In most vertebrate organs, epithelial tubes or sacs are surrounded by support and stromal tissues—including smooth muscle, cartilage, pericytes, fibroblasts, and mesothelium—that form during development from a loose collection of undifferentiated progenitor cells called mesenchyme. Although the behavior and regulation of epithelial progenitors and their niches have begun to be elucidated, much less is known about the identity and behavior of the progenitors of support and stromal tissues.
