Over 15 years ago, the Human Genome Project was completed. The project cataloged the first full human DNA sequence, which has gone on to transform biology and medicine. Of a similar scope, the Human Cell Atlas, launched almost four years ago, aims to map every single one of the trillions of cells that make up the human body. Ultimately, this would revolutionize how doctors and researchers understand, diagnose, and treat disease.
A new paper, published in Nature, has contributed some breakthrough data towards this goal, by describing the cell-type composition of all major human organs. Over 50,000 single cells from Chinese Han donors were sequenced by corresponding author Guoji Guo from the Zhejiang University School of Medicine in Hangzhou, China and colleagues. This data was then categorized by cell-type, offering potentially the most comprehensive cell-type repertoire described for humans ever reported.
As cells divide and grow in the embryo they start to perform different functions, despite carrying the same genetic information. As the cells differentiate they form various tissues of the body, which in turn make up organs. To better understand the function of the different cell-types present in each organ, and the factors that influence their activity, scientists need to determine which genes are expressed by each individual cell.
In this study, the team of researchers from China used their own tool, Microwell-seq, to sequence the RNA of the separated cells found in 60 tissue types, from both adult and fetal samples. This allowed the researchers to decipher each cell’s transcriptome. These are pieces of RNA messages that can be accurately used to distinguish the identity of one cell from another and provide information on the active genes within the cell.
Having sequenced hundreds of thousands of cells, the team then grouped them into 102 clusters, divided in line with cell type rather than the organs from which the sample originated, including endothelial cells (cells which line the interior surface of blood vessels), and stromal cells (connective tissue cells). From here, a comparative analysis of cell types between organs, ages (as fetal and adult samples were used) and different species (using data previously gained from mouse organs), could be carried out.
In the paper, the authors note that “the scale of the current analysis is limited in sequencing depth and cell number for each individual tissue.” However, their pilot study can serve as a solid starting point for future studies to build on, and will provide a significant contribution towards the eventual completion of the international Human Cell Atlas.