The Science group of journals publishes several parts of a high-resolution atlas of human brain cells, the largest 'map' of its kind to date. It is a series of papers from an international mega-project, the Brain Research through Advancing Innovative Neurotechnologies (BRAIN) Initiative - Cell Census Network (BICCN), launched in 2017 to study brain cell types and their functions in humans, non-human primates and rodents. The data "will now allow researchers to address fundamental scientific questions about the human brain and its genetic organisation," states the introduction to the journal's special issue.
An abstract representation of cell diversity in the brain. Individual nuclei are colored in the bright hues of t-SNE plots used in epigenomics analysis to distinguish individual brain cell types. Layers of background color represent the local environmental factors of each brain region that influence cell function. Credit: Michael Nunn.
Researchers from the Karolinska Institute (Sweden) and the University of California, San Diego (USA) have published in the prestigious research journal Science the most comprehensive catalogue to date of the cellular content of the human brain during its formation, specifically during the first trimester of life (the study by Braun et al.) and in adulthood (the study by Siletti et al. and Li et al.), thanks to the use of state-of-the-art omics techniques. These studies not only describe the spatial distribution of these cell types (the study by Braun et al. and Siletti et al.), but also determine the gene programmes that are specific to each cell type (the study by Li et al.).
These studies represent a milestone in the history of biology, on a par with the sequencing of the human genome in 2000, and could provide a gateway to understanding the causes of diseases such as autism or psychiatric disorders, which have an embryonic origin, or neurodegenerative diseases such as dementia, Parkinson's or Alzheimer's, which manifest themselves in old age.
[The two papers published by the Braun and Siletti teams] come from a pioneering research institute of great importance in the field of omics, Karolinska, and are associated with one of the leading and reference groups in the field, led by Sten Linnarsson. Therefore, I believe that the articles present a good scientific rigour, since many of the new technologies that have been developed in the field of omics come from these authors.
On the other hand, I think it is a great advance to be able to obtain transcriptomic information, which reflects which genes each cell is expressing, from the human brain, both in its adult state and, above all, in development, as these articles show. When it comes to extrapolating to humans, most studies are based on experimental models to try to understand or replicate what might happen. Therefore, being able to create these atlases of cell types in different brain areas at different times seems to me to be a great tool to bring us closer to a better understanding of the human brain and to more accurately match their homology with the experimental models we work with.
In addition, much of the single-cell transcriptomic sequencing work lacks spatial information in the tissue itself, making it difficult to relate the massive sequencing data to the location and dispersion of cells in situ. The work of Braun et al. makes this effort and not only collects gene expression data, but also employs techniques to identify in which specific areas of the developing brain different cell types are located.
Undoubtedly, there is still some way to go in the field of human genetics, especially in collecting data from diverse populations to avoid bias. Overall, however, this material provides a valuable starting point for understanding the heterogeneity and variability present in the human brain.