The panels represent a 3-dimensional model of foetal blood formation, built from mouse embryonic stem cells. We call these models haemogenic, or blood-forming, gastruloids.The mouse embryonic stem cells have been engineered to express a green fluorescent protein (GFP) gene in blood vessel-lining endothelial cells, some of which specialise to form blood cells, including blood stem cells. The top 3 panels show emergence of the first GFP-positive endothelial cells and progressive development of GFP-positive vascular networks in haemogenic gastruloids. Photographs were obtained at 4, 6 and 8 days of culture in the laboratory.At the bottom left, emergence of blood cells from the GFP-positive endothelial cells was captured by confocal microscopy. Round cells pseudo-coloured in red can be observed in bunches, or clusters, adjacent to flat endothelial cells shown in green. The arrangement closely mimics budding of blood cells in vivo from the vessel-lining endothelium. Round cells are labelled with an antibody against a molecule present in all blood cells, named CD45. Some of the CD45-positive cells are also pseudo-coloured yellow: this reveals the presence of another molecule named C-Kit, characteristic of immature blood cells, including blood stem cells. Blue stains all types of cells formed in the gastruloid model.Bottom right depicts a detailed analysis of the genes expressed by all blood and endothelial cells present in the day 8 haemogenic gastruloids. Gastruloids were dissociated into individual cells, and each cell was sequenced to catalogue the genes it expresses – this is called single-cell RNA sequencing, and informs our understanding of the identity and function of each cell. Individual cells are represented on a plot, where each dot corresponds to a single cell. Cells expressing similar genes in similar amounts are represented together in groups, or clusters, which are numbered. Endothelial GFP-positive cells are present in clusters 0 and 3, and blood CD45-positive cells are in clusters 1, 4, 5, and 6, with the most immature cells present in cluster 4.We are investigating cluster 4 cells for the presence of blood stem cells, which may be later expanded and used in bespoke bone marrow transplants.