That lymphatics can form from blood vessels is textbook knowledge, but at least for zebrafish, the textbooks need to be re-written: A sizable fraction of the cardiovascular system in zebrafish seems to form from lymphatics. Notably, the blood vessels of the so-called secondary vascular system (SVS) appear to form by transdifferentiation from lymphatics. This ground-breaking research was done by Das et al. from the Yaniv lab at the Weizmann Institute of Science (https://www.weizmann.ac.il/Biological_Regulation/Yaniv). Although the SVS is the central topic of the publication, the SVS is mentioned only twice in some subordinate clauses. Therefore Kari Alitalo and I thought it would be a good idea to spell out explicitly what this paper describes: the embryonic development of the SVS. Have look at the Das et al. Nature paper and Kari's and my Nature Cardiovascular Research piece.