Glossary of
Enamel Microstructures

© 7 June 1999 W. v. Koenigswald


A modified version of W.v. Koenigswald & P.M. Sander 1997; Glossary . 267-297 in Koenigswald W.v. & Sander, P. M.: TOOTH ENAMEL MICROSTRUCTURE. - Balkema, Rotterdam

In every field of research the consistent and correct use of technical vocabulary is essential to ensure efficient communication and to avoid misunderstandings and duplication of work. We felt that "Tooth Enamel Microstructure" would benefit greatly from a glossary. This is the first such exclusive glossary for enamel terminology. We believe it to contain all important terms but we did not strive for completeness. However, we took special care to emphasize a number of central concepts, i.e. Hunter-Schreger bands, prism, prism type, enamel type, and schmelzmuster.

For each term we provide a comprehensive explanation and, where appropriate, a definition based on our understanding. We always attempt to describe the structures in three dimensions as this is how they must be reconstructed from the different planes of section. Thus, we would like to discourage descriptions of two dimensional phenomena based on single sections only. Of course, every structural element has its ontogenetic and phylogenetic history. We avoid linking ontogeny or phylogeny to the definition of structural terms, however, since frequently only the structure is known but not its ontogeny nor phylogeny. We comment on these aspects if we feel justified to do so from the available evidence.


Table 1. Hierarchical levels of complexity in mammalian and reptilian enamel (Koenigswald & Clemens 1992, Sander 1995)

COMPLEXITY OF AMNIOTE ENAMEL

LEVELS

CHARACTERIZATION

 

Prismatic enamels (mainly in mammals) Prismless enamels
(mainly in reptiles)
Dentition Variation in schmelzmuster throughout the dentition As in prismatic enamel
Schmelzmuster Three-dimensional arrangement of enamel types in one tooth Three-dimensional arrangement of enamel types and major structural discontinuities in one tooth
Enamel type Orientation of prisms relative to the EDJ and to one another (parallel or decussating, and nature of the decussation). Differences in orientation of IPM crystallites relative to prisms are recognized as subtypes. Large volume of enamel which consists of crystallites or modules of the same type and of similar orientation.
Prism/module type Prism type describes the characters of a prism (particularly cross section, absolute prism size, prism density, crystallite orientation inside prisms, IPM surrounding prisms, and seam) Module type describes a repeatable volume of enamel that is delimited by crystallite discontinuities or by zones of changing crystallite orientation.
Crystallite Orientation of crystallites As in prismatic enamel

Note: The symbol ® indicates terms that are defined elsewhere in this glossary.

Aprismatic enamel: Term synonymous with ® parallel crystallite enamel. This term, as well as "nonprismatic enamel", should not be used because they imply all kinds of enamel without prisms. All enamel types without prisms are best described as ® prismless enamel.

Arcade enamel: Prism type with open prisms in horizontal rows, synonymous with ® Pattern 3 and partially with ® key hole pattern.

Banded enamel: Synonymous with Hunter-Schreger bands (Wahlert 1989).

Central bend: Specific feature in ® uniserial HSB in the lower incisors of some cricetid rodents. HSB are narrowly arched along a parasagittal plane between two symmetrical fields of obliquely or vertically oriented uniserial HSB (Kalthoff in press).

Compound unit: Enamel module in prismless enamel. Compound units are groupings of ® columnar units or ® microunits into higher order units.

Columnar unit: Enamel module in prismless enamel. Columnar divergence units (abbrev.: columnar units) consists of a linear zone of crystallite divergence surrounded by planes of crystallite convergence. The cross section of these columnar units in usually irregularly polygonal. The various kinds of columnar units differ in absolute size, cross sectional shape, relative length, angle of crystallite divergence, presence or absence of central lines or planes of crystallite divergence, and in the completeness of their boundary planes (Sander 1996).

Columnar units are the most common structural feature in prismless enamel.

Crystallite discontinuities: If the angle of divergence or convergence of crystallites surpasses a threshold value, crystallite discontinuities will arise. These discontinuities may either be planar or linear features and are the major components of features at the module level.

