Alkaline phosphatase activity at unusual sites of human foetal kidney.

Frozen sections of human foetal kidneys were treated by the lead citrate method in order to demonstrate alkaline phosphatase (ALP) activity. Unexpectedly, ALP activity was observed over the parietal layer of Bowman's capsule, in addition to usual localization in proximal tubules and blood vessels. The enzyme was confined to the membrane of microvilli belonging to tall columnar cells. Normally, in the mature nephron, these high cells are absent from the parietal layer that is instead entirely composed of squamous epithelium. ALP activity was also revealed at another unusual site in the kidney medulla. A collar of mesenchymal cells encapsulating groups of tubules and calyces gave an intense enzymatic reaction. The activity was present over the membrane of long cytoplasmic processes. The cytoplasm showed a well developed rough endoplasmic reticulum, an indication of a high rate of protein synthesis. The nearby presence of numerous collagenous fibrils is concordant with this assumption. Moreover, these ALP-positive cells might represent an intermediary stage through which cells have to pass before differentiating.