Ultracytochemistry of ouabain‐sensitive K+‐dependent p‐nitrophenyl phosphatase in rat incisor enamel organ

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Abstract
Sprague‐Dawley strain rats of 4–5 weeks old were perfusion‐fixed with either a mixture containing 0.1 or 0.25% glutaraldehyde and 2% formaldehyde, or a 2% formaldehyde in 0.1 M sodium cacodylate buffer for 10 minutes. Non‐decalcified 30–50‐μm sections of the enamel organ taken from lower incisors were then processed for ultracytochemical demonstration of ouabain‐sensitive, K+‐dependent, p‐nitrophenyl phosphatase, by use of the one‐step lead method, representing the second dephosphorylative step of Na+‐K+‐ATPase. Throughout the secretory, transition, and maturation stages of amelogenesis, the enzymatic activity was demonstrated along the cytoplasmic side of the plasma membranes of the stratum intermedium and the papillary layer cells, especially along their numerous micro‐villi. The plasma membranes forming gap junctions and desmosomes were free of reaction or showed slight focal precipitates of reaction products. The stellate reticulum and the outer enamel epithelium exhibited either a weak reaction or were reaction negative. Secretory ameloblasts showed a weak trace‐like reaction along the basal and lateral cell surfaces; however, the latter surfaces were sometimes completely free of reaction. Tomes' processes were usually reaction negative. Ameloblasts in the transition and maturation stages were devoid of enzymatic activity, except for a slight reaction along the plasma membranes of the basal cell surfaces of transition ameloblasts facing the papillary layer. The enzymatic activity described above was completely dependent on the presence of potassium and substrate in the incubation media and was almost completely inhibited by an addition of 10 mM ouabain to the incubation media.