Primary chronic TIN. The arrow indicates a normal glomerulus. Apart from providing structural support, the interstitium serves as a conduit for solute transport and is the site of production of several cytokines and hormones (erythropoietin and prostaglandins). For the exchange processes to occur between the tubules and vascular compartment, the absorbed or secreted substances must traverse the interstitial space. The structure, composition, and permeability characteristics of the interstitial space must, of necessity, exert an effect on any such exchange. Although the normal structural and functional correlates of the interstitial space are poorly defined, changes in its composition and structure in chronic TIN are closely linked to changes in tubular function. In addition, replacement of the normal delicate interstitial structures by fibrosclerotic changes of chronic TIN would affect the vascular perfusion of the adjacent tubule, thereby contributing to tubular dysfunction and progressive ischemic injury.

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