Differentiation of kidney cortex peroxisomes in fetal and newborn rats

Peroxisomal enzyme assays as well as cytochemical detection of catalase were carried out on fetal and newborn rat kidney cortex throughout the last 3 days of prenatal life and the first month of postnatal development. Concerning the patterns of peroxisomal enzymes, catalase activity, hardly detectable in the fetus, shows the strongest increment after the second week of postnatal life; beta-oxidation system and D-amino acid oxidase increase soon after birth; urate oxidase activity, detected in fetal life, rapidly decreases after birth; dihydroxyacetone phosphate-acyltransferase activity doubles at birth, remaining constant thereafter. Since by cytochemistry no catalase particles were detected in fetal kidneys, morphometric parameters were studied only postnatally. The numerical density shows only minor variations, mainly at day 3; the mean diameter remains practically unchanged between birth and day 14 but strongly increases later. The volume density pattern correlates in the early phase with the numerical density and later with the profile mean diameter. The results suggest that enzymes are asynchronously incorporated into pre-existing peroxisomes; that this import is faster in smaller organelles than in the larger, adult ones; that catalase increases after the H2O2-producing oxidases; and that the abrupt rise of beta-oxidation capacity and DH-APAT is related to the increased renal work immediately after birth.