Postnatal maturation of renal cortical peritubular fibroblasts in the rat

The stromal cells in the renal cortex and medulla of adult rats reveal different phenotypes. Cortical peritubular fibroblasts are ecto-5'nucleotidase (5'NT)-positive and lack alpha-smooth muscle actin (alphaSMA) and vimentin immunoreactivity, whereas medullary fibroblasts are 5'NT-negative and vimentin-positive. We have studied by immunohistochemistry the postnatal (neonatal up to 8 weeks) development of renal cortical stromal cells with respect to 5'NT and to the cytoskeletal proteins alphaSMA and vimentin. Both alphaSMA and vimentin are characteristic for the renal myofibroblasts that replace stromal fibroblasts in interstitial nephritis. In new-born and 1-week-old rats, stromal cells in the cortex and medulla display alphaSMA and vimentin, but lack 5'NT. During the second postnatal week, alphaSMA and vimentin immunoreactivity in cortical interstitial cells gradually declines, whereas 5'NT reactivity becomes progressively apparent between the convoluted tubules in the juxtamedullary labyrinth. For a short time, all three proteins are found to be coexpressed in the same cells. At the end of the third week, interstitial 5'NT-immunoreactivity becomes evident also in the superficial cortical labyrinth, and alphaSMA and vimentin are no longer detectable in cortical peritubular cells. From the fourth week on, the distribution pattern and phenotype of 5'NT-positive cortical fibroblasts correspond to that in adult rats. The temporal pattern of maturation of cortical peritubular fibroblasts seems to parallel the functional maturation of cortical tubules. It is suggested that the local phenotype of peritubular fibroblasts in healthy and possibly also in injured kidneys may be controlled, at least in part, by the local tubular environment, conditioned by tubular metabolism and function.