Facts, fantasies and fun in epithelial physiology.

The hallmark of epithelial cells is their functional polarization. It is those membrane proteins that are distributed differentially, either to the apical or to the basal surface, that determine epithelial physiology. Such proteins will include 'pumps', 'channels' and 'carriers', and it is the functional interplay between the actions of these molecules that allows the specific properties of the epithelium to emerge. Epithelial properties will additionally depend on: (a) the extent to which there may be a route between adjacent cells (the 'paracellular' route); and (b) the folding of the epithelium (as, for example, in the loop of Henle). As for other transporters, there is polarized distribution of amino-acid carriers; the molecular basis of these is of considerable current interest with regard to function, including 'inborn errors' (amino-acidurias); some of these transporters have additional functions, such as in the regulation of cell fusion, in modulating cell adherence and in activating intracellular signalling pathways. Collaboration of physiologists with fly geneticists has generated new insights into epithelial function. One example is the finding that certain amino-acid transporters may act as 'transceptors' and play a role as sensors of the extracellular environment that then regulate intracellular pathways controlling cell growth.