Kidneys of timed fetal and newborn mice of the C57R1, stock were sectioned and stained with a modified Bowie technique. The juxtaglomerular cells, identifiable by their granularity and position, were recognized rather late in fetal lifethree or four days before birth. By the eighth day postpartum, the granular cells of the JGA had reached a peak of differentiation and closely resembled the adult JGA in their number, sire, granularity, and position. The mesangial cell was often but not always granulated. The granular cell indices (G.C.I.) of fetal, newborn, and adult mice are compared. The high G.C.I. in fetal kidneys in our studies should account for the high renin titre found by others in fetal kidney extracts.
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