Three-Dimentional Architecture of the Renal Inner Medulla of the Desert Rodent Dipodomys Merriami: Potential Impact on the Urinary Concentrating Mechanism

Abstract

The objective of the following research project was to analyze the methods behind the urinary concentrating mechanism in Dipodomys merriami, Merriam's kangaroo rat. We hypothesize that the inner medulla of Dipodomys merriami contains extreme examples of various architectural features as well as transport properties which enable it to produce concentrated urine at over 6000mOsm/Kg water. The three-dimensional architecture of the vasculature and nephron segments inside the renal inner medulla (IM) was assessed using digital reconstruction from tissue sections. Descending thin limbs (DTLs), ascending thin limbs (ATLs), collecting ducts (CDs), and ascending vasa recta (AVR) were identified with indirect immunofluorescence using antibodies and lectins that recognize segment-specific proteins associated with solute and water transport (AQP1, ClC-K1, and AQP2). Electron microscopy shows close contact between CDs, AVR, and ATLs at adherence areas. The CDs, AVR, and ATLs are sufficiently close together to form discrete interstitial compartments. This architectural arrangement and apparent isolation of these compartments raise questions regarding their function. One possibility is that lateral solute diffusion from ATLs and CDs into AVR could be preferentially restricted to these areas. Interstitial cell architecture, which could increase and define compartmentalization, may further restrict diffusive exchange.