SEM Survey of Human, Shark, and Rodent Teeth

James Yuzawa
University of Rochester, Department of Computer Science
Rochester, NY 14627 -

Project Overview


Abstract: The scanning electron microscope (SEM) and X-ray spectrometer are valuable tools for examining microstructures and determining composition, respectively. In this survey, human, shark, woodchuck, and raccoon teeth were all examined in the SEM. Qualitative analysis showed that all of the teeth were composed of the same calcium phosphate mineral. Additionally, the enamel structures all generally followed the same stratified layered structure. Variation in dentin tubule density and size was also seen.

Introduction

The scanning electron microscope is a good tool for examining the microstructures of dental tissue. The field of view of a light microscope at high magnifications is not good enough to view the fine details and defects of dental material. Additionally, the usage of three different electron detectors in the SEM and an X-ray spectrometer can all allow for additional analysis to be done outside of the realm of light microscopy. With these tools, the teeth of four different animals will be examined: Human, shark, raccoon, and woodchuck. The composition of each type of animal's teeth and the microstructures of the enamel and dentin will be examined and compared.

Teeth consist of three major layers of tissue. The innermost layer is the pulp, which is living tissue. In the samples examined, this has either decayed away or dried up. The outermost layer, which is normally visible and experiences wear, is the enamel. The layer in between is the dentin. Both the enamel and dentin are made of similar material called hydroxyapatite. All of the samples have been outside of their living hosts for more than a few months. They were all air-dried, so sensitive biological cellular structure of bacteria, surrounding tissue, and pulp has been damaged.

Methods & Sample Preparation

In order to be imaged in the SEM, the teeth needed to be properly mounted on SEM stubs and possibly sectioned and polished. Both intentionally fractured and polished cross sections were imaged. The fractured samples were broken by applying slight force from a pair of pliers.

All samples were mounted on SEM stubs using thermoplastic to ensure they would not fall off. The conductive tape did not do a good job holding the samples securely. The SEM stubs were heated on a hot plate on its lowest setting. After the several minutes, a piece of thermoplastic was rubbed or pushed onto the hot surface of the SEM stub until an appropriately sized drop was accumulated in its molten form. At this point, the stage was removed from heat and the sample was immediately placed and held in the molten drop until the drop hardened. This effectively welded the bottom of the sample to the SEM stub.

After the samples were mounted, some were sanded and polished to reveal a cross section. The cross sections were prepared using a water-cooled metallurgical grinder/polisher (a South Bay Technology Model 910) at medium speed. Low grit sand paper was used to wear away the desired amount of material. Then very high grit diamond impregnated plastic was used on the wheel to polish the surface. The gradually finder diamond plastic paper was used to complete the polishing process (15 μm, 6 μm, 3 μm, 1 μm). After this was completed, the cross-section samples were rinsed and then air-dried. These samples appeared to be polished and were reflective.

Before sputter coating, as described below, the stubs were swabbed in isopropanol to clean them.

Microscopic Methods

A Zeiss Auriga CrossBeam SEM-FIB with an attached EDAX X-ray spectrometer was used to collect all of the micrographs in this survey.
The following microscopic methods were used:

Non-Microscopic Methods

The following methods were done as preparation or post-processing:

Results

The resulting images and spectra for each type of animal are included on their respective pages. Details on enamel structure, dentin structure, and composition are included.

Discussion

There was quite a variation in enamel thickness across the different animals. Humans had the thickest relative enamel size and sharks had the thinnest relative enamel size. The rodents were in between. Humans live the longest and practice dental hygiene, so a strong enamel layer would be very good in order to properly masticate food over the course of a lifetime. Sharks have multiple rows of teeth, which fall out and are replaced when damaged. Due to the replaceable nature of shark teeth, it does not seem worth the time and effort to develop a tooth with a lot of enamel. The rodent teeth appear to be eroded away by tooth decay, so it is unclear whether they originally have a thick layer of enamel and then live their lives getting it eroded away by decay. The microstructure of the enamel was generally similar across animals as well. The layered stratification was seen in the fractured samples from humans (Figure 1), sharks (Figure 9), and rodents (Figure 15). It was most visible in the human sample.

All of the animals had similar dentin structures. It appeared as a spongy material filled with tubules in all three types of animals. The size and density had a larger variation. It is unclear whether there is variation across species, type of tooth, or individual, or some combination of the three. A larger sample size with different types of teeth (molars, incisors, canine) from multiple individuals and species would be needed to conclude this.

The composition of the teeth in humans, sharks, and rodents is almost identical, as indicated by the similar spectra collected from teeth of each animal. The X-ray spectra were consistent with the teeth containing a type of calcium phosphate mineral called hydroxyapatite. No trace metals or other elements could be discerned from the spectra. Any heavy metals in an animal's diet would accumulate in the teeth and bones. The resolution on the EDAX is not good enough to capture trace elements. A wavelength dispersive spectrometer (WDS) could have the resolution to detect trace elements. However, such traces may be so low that they are obscured by the noise from the bremsstrahlung radiation.

Conclusion

The SEM proved to be a good tool in comparing the microstructure and composition of the teeth of different species. The main difference between species was in regards to enamel thickness. The origin of the variation of dentin tubule size and density is still unknown. The spongy, tubule-filled nature of dentin was seen across all species examined. The resolution of the EDS X-ray spectrogram was not good enough to provide details about trace elements or heavy metals. However, the X-ray spectra were consistent with the teeth containing the mineral hydroxyapatite.

Acknowledgements