HMOX1 and systemic sclerosis: To determine whether the observed decreased in HO-1 expression induced by aberrant TGF-β signaling contribute to defective angiogenesis, we performed endothelial-fibroblasts organotypic angiogenesis in vitro assays in which endothelial cells form tubules highly reminiscent of capillaries formed during angiogenesis in vivo, embedded in a natural matrix produced by SSc fibroblasts.