Effects of season on kidney morphology in house sparrows

Seasonal variability in kidney morphology of the house sparrow Passer domesticus was examined using light microscopy. Sparrows were captured from the wild in winter, spring, summer and autumn. The kidneys were perfused with half-strength Karnovsky's fixative and processed for light microscopy by embedding in either paraffin wax or JB4 acrylic resin. Absolute volumes of the kidneys, their components (cortex, medulla and blood vessels), components of the nephron (renal corpuscles, proximal tubules, loops of Henle, distal tubules and collecting ducts) and the capillaries surrounding the nephron were quantified using stereology. Tissue processed in paraffin wax had a mean shrinkage of 17.7 % compared with 10.1 % in JB4 resin. Absolute volumes of the kidneys and nephrons were compared statistically between tissue processing methods and among seasons. Absolute volumes of the structures within the kidneys were not significantly different between treatments or among seasons. Within the nephron, the only measured variables to show significant differences were the absolute volumes of the distal tubules and cortical collecting ducts between tissue processing treatments. Thus, kidney morphology was relatively unaffected by changes in season. In addition, the results show that embedding tissue using acrylic resin causes less shrinkage, and it should therefore be the preferred embedding medium for quantitative morphologists.