Decussating prisms: Prisms that decussate in a regular fashion (® HSB) or irregular fashion (® irregular enamel). "Decussating prisms" is sometimes used synonymously with Hunter-Schreger bands. This term obviously should be avoided as HSB are but one instance of decussating prisms.

Diazone and parazone: purely an artifact of sectioning in Hunter-Schreger bands, not a true structure.

In sections vertical to HSB, decussating prisms are intersected at different angles. Layers in which prisms are truncated were called diazones by Preiswerk (1894, 1896), while layers in which prisms are intersected at low angles or longitudinally were called parazones.

Three-dimensional understanding of Hunter-Schreger band structure makes it clear that the description of a particular prism layer as a diazone or parazone solely depends on the orientation of section and not on structural differences. Although layers of decussating prisms are of uniform thickness in all HSB, the thickness of the diazones may differ from that of the parazones at certain angles of section due to optical phenomena.

The terms diazone and parazone were often used in the old literature, but today these and other descriptions of sectioning artifacts should be avoided.

Enamel dentine junction (EDJ): The boundary between the enamel and the dentine of a tooth. Amelogenesis begins at the EDJ and is directed outward. A synonym is DEJ.

Enamel layer versus enamel zone: The enamel cover of a tooth may consist of different enamel types superimposed on one another. The term layer is used when the boundary between enamel types is very sharp, but the term zone if there is a gradual transition between enamel types without a distinct boundary. In general, boundaries between successive enamel types are more distinct in thin enamel (< 200 μm) than in thick enamel (see Koenigswald et al. 1994).

Enamel module: In the hierarchical terminology, a module is a repeatable volume of enamel that is delimited by crystallite discontinuities or by zones of changing crystallite orientation. Modules may form more than one level of the hierarchy, i.e. the modules themselves may consist of smaller modules. Accordingly, modules are scale-independent and may range in size from a few μm to more than one mm.

Several specific types of modules can be distinguished, most prominently the ® enamel unit and the ® enamel prism.

Enamel prisms: There is not one single feature that can distinguish prisms from various kinds of ® columnar units. Only a suite of features can be described.

Prisms are bundles of crystallites which extend from the EDJ close to the OES without interruption. Prisms do not split or merge. Prisms are of equal size. Prisms have a well defined boundary plane (prism sheath). In the primitive state, the mammalian prism sheath is open basally. Prisms are also surrounded by interprismatic matrix in the primitive state. Prism cross section are never polygonal. Primitively, prism cross sections are rounded but may vary in shape depending on taxonomic affinity and enamel type. Prisms have the ability to decussate in layers (® HSB), in groups, and individually (® irregular enamel).

All of the characters in this structural prism definition are explained by the formation of a single prism by a single ameloblast, but it should be noted that this causal link does not enter into the definition of a prism.

Enamel rod: Term used synonymously with enamel prism. Although less frequently observed than prisms, rod is the better term because there is no danger of confusion with the prisms in mollusc shells which are structurally very different, nor with crystallographic terminology. In the interest of nomenclatural stability, however, we prefer "prism" over "rod".

Enamel tubules: Unmineralized passages in the enamel originating at the EDJ. In prismatic enamel, tubules follow the prism course.

Tubules are common in many marsupials with the exception of wombats. In placentals, tubules are not as widespread. If they do occur, they do not extend very far into the enamel (Boyde & Lester 1967, Gilkeson 1997). Enamel tubules also commonly occur in prismless and prismatic reptile enamel (e.g. in Uromastyx). The developmental origin of enamel tubules is still rather controversial as some workers believe them to be odontoblast processes extending into the enamel while others favor ameloblast affinities (Gilkeson 1997).

Enamel type (in prismatic enamel): Volume of enamel in which the prisms are either parallel to each other (radial and tangential enamel), or decussate in a regular fashion (Hunter-Schreger bands, Zipper enamel) or in an irregular fashion (irregular enamel). Since several enamel types commonly follow one another in succession from the EDJ to the OES, prisms continue through different enamel types. The boundaries between different enamel types are thus delimited by changing prism directions.

