Appositional enamel growth in molars of South African fossil hominids

Enamel is formed incrementally by the secretory activity of ameloblast cells. Variable stages of secretion result in the formation of structures known as cross striations along enamel prisms, for which experimental data demonstrate a correspondence with daily periods of secretion. Patterns of variation in this daily growth are important to understanding mechanisms of tooth formation and the development of enamel thickness. Transmitted light microscopy (TLM) of histological ground sections and scanning electron microscopy (SEM) of bulk specimens or their surface replicas are the usual methods for investigating cross striations. However, these methods pose some constraints on the study of these features in Plio-Pleistocene hominid enamel, the specimens of which may only rarely be sectioned for TLM or examined on only their most superficial surfaces for SEM. The recent development of portable confocal scanning optical microscopy (PCSOM) resolves some of the restrictions on fractured enamel surfaces, allowing the visualization of cross striations by direct examination. This technology has been applied here to the study of Australopithecus africanus and Paranthropus robustus hominid molars from the Plio-Pleistocene of South Africa. We hypothesize that these taxa have increased enamel appositional rates compared with modern humans, because despite having thicker enamelled molars (particularly P. robustus), the enamel crowns of these fossil taxa take an equivalent or reduced amount of time to form. Cross striations were measured in cuspal, lateral and cervical regions of the enamel crowns, and, within each region, the inner, middle and outer zones. Values obtained for A. africanus outer zones of the enamel crown are, in general, lower than those for P. robustus, indicating faster forming enamel in the latter, while both taxa show higher rates of enamel growth than modern humans and the African great apes. This demonstrates a relatively high degree of variability in the mechanisms underlying the development of enamel across taxa

Micro‐computed tomography analysis of enamel structure after different bleaching treatments

The aim of this investigation is to assess effect of three different bleaching techniques on structural integrity, mineral density and volume of enamel via micro-computed tomography (micro-CT) and examine color changes by CIELAB and CIEDE2000 formulas. Twenty-four upper premolar teeth were randomly separated into three different groups (n = 8) (group 1, 35% hydrogen peroxide (HP) gel with Er,Cr:YSGG laser activation; group 2, 35% HP gel with diode laser activation; group 3, 35% HP gel with chemically activation). High-resolution micro-CT system (Bruker Skyscan 1275, Kontich, Belgium) was carried out to scan samples. Each tooth was scanned twice before-after application of bleaching with same scanning parameters. Structural thickness, structural separation, fragmentation index, mineral density, and mineral volume of enamel were calculated for each region of interest (ROI). Color changes were measured with spectrophotometer and calculated with CIELAB and CIEDE2000 formulas. Data analysis was made with one-way ANOVA with post-hoc LSD. Micro-CT analysis indicated that statistically significant differences were found in structural thickness, and separation, mineral density, and mineral volume of enamel for different ROIs before and after whitening for whole groups (P .05). All bleaching applications exhibited similar efficiency on the enamel surface. Nevertheless, bleaching process with Er,Cr:YSGG laser had less negative effects on enamel as it showed less change in enamel compared to other bleaching treatments. Micro-CT analysis of histomorphometric parameters can be useful for further mineralization studies of dental hard tissues