Journal
ACS NANO
Volume 13, Issue 3, Pages 3151-3161Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acsnano.8b08668
Keywords
enamel; apatite; crystal growth; graphene liquid cell; atomic scale microscopy
Categories
Funding
- National Institute for Dental and Craniofacial [DE018900]
- UIC Department of Physics
- Scientific Imaging and Nanotechnology Division of the UIC Research Resources Center
- Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource [NSF ECCS-1542205]
- MRSEC program at the Materials Research Center [NSF DMR-1720139]
- International Institute for Nanotechnology (IIN)
- Keck Foundation
- State of Illinois, through the IIN
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Tooth enamel is a hard yet resilient biomaterial that derives its unique mechanical properties from decussating bundles of apatite crystals. To understand enamel crystal nucleation and growth at a nanoscale level and to minimize preparation artifacts, the developing mouse enamel matrix was imaged in situ using graphene liquid cells and atomic resolution scanning transmission electron and cryo-fracture electron microscopy. We report that 1-2 nm diameter mineral precipitates aggregated to form larger 5 nm particle assemblies within ameloblast secretory vesicles or annular organic matrix subunits. Further evidence for the fusion of 1-2 nm mineral precipitates into 5 nm mineral aggregates via particle attachment was provided by matrix-mediated calcium phosphate crystal growth studies. As a next step, aggregated particles organized into rows of 3-10 subunits and developed lattice suprastructures with 0.34 nm gridline spacings corresponding to the (002) planes of apatite crystals. Mineral lattice suprastructures superseded closely matched organic matrix patterns, suggestive of a combination of organic/inorganic templates guiding apatite crystal growth. Upon assembly of 2-5 nm subunits into crystal ribbons, lattice fringes indicative of the presence of larger ordered crystallites were observed surrounding elongating crystal ribbons, presumably guiding the c-axis growth of composite apatite crystals. Cryo-fracture micrographs revealed reticular networks of an organic matrix on the surface of elongating enamel crystal ribbons, suggesting that protein coats facilitate c-axis apatite crystal growth. Together, these data demonstrate (i) the involvement of particle attachment in enamel crystal nucleation, (ii) a combination of matrix-and lattice-guided crystal growth, and (iii) fusion of individual crystals via a mechanism similar to Ostwald ripening.
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