The endothelium, a key actor in organ development and hPSC-derived organoid vascularization

Over the last 4 decades, cell culture techniques have evolved towards the creation of in vitro multicellular entities that incorporate the three-dimensional complexity of in vivo tissues and organs. As a result, stem cells and adult progenitor cells have been used to derive self-organized 3D cell aggregates that mimic the morphological and functional traits of organs in vitro. These so-called organoids were first generated from primary animal and human tissues, then human pluripotent stem cells (hPSCs) arose as a new tool for organoid generation. Due to their self-renewal capacity and differentiation potential, hPSCs are an unlimited source of cells used for organoids. Today, hPSC-derived small intestinal, kidney, brain, liver, and pancreas organoids, among others, have been produced and are promising in vitro human models for diverse applications, including fundamental research, drug development and regenerative medicine. However, achieving in vivo-like organ complexity and maturation in vitro remains a challenge. Current hPSC-derived organoids are often limited in size and developmental state, resembling embryonic or fetal organs rather than adult organs. The use of endothelial cells to vascularize hPSC-derived organoids may represent a key to ensuring oxygen and nutrient distribution in large organoids, thus contributing to the maturation of adult-like organoids through paracrine signaling. Here, we review the current state of the art regarding vascularized hPSC-derived organoids (vhPSC-Orgs). We analyze the progress achieved in the generation of organoids derived from the three primary germ layers (endoderm, mesoderm and ectoderm) exemplified by the pancreas, liver, kidneys and brain. Special attention will be given to the role of the endothelium in the organogenesis of the aforementioned organs, the sources of endothelial cells employed in vhPSC-Org protocols and the remaining challenges preventing the creation of ex vivo functional and vascularized organs.