AbstractFifty day old Sprague‐Dawley rats were given sodium fluoride (50 mg/kg) and killed at times between one half hour and 32 days after injection. Transverse sections of the upper incisor 40 μ thick were radiographed, demineralized and stained with hematoxylin and eosin. The response seen in hematoxylin and eosin stained material agreed with previous descriptions of the “calciotraumatic response” and consisted of three components. However, in the radiogram only two components could be distinguished: an external hypermineralized zone and an adjacent hypomineralized zone. The radiographic response was first recognized four hours after injection as a narrow zone of hypermineralized dentin at the dentin‐predentin junction. When the radiograms were compared to the H and E stained sections in animals killed up to 19 hours after injection, two important features were observed: (1) the width of the predentin had increased markedly, and (2) the hypermineralized zone had increased to its final width. This was interpreted as concurrent formation of both hypermineralized and hypomineralized components. At 24 hours normal dentin formation had resumed. Since both components were forming simultaneously it is believed that sodium fluoride does not cause the response by interfering with the humoral mechanisms of mineralization, but, rather, its effect is most likely on collagen or ground s
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