Tubulo-vascular relationships in the developing kidney.

The postnatal development of the tubulo-vascular relationships in the kidney was studied in 60 rats. The kidney is relatively immature at birth and only the juxtamedullary glomeruli are present. As their loops of Henle develop they grow down into the medulla around the previously developed collecting ducts, forming tubulovascular 'units' around whose periphery the shorter loops of the more peripheral glomeruli grow down. The bundles of vasa recta develop in loose interstitial tissue which lies between the tubulo-vascular units so that the peripheral vessels of the bundles, which are ascending vasa recta, lie in close relationship with the descending short loops of Henle. Thus, the vascular bundles do not form the central axis around which the medullary components develop but rather take up the available space left after the tubulo-vascular units have developed. Development is virtually complete by the fourth week of postnatal life.