While formerly viewed as merely connective tissue, the emerging view of the arterial adventitia is that of a complex and organized vascular progenitor cell niche. Previously, our lab has identified a domain of Sonic Hedgehog (Shh) signaling restricted to the adventitia. This domain is locally produced and maintained, with the peak of activity being during embryonic and neonatal development of the vessel from embryonic day 14.5 (e14.5) to postnatal day 14 in the mouse. Developmental studies suggest that the mesenchyme continuous with the perichondrium of the axial skeleton contains Shh responsive cells that may contribute to adventitial development around e14.5. The cell types that participate in this Sonic Hedgehog adventitial signaling community are CD68-positive macrophages, Sca1-positive progenitor cells, and Perilipin A-positive adipocytes that make and/or respond to Shh protein as shown using transgenic reporter mice. Thus, we characterize the adventitia as a Shh responsive vascular niche
