Maturation of the proximal tubule in the [beagle] puppy kidney: a comparison to the adult

Two‐ to four‐ day‐ old beagle puppy kidneys were prepared fortransmission and scanning electron microscopy and compared to similarly prepared adult tissues. proximal tubules of puppy kidneys which contained nephrons in various stages of differentiation were examined andmaturational changes were described. Lateral surface contours of proximal tubular cells were initially smooth and relatively unfolded, but progressively acquired complex processes that may be recognized as lateral ridges and lateral‐basal processes. Basal projections began as short, stubby processes and gradually took on either a narrow, plate‐like or finger‐like appearance. Mitochondria, lysosomes and apical vacuoles increased in number as the tubules matured. Mitochondria lacked orientation in outer cortical tubules, but exhibited some vertical arrangement within basal processes in inner cortical tubules. Despite features indicating advanced maturation of tubules in the inner cortex, puppy kidneys lacked the lipid droplets characteristic of the adult. Thus, differentiation of this portion of the developing nephron into S, S2 and S, segments was notpossible at day 4.Morphometric analyses of the lateral and basal membrane surface concentrationof proximal convoluted tubules from the puppy revealed all cells to have significantly smaller membrane area than that of the adult. However, the inner cortical cells of the puppy had a greater surface concentration than those of the outer cortex. The reduced transport capacity of the puppy proximal tubule may be related to the lack of segmentation and/or reduced lateral‐basal surface area.