Abstract
The activities of various enzymes were studied histochemically in two species which at birth have a kidney with an active nephrogenic zone, the rat and rabbit, and one (the guinea pig) in which this organ is at this time apparently fully matured. The histochemical reactions, in general, reflect the degree of maturity found in kidneys of newborn and growing animals. Immature proximal convoluted tubules lack enzymatic activity or show only minimal amounts. As these tubules mature, the adult pattern is noted at about the 14th to 16th day after birth in rat, and after 21 to 28 days in rabbit. Within this general pattern, however, considerable variations are noted. Glucose-6-phosphatase, e.g., is less active at birth, even in mature tubules, while acid phosphatase localized in granular "lysosomal" bodies is as prominent in newborn kidney as in adult. Newborn guinea pig kidney lacks glomerular adenosine triphosphatase activity in spite of its general enzymatic maturity, while rat kidney at birth has no tubular adenosine triphosphatase activity, even in more mature proximal convolutions.
Oxidative enzymes, particularly succinic dehydrogenase, and acid phosphatase are active in tubules of the inner portion of medulla in rat and rabbit at birth. This appears to be an expression of the immaturity of newborn kidney. With the progress of zonal differentiation, this enzyme activity is no longer found in the papillary portion of medulla where thin limbs of Henle's loop are now located. In rat kidney, best seen in cryostat sections briefly postfixed in very cold neutral formalin, single cells are found in the collecting ducts with striking 5-nucleotidase activity. The number of these cells is greater in neonatal kidney than in adult kidney. The physiological significance of many of the findings described in this report has still to be clarified.