Carbonic anhydrase in the human fetal kidney.

Kidney tissue from human fetuses of gestational ages ranging 12--26 wk were studied by histochemical and biochemical methods. Clear staining for carbonic anhydrase activity was found in both proximal and distal parts of some nephrons already at gestational age of 12--15 wk. The staining pattern in the inner cortex of the fetuses of 24 and 26 wk did not differ markedly from that in adult renal cortex, except for the absence of capillary staining; proximal and distal convoluted tubules as well as initial collecting tubules exhibited distinct enzyme activity. In the outer cortex, where the nephronogenic zone is located, newly formed nephrons with no or little staining were found in all kidneys. Loops of Henle were absent or sparse in the medulla of the fetuses of 12--17 wk. Fully developed loops of Henle were relatively few also in the fetuses of 24 and 25 wk; the thick ascending limb and part of the thin limb were stained in such loops. The collecting ducts in all kidneys contained a varying number of intensely stained cells interspersed among unstained or weakly stained ones. The catalytic activity and the immunoassayable amount of carbonic anhydrase were determined in homogenates of renal cortex from three fetuses (19, 21, and 26 wk). The kidney from the fetus of 26 wk showed the highest values with an enzyme activity and an immunoassayable amount of HCA-C corresponding to 204 enzyme units and 0.14 mg enzyme per g wet weight of tissue, respectively. These values are similar to those of adult renal cortex and indicate that all of antigenically determined HCA-C is also catalytically active in the kidney at this gestational age. As judged from their content of carbonic anhydrase many nephrons might be fully able to reabsorb bicarbonate and secrete hydrogen ions in human fetuses of 24--26 wk. Other nephrons are much less developed at this stage, however. This nephronic heterogeneity might at least partly explain the reduced capacity for urinary acidification which has been reported for premature newborn infants.