Architecture of kangaroo rat inner medulla: segmentation of descending thin limb of Henle's loop

Abstract

We hypothesize that the inner medulla of the kangaroo rat Dipodomys merriami, a desert rodent that concentrates its urine to more than 6,000 mosmol/kgH(2)O water, provides unique examples of architectural features necessary for production of highly concentrated urine. To investigate this architecture, inner medullary nephron segments in the initial 3,000 μm below the outer medulla were assessed with digital reconstructions from physical tissue sections. Descending thin limbs of Henle (DTLs), ascending thin limbs of Henle (ATLs), and collecting ducts (CDs) were identified by immunofluorescence using antibodies that label segment-specific proteins associated with transepithelial water flux (aquaporin 1 and 2, AQP1 and AQP2) and chloride flux (the chloride channel ClC-K1); all tubules and vessels were labeled with wheat germ agglutinin. In the outer 3,000 μm of the inner medulla, AQP1-positive DTLs lie at the periphery of groups of CDs. ATLs lie inside and outside the groups of CDs. Immunohistochemistry and reconstructions of loops that form their bends in the outer 3,000 μm of the inner medulla show that, relative to loop length, the AQP1-positive segment of the kangaroo rat is significantly longer than that of the Munich-Wistar rat. The length of ClC-K1 expression in the prebend region at the terminal end of the descending side of the loop in kangaroo rat is about 50% shorter than that of the Munich-Wistar rat. Tubular fluid of the kangaroo rat DTL may approach osmotic equilibrium with interstitial fluid by water reabsorption along a relatively longer tubule length, compared with Munich-Wistar rat. A relatively shorter-length prebend segment may promote a steeper reabsorptive driving force at the loop bend. These structural features predict functionality that is potentially significant in the production of a high urine osmolality in the kangaroo rat.

Architecture of the human renal inner medulla and functional implications

“…The renal medulla was isolated in PBS, trimmed to a size of ϳ3 ϫ 3 mm 2 and variable lengths, and immersion-fixed with periodate-lysine-paraformaldehyde (PLP; 0.01 M, 0.075 M, 2%) in PBS (pH 7.4) for 3 h at 4°C, or in 10% neutral buffered formalin overnight at 4°C, washed in PBS, and dehydrated through an ethanol series (47). Tissue pieces were dissected from near the outer medullary-inner medullary border (upper inner medulla) and from the terminal 50% of the inner medulla (lower inner medulla).…”

“…All tubules and blood vessels were labeled nonselectively with fluorescein-conjugated wheat germ agglutinin (FL-1021, Vector Laboratories) or with ␣B-crystallin (mouse, SPA-222, Stressgen). Secondary antibodies conjugated to fluorescent probes (Invitrogen/Molecular Probes or Jackson ImmunoResearch) were applied as described previously (47). Sections were mounted with Dako fluorescent mounting medium (Carpinteria, CA) and were viewed with epifluorescence microscopy (Applied Precision, DeltaVision).…”

Architecture of vasa recta in the renal inner medulla of the desert rodent Dipodomys merriami: potential impact on the urine concentrating mechanism

“…Kidneys were prepared for immunohistochemistry as described previously (36). Briefly, kidneys are perfused through the aorta with phosphate-buffered saline (PBS) (pH 7.4) for 5 min, followed by periodate-lysine-paraformaldehyde (PLP) (0.01 M, 0.075 M, 2%) in PBS (pH 7.4) for 5 min; some kidneys were fixed without perfusion.…”

“…FL-1021). Secondary antibodies conjugated to fluorescent probes (Invitrogen/Molecular Probes or Jackson ImmunoResearch) were applied as described previously (36). Sections were mounted with Dako fluorescent mounting medium (Carpinteria, CA) and were viewed with epifluorescence microscopy (Applied Precision, DeltaVision).…”

“…Consequently, AVR were identified in transverse sections from nonreconstructed tissue by two methods. For the first method, AVR were identified simply on the basis of morphological criteria (wall thickness and vessel diameter) (23,36) and absence of UT-B expression. For the second method, the total number of thin limb segments was determined from ClC-K expression, estimated prebend density (0.9%) (36), and the 1:1 ratio of DTLs and ascending thin limbs (ATLs), and the number of DVR were subtracted from the total number of tubular and vascular structures to obtain the number of AVR.…”