Freeze‐fracture studies on the thin limbs of Henle's loop in Psammomys obesus

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Abstract
The thin limbs of short and long loops of Henle of the desert rodent Psammomys obesus were studied by freeze‐fracture techniques. Intercellular junctions and internal membrane characteristics of thin‐limb epithelia are of interest with regard to the high urine‐concentrating capacity of this animal. The epithelium of the descending thin limbs of short loops is remarkably undifferentiated and equipped with multistrand tight junctions. In the descending thin limb of long loops, two segments are to be distinguished. The upper parts are characterized by an extensive cellular interdigitation and single‐strand tight junctions. Thus, the paracellular pathways are prominent from two aspects: the junctional belt is elongated by interdigitation, and its apico‐basal depth is shallow. The transition from the upper to the lower part appears to be abrupt, as indicated by the change in intramembrane particle density. The lower parts are characterized by a noninterdigitating epithelium with junctions consisting of few, but always more than two, strands. In addition, this thin‐limb segment is characterized by regularly distributed infoldings of the basal cell membrane. The ascending thin limbs are established by an interdigitating epithelium, with junctions generally consisting of one strand. Once again, the elongated junctional belt is shallow. This study presents further evidence that remarkable species differences occur among thin‐limb epithelia, especially concerning the descending thin limbs of long loops. Those differences may well explain discrepant functional findings concerning the transport properties of this segment in various species.