A proposal for a coordinated effort for the determination of brainwide neuroanatomical connectivity in model organisms at a mesoscopic scale

In this era of complete genomes, our knowledge of neuroanatomical circuitry remains surprisingly sparse. Such knowledge is however critical both for basic and clinical research into brain function. Here we advocate for a concerted effort to fill this gap, through systematic, experimental mapping of neural circuits at a mesoscopic scale of resolution suitable for comprehensive, brain-wide coverage, using injections of tracers or viral vectors. We detail the scientific and medical rationale and briefly review existing knowledge and experimental techniques. We define a set of desiderata, including brain-wide coverage; validated and extensible experimental techniques suitable for standardization and automation; centralized, open access data repository; compatibility with existing resources, and tractability with current informatics technology. We discuss a hypothetical but tractable plan for mouse, additional efforts for the macaque, and technique development for human. We estimate that the mouse connectivity project could be completed within five years with a comparatively modest budget.

💡 Research Summary

In the era of fully sequenced genomes, our understanding of the brain’s wiring diagram remains surprisingly limited, yet such knowledge is essential for both basic neuroscience and clinical research. This paper makes a compelling case for a coordinated, large‑scale effort to map brain‑wide neuroanatomical connectivity at a mesoscopic scale—roughly a few hundred micrometers—using systematic tracer injections or viral vectors. The authors argue that this intermediate resolution strikes a pragmatic balance: it is fine enough to quantify inter‑regional connection strength and directionality, while being feasible for whole‑brain coverage across many specimens.

The manuscript first reviews the current landscape. High‑resolution electron‑microscopy connectomics provides synapse‑level detail but is prohibitively expensive and slow for whole‑brain studies. Conventional tracer studies, although valuable, have been limited to isolated regions and small sample sizes, leaving the brain’s global network largely uncharted. Consequently, a “mesoscopic” approach is proposed as the missing link between these extremes.

Five core desiderata are defined: (1) Brain‑wide coverage – every anatomical region must be sampled uniformly, requiring on the order of 10,000–20,000 injections to achieve statistical power; (2) Standardized, automatable experimental pipelines – precise delivery of anterograde and retrograde tracers (e.g., BDA, CTB) and recombinant viruses (AAV, rabies) combined with robotic injection, sectioning, high‑resolution scanning, and quantitative image analysis; (3) Centralized open‑access repository – raw images, processed connectivity matrices, and rich metadata stored in a public database with APIs for unrestricted download and re‑analysis; (4) Compatibility with existing resources – seamless integration with atlases such as the Allen Brain Atlas and the Mouse Brain Connectivity Atlas, ensuring multimodal data fusion; and (5) Feasibility with current informatics – leveraging cloud computing, machine‑learning‑based segmentation, graph databases, and scalable storage solutions already available to the research community.

The authors outline a concrete five‑year roadmap focused on the mouse as a proof‑of‑concept model. Each year would involve 2,000–3,000 tracer injections, with 3–5 serial sections per injection scanned at sub‑micron resolution. An automated pipeline would perform segmentation, labeling, and quantitative estimation of projection density, outputting standardized NIfTI and JSON files. A web‑based viewer and analysis suite would allow real‑time quality control and community feedback.

Success in the mouse would pave the way for a comparable effort in the macaque, whose brain size and cortical organization are more analogous to humans, thereby providing a translational bridge for neuropsychiatric disease modeling. For humans, the authors acknowledge the scarcity of living tissue and propose a hybrid strategy: post‑mortem specimens, intra‑operative biopsies, and high‑resolution MRI‑derived tractography would be combined with the same computational framework to generate a provisional mesoscopic connectivity map.

Budget considerations are addressed transparently. The projected annual cost—approximately $200–300 million—covers robotic injection platforms, high‑throughput slide scanners, personnel (experimentalists, data scientists, software engineers), and data storage/management. Compared with existing large‑scale initiatives such as the BRAIN Initiative, this figure is modest, and the authors argue that the long‑term return on investment—through open data, reproducibility, and accelerated hypothesis testing—will far outweigh the expense.

In conclusion, the paper presents a detailed, technically viable blueprint for constructing a brain‑wide mesoscopic connectivity atlas. By standardizing experimental protocols, automating data acquisition, and committing to open, interoperable data sharing, the proposed effort promises to transform neuroscience from a collection of isolated circuit studies into a cohesive, quantitative network science. The implications span basic research, disease modeling, and even the development of biologically inspired artificial intelligence, making the initiative both scientifically compelling and societally valuable.