Ameloblastin-rich enamel matrix favors short and randomly oriented apatite crystals

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Abstract

Molecular evolution studies suggest that amelogenin (AMELX), the principal component of the mammalian enamel matrix, emerged considerably later than ameloblastin (AMBN), and enamelin. Here, we created a transgenic mouse model to ask the question how a conceivable basal enamel lacking AMELX and enriched in the more basal AMBN might compare with recent mouse enamel. To answer this question we overexpressed AMBN using a keratin 14 (K14) promoter and removed AMELX from the genetic background by crossbreeding with amelx−/− mice. Enamel coverings of amelx−/− mice and of the squamate Iguana iguana were used for comparison. Scanning electron microscopic analysis documented that AMBN transgenic (TG) × amelx−/− mouse molars were covered by a 5 μm thin ‘enameloid’ layer resembling the thin enamel of the Iguana squamate. Transmission electron microscopy revealed that the enamel of developing AMBN TG × amelx−/− mouse molars contained short (approximately 70 nm) and randomly oriented crystals, while WT controls, AMBN overexpressors, and AMELX−/− mice all featured elongated and parallel oriented crystals measuring between 300 and 600 nm in average length. Together, these studies illustrate that AMBN promotes the growth of a crystalline enamel layer with short and randomly oriented crystals, but lacks the ability to facilitate the formation of long and parallel oriented apatite crystals.

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