Heterogeneity of the descending thin limb of Henle's loop

Although no unified theory has been established to account for the mechanisms of formation of concentrated urine, it has been generally accepted that the loops of Henle play a crucial role in the generation and maintenance of a steep osmotic gradient in the renal medulla [1]. Although the countercurrent systems in the renal medulla, in principle, is essential for the urine concentrating mechanism, detailed morphological analyses [1–4] have revealed that the construction of the counterflow system of the Henle's loop is not simple as was originally proposed by Kuhn and his associates [5–7]. Complicated features exist not only in the architectural organization but also in the heterogeneity of epithelial morphology [1–4]. The latter features are the most prominent in the descending limb.The technique of in vitro microperfusion provides a useful tool to clarify the functional significance of cellular heterogeneity. In this article we will review the accumulated data on the functional heterogeneity of the descending limbs of Henle's loop. Special attention will be focused on the function of the upper portion of the long-loop descending limb