Diverse axonal morphologies of individual callosal projection neurons reveal new insights into brain connectivity

Traditions of Excellence: neuroanatomy at the forefront of the new era

Neuroanatomy is emerging from decades of neglect, where the synthetic approach, a neuroanatomical hallmark, was too often, as an aspersion, dismissed as "descriptive." The resurgence is in part driven by new technical advances (e.g., better visualization tools; larger sample sizes), many of which are briefly described here and treated in more detail elsewhere in this Special Issue. Another factor is the over-due recognition that a seemingly "descriptive" approach can be positively compatible with synthesis, integration, and conceptual formulation. The first two sections of this brief overview highlight advances in the level of microscopic visualization and the increasing availability of large data sets ("big data"). Illustrative examples of specific applications are drawn from the literature. A last section briefly discusses how neuroanatomy might be expected to develop further as the field continues to move forward.

Layer 6 corticocortical neurons are a major route for intra- and interhemispheric feedback

The neocortex comprises anatomically discrete yet interconnected areas that are symmetrically located across the two hemispheres. Determining the logic of these macrocircuits is necessary for understanding high level brain function. Here in mice, we have mapped the areal and laminar organization of the ipsi- and contralateral cortical projection onto the primary visual, somatosensory, and motor cortices. We find that although the ipsilateral hemisphere is the primary source of cortical input, there is substantial contralateral symmetry regarding the relative contribution and areal identity of input. Laminar analysis of these input areas show that excitatory Layer 6 corticocortical cells (L6 CCs) are a major projection pathway within and between the two hemispheres. Analysis of the relative contribution of inputs from supra- (feedforward) and infragranular (feedback) layers reveals that contra-hemispheric projections reflect a dominant feedback organization compared to their ipsi-cortical counterpart. The magnitude of the interhemispheric difference in hierarchy was largest for sensory and motor projection areas compared to frontal, medial, or lateral brain areas due to a proportional increase in input from L6 neurons. L6 CCs therefore not only mediate long-range cortical communication but also reflect its inherent feedback organization.

Orderly specification and precise laminar deployment of mouse cortical projection neuron types through intermediate progenitors

The cerebral cortex comprises diverse types of glutamatergic projection neurons (PNs) generated from radial glial progenitors (RGs) through either direct neurogenesis (dNG) or indirect neurogenesis (iNG) via intermediate progenitors (IPs). A foundational concept in corticogenesis is the "inside-out" model, whereby successive generations of PNs sequentially migrate first to deep and then progressively to more superficial layers. However, its biological significance remains unclear, and the role of iNG in this process is unknown. Using genetic strategies linking PN birth dating to projection mapping in mice, we found that the laminar deployment of IP-derived PNs substantially deviates from a stringent inside-out rule: PNs destined to non-consecutive layers are generated at the same time, and different PN types of the same layer are generated at non-contiguous times. The overarching scheme of iNG is the sequential specification and precise laminar deployment of projection-defined PN types, which may contribute to the orderly assembly of cortical output channels and processing streams.

Dimensionality reduction simplifies synaptic partner matching in an olfactory circuit

The distribution of postsynaptic partners in three-dimensional (3D) space presents complex choices for a navigating axon. Here, we discovered a dimensionality reduction principle in establishing the 3D glomerular map in the fly antennal lobe. Olfactory receptor neuron (ORN) axons first contact partner projection neuron (PN) dendrites at the 2D spherical surface of the antennal lobe during development, regardless of whether the adult glomeruli are at the surface or interior of the antennal lobe. Along the antennal lobe surface, axons of each ORN type take a specific 1D arc-shaped trajectory that precisely intersects with their partner PN dendrites. Altering axon trajectories compromises synaptic partner matching. Thus, a 3D search problem is reduced to 1D, which simplifies synaptic partner matching and may generalize to the wiring process of more complex brains.

Orderly specification and precise laminar deployment of cortical glutamatergic projection neuron types through intermediate progenitors

The cerebral cortex comprises diverse types of glutamatergic projection neurons (PNs) generated from radial glial progenitors (RGs) through either direct neurogenesis or indirect neurogenesis (iNG) via intermediate progenitors (IPs). A foundational concept in corticogenesis is the "inside-out" model whereby successive generations of PNs sequentially migrate to deep then progressively more superficial layers, but its biological significance remains unclear; and the role of iNG in this process is unknown. Using genetic strategies linking PN birth-dating to projection mapping in mice, we found that the laminar deployment of IP-derived PNs substantially deviate from an inside-out rule: PNs destined to non-consecutive layers are generated at the same time, and different PN types of the same layer are generated at non-contiguous times. The overarching scheme of iNG is the sequential specification and precise laminar deployment of projection-defined PN types, which may contribute to the orderly assembly of cortical output channels and processing streams.

HIGHLIGHTS

- Each IP is fate-restricted to generate a pair of near-identical PNs - Corticogenesis involves the orderly generation of fate-restricted IP temporal cohorts - IP temporal cohorts sequentially as well as concurrently specify multiple PN types - The deployment of PN types to specific layers does not follow an inside-out order.