The number of different enamel types is restricted by the limited constructional abilities of the ameloblasts.

® Radial enamel is regarded as the most primitive enamel type because the fossil record documents the evolution of the other enamel types from radial enamel.

Enamel type (in prismless enamel): In prismless enamel, an enamel type is a large volume of enamel that consist of crystallites or modules of the same type and of similar orientation. The enamel of a tooth may possess more than one enamel type, and enamel types may be arranged in layers from the EDJ to the OES or may be distributed in other ways.

Enamel unit: At the module level, the enamel unit is the smallest repeatable volume of enamel bounded by planar crystallite discontinuities.

Gigantoprismatic enamel: Enamel formed by an unusual prism type with very large diameters of about 8 μm instead of the usual 4-5 μm in other mammals. Gigantoprismatic enamel is found only in taeniolaboid multituberculates (Fosse et al. 1978, Carlson & Krause 1985, Clemens 1997). Currently, gigantoprismatic enamel is only known as radial enamel.

Growth line: Growth lines (synonyms: incremental lines, striae of Retzius) are the sectional traces of surfaces in the enamel resulting from an interruption or a slowing in enamel growth.

These surfaces form a series of stacked conical layers on the dentine core of a tooth and reflect the appositional growth of enamel. Each successive growth interval extends farther basally on the tooth which, in longitudinal section, is expressed by the fact that the incremental lines form an acute angle with the EDJ. If the most apical cells in an enamel organ cease secretion before the basal cells, growth lines will intersect with the outer enamel surface as well. They may then be visible on the OES as circumferential ridges called perikymata.

Growth lines forming an angle with the EDJ are thus a reliable criterion for appositional growth indicative of monotypic ("true") enamel (Smith 1989).

Hunter-Schreger bands (HSB): Specific mode of prism decussation. Prisms decussate in layers, also called bands. Layer bifurcations are always present. Each set of layers bifurcate in only one direction. Transitional zones may occur between bands.

The spatial arrangement of the layers relative to the EDJ is described by two parameters, the ®inclination and the orientation in tangential view. In vertical sections the inclination can be observed. Primitively, HSB are roughly perpendicular to the EDJ i.e. they are not inclined. In derived states, HSB may be inclined upwards or occasionally downwards to the OES.

The orientation in tangential view (in earlier works called lateral orientation) describes the direction of the bands as seen tangential sections or from the outside. Primitively, band or layer orientation is transverse to the growth axis of the tooth and is slightly undulating. In derived states, the amplitude of undulation may increase or the transverse orientation can be modified to arched, u-shaped, vertical, or oblique.

Thickness of the bands can vary from more than 30 prisms to one prism. Subtypes of HSB are based on band thickness and IPM orientation. In rodent incisors, ® pauciserial, ® uniserial, and ® multiserial HSB are distinguished (Korvenkontio 1934, Wahlert 1989). Prism cross sections often are elliptical in tangential sections. The long axis of the ellipse is inclined in opposite directions in alternating bands. In combination with transitional zones between bands, this feature causes undulation of the IPM in tangential view. Such undulation is often regarded as typical for Lagomorpha but is a general feature of HSB.

Hunter (1778) and Schreger (1800) figured the bands and described some details (Fig. 1). Thus HSB were originally described as an epiphenomenon. However,Preiswerk (1894, 1896) had already begun to use the term for the structure of ® diazones and parazones and not for optical phenomenon.

Inclination: Defined by Korvenkontio (1934) as the angle between prisms or HSB and the normal on the EDJ in vertical sections. The inclination is zero when HSB are perpendicular to the EDJ. Positive values describe prisms or HSB that are directed toward the tooth apex.

Interprismatic matrix (IPM): Hydroxyapatite crystallites that are not part of a prism. Primitively, the crystallites of the IPM diverge and include different angles with the prism direction. In derived enamels, the IPM crystallites are parallel to one another but at an angle to prism direction. Synonyms: interprismatic substance, interprismatic phase.

