RESUMEN
Osteopontin (OPN), originally considered to be a bone protein, is now reported to be expressed in other tissues, notably in kidney. OPN has been demonstrated in the kidney by Northern and Western analyses, immunohistochemistry and in situ hybridization. However, studies of the cellular distribution of OPN in the kidney, especially in normal condition, have given highly conflicting results. This study is designed to establish the expression of OPN in kidney from the fetus to adult. Kidneys from 16-(F16), 18-(F18), and 20 day-old (F20) fetuses and 1-(P1), 3-(P3), 7-(P7), 14-(P14), 21 day-old pups, and adult were studied by in situ hybridization and immunohistochemistry. OPN mRNA and protein were expressed in the thick ascending limb (TAL) at F18 and F20, but disappeared gradually after birth. In the collection duct (CD), weak labeling appeared in a few cells at F20. After birth cells with strong labeling were scattered throughout the CD in the medulla and inner cortex at P1-P7 and in the outer cortex at P14. There was little OPN expression in the CD at P21. OPN mRNA and immunoreactivity appeared in the papillary surface epithelium (PSE) at F20 and in the descending thin limb (DTL) at P1. After birth OPN expression gradually increased to adult levels in the PSE. In the DTL, adult levels of expression were reached at P21. Proximal tubules exhibited a punctate subapical OPN immunos-taining from F18, but no hybridization signal. During renal development, the transient expression of OPN in the TAL and CD suggests that OPN may have a role in the developing kidney, and from P21 OPN expression was localized at DTL and PSE exclusively, which was similar to adult OPN expression pattern.