Architecture of the human renal inner medulla and functional implications.
The architecture of the inner stripe of the outer medulla of the human kidney has long been known to exhibit distinctive configurations; however, inner medullary architecture remains poorly defined. Using immunohistochemistry with segment-specific antibodies for membrane fluid and solute transporters and other proteins, we identified a number of distinctive functional features of human inner medulla. In the outer inner medulla, aquaporin-1 (AQP1)-positive long-loop descending thin limbs (DTLs) lie alongside descending and ascending vasa recta (DVR, AVR) within vascular bundles. These vascular bundles are continuations of outer medullary vascular bundles. Bundles containing DTLs and vasa recta lie at the margins of coalescing collecting duct (CD) clusters, thereby forming two regions, the vascular bundle region and the CD cluster region. Although AQP1 and urea transporter UT-B are abundantly expressed in long-loop DTLs and DVR, respectively, their expression declines with depth below the outer medulla. Transcellular water and urea fluxes likely decline in these segments at progressively deeper levels. Smooth muscle myosin heavy chain protein is also expressed in DVR of the inner stripe and the upper inner medulla, but is sparsely expressed at deeper inner medullary levels. In rodent inner medulla, fenestrated capillaries abut CDs along their entire length, paralleling ascending thin limbs (ATLs), forming distinct compartments (interstitial nodal spaces; INSs); however, in humans this architecture rarely occurs. Thus INSs are relatively infrequent in the human inner medulla, unlike in the rodent where they are abundant. UT-B is expressed within the papillary epithelium of the lower inner medulla, indicating a transcellular pathway for urea across this epithelium.
Department of Physiology, University of Arizona Health Sciences Center, Tucson, Arizona; and.
Guojun
G
Wei
Department of Pathology, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, Massachusetts.
Seymour
S
Rosen
Department of Physiology, University of Arizona Health Sciences Center, Tucson, Arizona; and.
William H
WH
Dantzler
Department of Physiology, University of Arizona Health Sciences Center, Tucson, Arizona; and pannabec@email.arizona.edu.
Thomas L
TL
Pannabecker
D051398:Aquaporin 1 / Q000378:metabolism; D002196:Capillaries / Q000378:metabolism; D004848:Epithelium / Q000378:metabolism; D006801:Humans; D021621:Imaging, Three-Dimensional; D007150:Immunohistochemistry; D066298:In Vitro Techniques; D007671:Kidney Concentrating Ability / Q000502:physiology; D007679:Kidney Medulla / Q000033:anatomy & histology* / Q000502:physiology*; D007684:Kidney Tubules / Q000378:metabolism; D007685:Kidney Tubules, Collecting / Q000378:metabolism; D010101:Oxygen Consumption
D051398:Aquaporin 1
UT-B; aquaporin-1; aquaporin-2; blood flow; hemodynamics; urine concentrating mechanism