Enamel pits in hamster molars, formed by a single high fluoride dose, are associated with a perturbation of transitional stage ameloblasts

Abstract

Excessive intake of fluoride (F) by young children results in the formation of enamel subsurface porosities and pits, called enamel fluorosis. In this study, we used a single high dose of F administered to hamster pups to determine the stage of ameloblasts most affected by F and whether pit formation was related to F-related sub-ameloblastic cyst formation. Hamster pups received a single subcutaneous injection of either 20 mg or 40 mg NaF/kg body weight, were sacrificed 24 h later, and the number of cysts formed in the first molars were counted. Other pups were sacrificed 8 days after F injection, when the first molars had just erupted, to score for enamel defects. All F-injected pups formed enamel defects in the upper half of the cusps in a dose-dependent way. After injection of 20 mg NaF/kg, an average of 2.5 white spots per molar was found but no pits. At 40 mg NaF/kg, almost 4.5 spots per molar were counted as well as 2 pits per molar. The defects in erupted enamel were located in the upper half of the cusps, sites where cysts had formed at the transition stage of ameloblast differentiation. These results suggest that transitional ameloblasts, located between secretory- and maturation-stage ameloblasts, are most sensitive to the effects of a single high dose of F. F-induced cysts formed earlier at the pre-secretory stage were not correlated to either white spots or enamel pits, suggesting that damaged ameloblasts overlying a F-induced cyst regenerate and continue to form enamel.

Fig. 1

Fig. 1a. Erupted first mandibular molar of the hamster, top (occlusal) view. Hamster first molars have six upright cusps in three successive rows of two (either labial or lingual), including a mesial, central and distal pair. (Drawing adapted from Gaunt [1961] with permission.) Fig. 1b. Sagittal section of an unerupted mandibular molar 24 h after a single injection of 20 mg NaF/kg b.w. Arrow indicate cyst. Toluidine blue staining, x25. Fig. 1c. Schematic representation of the various tooth surfaces screened for enamel defects. The grey area indicates the occlusal surface, defined as the area within the lines that connect the cusp tips (small circles). Vestibular surfaces: 1a+2a+3a; lingual surfaces: 1c+2c+3c; anterior (mesial) surface: 1b; posterior (distal) surface: 3b; mesial cusps; 1a+1b+1c+4; central cusps: 2a+2b+2c; distal cusps: 3a+3b+3c+5; occlusal surface: 2b+4+5

Fig. 2

Fig. 2a–e: Erupted first molar from 12 day old postnatal pups, 8 days after injection of F. Fig. 2a, b: Control molar injected with NaCl seen under either white light (Fig. 2a) or laser light (QLF; Fig. 2b). Fig. 2c–d: Molar of a pup injected with 20 mg NaF/kg b.w. either under white light (Fig. 2c) or laser light (QLF, Fig. 2d). Fig. 2e: Molar of a pup injected with 40 mg NaF/kg b.w. under white light. White spots are located within white circles (Fig. 2b, c) and in Fig. 2e a large pit is indicated by white arrow heads. Note that around the pit defect the enamel surface is opaque white (white arrows).

Fig. 3

The distribution of white spots (percentages in black) and pits (percentages in red) over the various enamel surfaces show a preference for spots to form in lingual/palatal surfaces as compared to vestibular/labial surfaces (p< 0.05) and in the pair of distal cusps compared to mesial cusps (p< 0.05). The calculations were made from the collective data from upper and lower molars, treated with 20 mg and 40 mg NaF/kg b.w. Cumulative number of white spots: n=56. Cumulative number of pits: n=15.