Shapson-Coe, Januszewski, Berger et al. recently reported in Science a major milestone in neuroanatomy and connectomics, reconstructing a cubic millimeter of human cortex using electron microscopy to achieve nanometer resolution. The sample they analyzed was from a 45 year-old woman’s left anterior middle temporal gyrus, which was resected to gain access to an underlying epileptic focus. Machine learning-based classifiers identified 149,871,669 synapses (111,272,315 excitatory and 38,599,354 inhibitory); of these, 99.4% were on dendrites, whereas only 0.197% and 0.394% were on axon initial segments and somata, respectively. Analysis of all the cells with nuclei in the sample revealed that glia outnumbered neurons two to one and that the overall density of neurons was around 16,000 per mm3, which is lower than previous estimates derived from light microscopy. The authors have made their data available via an online tool, and users can apply to become ‘proofreaders’ and correct errors that were made during the automated segmentation. This work represents a valuable resource for further deep dives into human cortical neuroanatomy and heralds the possibility of mapping human brain circuits in exquisite detail.

Original reference: Science https://doi.org/10.1126/science.adk4858 (2024)