of postnatal development). MZ, marginal zone; CP, cortical plate; WM, white matter.

Figure 5.. Development of intracortical association networks of the rodent neocortex.

Recent graph-theoretical meta-analyses of cortical association connections in rats (Swanson et al., 2017) and comprehensive tracing of homologous connecitons in mice (Zingg et al., 2014) suggest a core-shell arrangement of cortical connections that consists of at least 3 networks distinguished by increased within network interconnectivity (white arrows). The core (orange) is surrounded by the shell, which is comprised of medial (blue) and lateral portions (green). Importantly, connections between the three distinct networks are common, but tend to be less pronounced, with the exception of those relayed through hub regions such as the entorhinal, temporal association, and posterior parietal cortices (between network connections denoted by black arrows). B) Restricted and selective axonal growth of developing intracortical neurons into the cortical targets that receive input from those neurons in adulthood. The green circles (green light beneath dimmed yellow and red circles) represent the presence of growth permissive or growth promoting signals for appropriate axons, whereas the red octagons (stop signs) indicate the absence of growth promoting signals for some axons or potential presence of axonal growth inhibiting signals. C) Widespread developmental axon outgrowth of intracortical projection neurons, followed by refinement of connectivity maps through selective axonal pruning.