Crossing the arterial wall with CARS.

Regulation of the flow and movement of water is essential to the health and well-being of our bodies. Involving a diverse array of structures that range from molecular gatekeepers to tissue barriers, water flow and concentration are maintained at precise levels. However, in many instances we do not have a complete picture of how this works. A case in point is the arterial wall, which separates the luminal space of the artery, where water flow is highly directional and at high pressure, from the surrounding tissue, where the movement of water is diffusive and pressures are lower. To maintain the pressure difference across the arterial wall, the permeability of water through this barrier is necessarily low. Although the limited water permeability of the arterial wall has been measured extensively, it has remained unclear how this dedicated tissue structure is able to limit the flow of water or even which layer of the wall is responsible for the decrease in permeability. To solve this puzzle, a closer look at the water barrier is needed to bring the location of the water-blocking component into sharper view. In PNAS, Lucotte et al. accomplish this feat by using coherent anti-Stokes Raman scattering (CARS) microscopy (1). By creating water concentration maps with submicrometer resolution, the authors are able to pinpoint the endothelial basolateral membrane as the key player in limiting water permeation across the arterial wall.