Tooth

A mineralized oral structure involved in food acquisition or processing. dental occlusion, replacement pattern and tissue distribution can be more informative than gross shape alone.

The dental crown is the coronal part of a tooth, anatomically distinguished from its root. Its external covering and surface geometry are adapted to contact and food processing. The visible clinical crown can differ from the anatomical crown when the gingival margin changes.

A specialized cell producing dentine. Its tissue contribution differs from that of an ameloblast.

A mineralized dental tissue formed by odontoblasts. It differs developmentally and structurally from the outer enamel coating of a typical human tooth crown.

A dentinal tubule is a microscopic channel in dentine, within the mineralized material formed by odontoblasts. The tubule network records the paths of odontoblast processes as dentine is deposited. A tubule is a structural channel; whether a prepared specimen contains fluid, a cellular process or an empty space depends on the tissue and its preparation.^[ref]

Elephant ivory provides a useful structural example. In a2017 study, observations of different cut planes and computed tomography using synchrotron radiation connected visible tubule profiles with their three-dimensional trajectories. A tube cut across, along or obliquely does not present the same two-dimensional profile. Consequently, a collection of oval profiles in a section need not consist of isolated oval cavities.^[ref]

A deliberately simplified transport example makes the channel size matter. For steady laminar flow through a straight circular tube, the Hagen-Poiseuille equation gives $Q=\pi r^4\Delta p/(8\mu L)$, where $r$ is tube radius, $L$ length and $\mu$ fluid dynamic viscosity. Doubling $r$ would multiply $Q$ by16 at fixed pressure difference and length. Real dentinal tubules branch, vary in width and may be obstructed, so this ideal calculation illustrates sensitivity to geometry rather than predicting a whole tooth's permeability.

Ivory is the dentine material of tusks and certain other large teeth. It is a mineralized biological tissue, rather than the enamel coating of a tooth or an undifferentiated piece of bone. In elephant ivory, mineralized collagen fibres form a spatially organized composite: identifying its chemical ingredients does not specify their arrangement.^[ref]

A 2018 structural study combined X-ray scattering, observations with polarized light and a scanning electron microscope. The measurements found changing fibre orientations from the outside toward the inside of the tusk. Mid-dentine fibres lay mainly along its length in layers with oscillating orientations. The authors related this arrangement and collagen birefringence to the visible Schreger pattern, rather than treating light and dark regions as necessarily different chemical substances.^[ref]

An illuminating comparison is to cut two sections of one oriented composite, one across its long direction and one along it. Their patterns can differ even if their average composition is the same. This geometrical example explains why a section's appearance must be interpreted together with its orientation; it is not a species-identification test.

The distinction between composition and organization also motivates solid-state nuclear magnetic resonance. Historical CPMAS demonstrations included an ivory billiard ball, showing how a rigid biological material could yield chemically informative spectra.^[ref]

The Schreger pattern is the recurring light-and-dark motif visible in sections of elephant ivory, the dentine of the tusk. Sections across the tusk can show a checkerboard-like motif, whereas lengthwise sections show alternating bands. This is an example of how the appearance of a three-dimensional biological tissue depends on the plane of observation.^[ref]

In2017, a study based on one elephant tusk compared several section orientations with microscopic observations and computed tomography using synchrotron radiation. The researchers related darker regions to dentinal tubules running obliquely to the cut surface and brighter regions to tubules running more nearly parallel to it. They proposed a three-dimensional tubule arrangement to connect those observations.^[ref]

The optical mechanism also involves the surrounding material: a2018 study combined X-ray scattering, polarized-light observations and scanning electron microscopy to investigate mineralized collagen orientations. Its authors proposed that the layered collagen architecture and birefringence contribute to the pattern. Thus a visible band is evidence about structure and light interaction, rather than a direct map of a single chemical constituent.^[ref]

A highly mineralized epithelial product formed by ameloblasts, distinct from dentine. fossil tissue distributions and enamel-matrix gene expression support an ancestry in dermal coverings deeper than its familiar deployment on teeth.^[ref]

An organized structural unit of mammalian crown enamel. Its architecture is a derived tissue organization, distinct from the deeper origin of the enamel tissue itself.

The arrangement and complement of teeth, including their classes, positions and replacement pattern.

A highly mineralized dental tissue. Its distribution and wear relative to other tissues help maintain functional tooth surfaces.

An anterior tooth class specialized in diverse ways for cropping, cutting or gnawing.

A posterior tooth class important in many forms of food processing. Molar form varies with lineage, position and wear.

A tooth class lying in front of the molars in the typical heterodont mammalian arrangement.

An occlusal construction combining shearing and crushing, with the lower molar talonid basin interacting with an upper cusp. Similar functional geometry does not alone settle whether all examples share one evolutionary origin.

Assessment of macroscopic wear-related cusp shape and relief. It integrates a different temporal scale of food processing from dental microwear.

Small wear features on tooth surfaces used to infer food processing near the end of the wear record. Diet, external grit, preparation and post-mortem damage must be distinguished.

Continued growth of a tooth, distinguished from a tall but finite reserve crown in hypsodonty.

A high-crowned tooth construction supplying a reserve against wear. It differs from hypselodonty, and is not by itself proof of a grass diet.

A dental construction with crescentic ridges, characteristic of many ruminants.

A dental construction in which cusps are connected into ridges. Ridge arrangement and wear influence the cutting and grinding performance of the surface.

A specialized shearing tooth. In Carnivora the principal opposing pair is the upper fourth premolar and lower first molar; other predatory lineages can specialize different teeth.

A replacement pattern with two successive generations for the replaced tooth classes. It should not be taken to mean every tooth in the jaw is replaced exactly once.

The relationship and contact of opposing teeth during jaw closure and chewing. Precise correspondence of opposing surfaces permits specialized cutting and grinding.