Interrow sheets: Layers of IPM between rows of prisms. Interrow sheets occur in radial enamel as well as in HSB (Boyde 1965).

Irregular enamel: Enamel type with irregular decussation of individual prisms or bundles of prisms. The descriptive term "irregular enamel" covers enamel types of different evolutionary origin. In placentals, most irregular enamels evolved from HSB. Marsupial irregular enamels, on the other hand, are derived from radial enamel.

Key hole pattern: Prism type with a basally incomplete prism sheath (= pattern 3). The prisms are arranged in horizontal rows and are densely packed (Boyde's "pattern 3B", Fig. 2). The "tail" which is traditionally regarded as part of the key hole prism actually consists of the IPM which separates the prisms in the next row towards the crown base. Ameloblasts other than the one producing the prism body are involved in the secretion of this tail.

Key hole prisms are found throughout the enamel of higher primates but in elephants and sirenians as well. In other taxa, this prism type is restricted to specific enamel types.

Lemming enamel: An enamel type, irregular enamel in lemming molars. The prisms decussate individually because lemming enamel is derived from uniserial HSB (Koenigswald & Martin 1984).

Levels of complexity: Hierarchical levels used in enamel microstructure descriptions (tab. 1). Levels of complexity integrate different research traditions and approaches (Koenigswald & Clemens 1992).

Light guide effect of prisms: The highly mineralized prisms function like fiber optics because light entering the prism is reflected from the surrounding prism sheath and cannot escape from the prism. Layers of prisms in Hunter-Schreger bands that intersect at different angles show up as light and dark bands. Light and dark bands are switched when the direction of illumination is reversed. The light guide effect can be used to identify prism directions under the light microscope (Koenigswald & Pfretzschner 1987).

Major structural discontinuity: Major structural discontinuities are planes of crystallite and module divergence (called major divergence) or convergence (called major convergence) that extend through much if not all of the enamel layer. Major discontinuities are common in prismless enamel but are rare in prismatic enamel.

Major structural discontinuity is not to be confused with the major discontinuity of Lester & Koenigswald (1989) which is basically synonymous with prism sheath.

Microunit: Microunits are small enamel units and consist of bundles of diverging crystallites only a few crystallites across and and a few crystallites long (Sander 1996).

Modified radial enamel: Subtype of radial enamel with vertical interrow sheets as thick or thicker as the prisms. This subtype occurs as an inner layer in the hypsodont molars of many large herbivores (Pfretzschner 1994).

Multiserial enamel: Originally, the term was used to denote HSB with thick bands in rodent incisors (Korvenkontio 1934). However, in some cases band thickness is insufficient in some cases to separate multiserial from pauciserial enamel. Defining structural characters are the angle between IPM crystallites and prism direction (which increases during phylogeny from oblique to rectangular), and the presence of ® transition zones between bands and high inclination of bands (Martin 1992, 1993).

Although "multiserial enamel" was used for thick HSB in other taxa than rodents, the use of this term should be restricted to this group, as band thickness is not the only determining character. ® uniserial and pauciserial enamel.

Nonprismatic enamel: See parallel crystallite enamel.

Outer enamel surface (OES): The outer limit of enamel, the surface of amelogenesis cessation.

Parallel crystallite enamel: Parallel crystallite enamel is an enamel type consisting of crystallites parallel to each other. It lacks any higher order structural features. Parallel crystallite enamel is equivalent to what has been called nonprismatic (Carlson 1990 a+b) or aprismatic enamel (Koenigswald & Clemens 1992) in mammals. The use of these terms for specific enamel types is discouraged because nonprismatic and aprismatic simply implies that the enamel in question lacks prisms (® prismless enamel).

