In the kidney of the adult mouse the tubular epithelium in the most proximal (Pr) segment, corresponding roughly to the convoluted portion of the proximal tubule, shows strong alkaline phosphatase activity, and the brush border is moderately reactive to the periodic acid Schiff (PAS) method. In the more distal (P2) segment of the proximal tubule there is no alkaline phosphatase activity, and the PAS reaction of the brush border is intense. Examination of a large number of young mice has now revealed a distinct pattern in the development of this adult condition.

Differentiation of the two segments on the basis of the PAS reaction of the brush border becomes apparent on about the 15th post-natal day. The effect is the result of a decrease in the reactivity of the Pi segment. Differentiation of the two segments on the basis of alkaline phosphatase activity develops gradually between about the 22nd and 36th post-natal days. During this period the alkaline phosphatase activity disappears progressively from the proximal to the distal end of the P2 segment.

The administration of testosterone or estradiol to either sex accelerates the differentiation with respect to alkaline phosphatase activity. Castration of male mice retards the completion of this process.

It is suggested that morphological changes of this type may provide the basis for some of the functional differences between the kidneys of young and mature animals.

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