Rat thin limbs of Henle were studied by freeze fracture electron microscopy. Thin limb segments in both short and long looped nephrons were identified by previously developed ultrastructural criteria, continuity with known thick segments, and architectural relationships in the outer medulla. Intramembrane particle (IMP) density and the number of intramembrane fibrils comprising the zonula occludens were determined for each morphologically identifiable thin limb segment. The IMP density on the protoplasmic faces of both the luminal and abluminal membranes of the upper portion of the descending thin limb (DTL) of the long looped nephron is quantitatively greater than in the short looped thin limb, lower portion of long looped DTL, and in the ascending thin limb. The zonula occludens in the long looped upper DTL consists of a single fibril; the long looped lower DTL contains 3.13 ± 0.14 fibrils; the ascending thin limb contains 1.31 ± 0.09 fibrils; and the short looped DTL contains 3.75 ± 0.19 fibrils. These studies further support the contention that there is anatomic heterogeneity among the limb segments. Because direct physiologic studies in thin limbs are incomplete and conflicting, the need for correlative physiologic studies on anatomically characterized structures is indicated.
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