Spontaneous differentiation of coculture spheroids of human umbilical vein endothelial cells (EC) and human umbilical artery smooth muscle cells (SMC). EC spheroids, SMC, and EC/SMC coculture spheroids were cultured for two days after which they were fixed, embedded, sectioned and stained for the expression of the endothelial cell marker CD31. The EC spheroids uniformly stained for CD31 whereas SMC in spheroids do not express CD31. EC/SMC coculture spheroids organize to establish a CD31 positive surface layer of EC and core of CD31 negative SMC. Only a few single CD31 positive EC are trapped in the center of the spheroid.EC cultured together with SMC in EC/SMC coculture spheroids show an increased formation of junctional complexes between EC, downregulate their PDGF-expression and show a enhanced resistance to apoptosis induced by serum starvation and growth factor deprivation