Embryonic and postnatal development of the rat renal interstitium

Whilst antihypertensive, structural and functional roles have been proposed for the cells of the renomedullary interstitium in the adult kidney, little is known about its role in renal development. Rat kidneys were studied throughout development, prenatally at gestational ages E14-E21 and postnatally at 0-28 days, by light microscopy, transmission and scanning electron microscopy and following immunocytochemistry. Renomedullary interstitial cells were observed as early as embryonic day E14, forming a loose, orderly network around branches of the ureteric bud. Paralleling the development of the first nephron structures, renomedullary interstitial cells were arranged in a concentric circular manner around collecting ducts. Following tubular and vascular growth from the cortex into the medulla, this arrangement resulted in the characteristic `rungs of a ladder' appearance of interstitial cells between tubules, blood vessels and the collecting ducts. Renomedullary interstitial cells were closely adherent to basement membranes of tubules, blood vessels and collecting ducts from early in development. Contacts were absent between renomedullary interstitial cells and tubular structures in the process of remodelling, such as the hair-pin bends of the loops of Henle. At these foci laminin, a basement membrane glycoprotein was specificially localised to intracellular epithelial sites, whereas in more developed areas, laminin was restricted to epithelial basement membranes. Associated with the more mature structures, laminin was also localised to intracellular granules of renomedullary interstitial cells. Thus, renomedullary interstitial cells are present prior to and appear to be actively associated with tubule repositioning in the medulla, establishing themselves as integral to the process of renal development.