Interstitial Spaces: A Basolateral Source of Structure and Signals

The mammalian interstitium is a body-wide network of fluid-filled, prelymphatic spaces. Recent studies demonstrate that it exists at three scales in continuity both within and between organs, comprising intercellular, pericapillary, and large (or fascial) interstitial spaces, the latter including fascia, dermis, organ submucosae and capsules, vascular adventitia, and perineurium. Hyaluronic acid fills all interstitial spaces, but large interstitial spaces also contain additional structurally complex and varied extracellular matrices that support soluble factor, mechanical, and potentially electrical signaling. Here we review areas where the new anatomic concept of the interstitium has led to the re-examination of previous findings, including data on interstitial matrix composition and cell trafficking. We also identify new questions arising specifically from the finding that the interstitium is multiscale and body-wide, including questions about the characteristics and drivers of interstitial fluid flow and the role of the interstitium as a rich and active basolateral signaling compartment.