Development of solute transport in rabbit proximal tubule. II. Morphologic segmentation.

We examined the segmental morphologic maturation of the proximal tubule in order to identify some of the factors influencing the development of solute transport. Proximal tubules from the outer, middle, and inner kidney cortex were obtained from rabbits during the entire period of development and studied using transmission and scanning electron microscopy, microdissection, and semiquantitative morphometric analysis. The location of the tubule in the cortex was established by identifying its own glomerulus. The 1-wk-old rabbit showed the most immature proximal tubules in the outer cortex and a gradation of more mature tubules toward the inner zone. Only the inner cortical tubules were segmented at this time. The outer cortical tubules showed limited development for the 1st 2-3 wk after birth, while the inner cortical tubules were morphologically mature by 3-4 postnatal wk. Morphometric analysis revealed that the areas of apical, basolateral, and mitochondrial membranes developed at similar rates. Clearly, morphologic maturation of proximal tubules was reached at different times depending on the cortical origin. We conclude that a serial developmental examination of transport in proximal tubules should require the careful identification of the origin of each tubule in the cortex; otherwise, time-dependent comparisons will be complicated by the inclusion of nephrons of diverse postnatal ages. In practice, at this time, the use of proximal tubules from the inner cortex throughout development, or from the outer cortex beyond 2-3 wk of age, permits such serial measurements and comparisons in the rabbit.