Multimodal imaging of fluid transport in living epithelial sheets

Epithelial tissues form the barrier between different fluid compartments throughout the body, lining and delineating the borders between tissues to control homeostasis and provide for the functions of secretion, absorption, and volume regulation. Key to these physiological roles is the transepithelial transport of fluid and solutes. Vectorial transport is possible due to the highly polarised cytoarchitecture, with different ion transporters and other proteins present in the apical vs basolateral membrane domains, which are separated by “tight junctions” which ring the lateral membranes of each cell. These separate the apical and basolateral compartments, providing intercellular adhesion and controlling permeation via the intercellular pathway. By varying the composition and function of the pools of transport proteins between these segregated membrane domains, directed transport is achieved thus promoting homeostasis. The retinal pigment epithelium (RPE) lies behind the neural retina, forming the blood-retinal barrier and providing for the homeostasitic and biochemical support of the photoreceptors and other neuronal layers. Much is known of RPE physiology at the macroscopic level since dysregulation or pathology have profound consequences for the visual system. Lacking is the detailed knowledge of biophysical mechanisms and local intercompartmental dynamics by which fluid and solute transport is achieved and regulated at the cellular and subcellular level.