Serially etched shark enameloid observed by incident light microscopy

Area of Science:

• * Ichthyology
• * Paleontology
• * Materials Science

Background:

• * The microstructure of shark teeth, particularly enameloid, is crucial for understanding feeding adaptations and evolutionary history.
• * Previous studies have utilized various techniques to investigate tooth structure, but detailed analysis of enameloid etching properties is limited.

Purpose of the Study:

• * To investigate the serial acid etching of Isurus oxyrinchus (mako shark) tooth sections.
• * To characterize the optical properties of etched enameloid surfaces and their relationship to crystallite orientation.
• * To compare the acid solubility of shark enameloid with human enamel.

Main Methods:

• * Longitudinal and transverse tooth sections of Isurus oxyrinchus were prepared.
• * Serial etching was performed using 2.6% nitric acid.
• * Optical properties of etched surfaces were observed, and enameloid descent was measured.

Main Results:

• * Shark enameloid exhibited lower etching effectiveness compared to human enamel, potentially due to fluorapatite vs. hydroxyapatite solubility differences.
• * Key structural information was obtained within the first five of ten serial etchings.
• * Crystallite orientation influenced light reflection: longitudinal sections reflected light better than transverse sections.
• * Dentinal extensions, continuous with dentine, and radial fibers contained organic material and crystallites.
• * Imbibing specimens with water enhanced the visibility of dentinal extensions and radial fibers.

Conclusions:

• * Serial acid etching is effective for revealing shark enameloid structure, though less so than for human enamel.
• * The orientation of crystallites is a critical factor in the optical properties of etched enameloid surfaces.
• * Dentinal extensions and radial fibers are integral components of the tooth's internal structure, with improved visualization upon hydration.