Methods
For positional mapping, the dimensions of each medulla were measured as a 2-dimensional matrix. Successive sections from each medulla (200 mm x 500 mm x medullary depth) were teased free, and accessible tubules were isolated. The length of each tubule was determined, and its position plotted on the matrix. For ultrastructural studies, thin limbs were teased from the inner medulla of male Munich-Wistar and Sprague-Dawley rats. Tubules were fixed in 3% glutaraldehyde, post-fixed in 2% osmium tetroxide, then embedded in 2% agarose. After ETOH dehydration, tubules were embedded in Spurr's. Ultra-thin sections were mounted on Formvar-coated copper grids and stained with 5% uranyl acetate.

Results
50% of the inner medullary thin limbs are of the mixed-type. Mixed thin limbs are localized predominantly within the outer two-thirds of the inner medulla. The ultrastructure of cells of DTL-type segments from mixed-type thin limbs appears identical to that of cells of pure DTL. The ultrastructure of cells of ATL-type segments from mixed-type thin limbs appears identical to that of cells of pure ATL.

Conclusions
Mixed-type thin limbs appear to be distributed throughout the inner medulla in a general pattern that resembles that of the pure thin limbs. On the basis of cellular, molecular, and ultrastructural studies, mixed-type thin limbs appear to be structurally and functionally unique segments, comprising the properties of pure DTL and ATL in a single segment. We hypothesize that heterogeneous expression of transport proteins along the axis of a mixed-type tubule significantly alters its function relative to that of pure inner medullary thin limbs. Discontinuous expression of transporters within single nephrons imposes novel constraints on the concentrating mechanism.(Supported by NIH DK 16294)