The major limitations of this in vitro assay relate to how reflective the assay model is to the in vivo state. Overall, the data and conclusions obtained over the years have been repeatedly demonstrated to be observed using mouse, zebrafish, or quail in vivo models [155]. One of the interesting issues is the state of differentiation of the EC that are used in these in vitro assays, and how similar are they to EC in vivo that are undergoing vasculogenesis or angiogenic sprouting. This is a very difficult question to answer and needs to be addressed in future studies. Another potential weakness of the current in vitro systems is that flow conditions have to be adapted or the gels have to be transplanted into a mouse model to interface the human vessels with an in vivo vascular system.