Pattern 1, pattern 2, pattern 3: Based on Boyde's classification of enamel microstructure (Fig. 2) (Boyde 1964, 1976, Boyde & Martin 1983, 1984), the terms pattern 1, 2, and 3 were widely used in enamel descriptions. In pattern 1, prisms are completely surrounded by the prism sheath and arranged in a hexagonal pattern. In pattern 2, the prism sheath is open basally, and prisms are arranged in approximately vertical rows divided by interrow sheets. In pattern 3, prisms are also open basally and arranged in horizontal rows but in alternating position (also termed arcade-shaped prisms). A derived form of pattern 3 is the ® key-hole pattern (Pattern 3B).Prisms with an open prism sheath (a type of pattern 2) are most likely the most primitive pattern (Koenigswald & Clemens 1992).

These three patterns insufficiently cover the great variety of structures in mammalian enamel. In terms of the levels of complexity, the patterns are not coherent. Mostly, the patterns can be regarded as prism types, but "pattern 2B" (Fig. 2) of Boyde (1964) and Boyde & Martin (1984), is not a prism type but a particular enamel type, uniserial HSB. We thus feel that the enamel pattern approach is inadequate for enamel microstructure descriptions.

Pauciserial enamel: The term was originally used to denote HSB with bands only a few prisms thick in rodent incisors (Korvenkontio 1934. However, band thickness may well increase to that of multiserial enamel. The defining character of pauciserial enamel are thus

IPM crystallites parallel to the prism direction and the lack of transition zones (Wahlert 1984). HSB are not or only slightly inclined. Pauciserial enamel is the most primitive type of HSB in rodent incisors (Martin 1992, 1993). ® uniserial and multiserial enamel.

Perikymata: Lines on the enamel surface related to striae of Retzius (Moss-Salentijn et al. 1997, also see growth lines).

PLEX: Acronym for "prismless external layer". The PLEX is formed by IPM and is found in many mammals at the OES and is usually very thin (Martin 1992, 1993).

Portio externa, portio interna: The enamel of rodent incisors is normally two-layered. Korvenkontio (1934) defined the inner layer with HSB as "portio interna" and the outer layer with radial enamel as "portio externa". Although two-layered enamel is common in other mammal teeth, these terms should be restricted to rodent incisor enamel.

Preprismatic enamel: Term referring to any one of a number of prismless enamel types that are structurally more complex than ® parallel crystallite enamel. This term should be avoided because it does not refer to a specific enamel type and carries unwanted evolutionary connotations as preprismatic enamel cannot be shown to be ancestral to prismatic enamel. The same applies to the terms "pseudoprismatic" and "protoprismatic" which are essentially synonymous to "preprismatic" (also Sander 1997).

Prism: See enamel prism

Prism cross section: Shape of the prism sheath in tangential section. The cross sections were regarded to have great systematic value. However, prism cross section is strongly dependent on enamel type and plane of section. Shobusawa (1952) and Boyde (1964, 1965, 1976) developed different terminologies (see Fig. 2). The mostly widely used terminology is ® pattern 1, 2, and 3. It should be noted that prism cross section is only one aspect of the ® prism type.

Prism direction: Prism direction is determined by the direction of the prism sheath, not by that of the crystallites. The crystallites within a prism may be set at an angle variable or constant to prism direction. The angle of the IPM is measured against prism direction, not against the crystallite direction within the prism. Prism direction does not necessarily apply to the entire length of the prisms but can be determined for specific intervals of a prism, for example in different enamel types.

Prism packing pattern: See pattern 1, 2, and 3.

Prism type: Describes the characters of a prism. Most important is the cross section, i.e. the shape of the prism sheath. We suggest, however, that absolute prism size, prism density, crystallite orientation within the prism, the IPM surrounding the prism, and the presence of the seam be incorporated into prism type descriptions.

Prism cross sections figure in the definition of ® pattern 1, 2, 3 (Boyde 1964, 1976 Boyde & Martin 1983, 1984, Carlson 1990 a+b). However, prism cross section may vary as a prism passes through different enamel types. Nevertheless, certain prism types may be characteristic for some mammalian taxa of higher rank.

Prismless enamel: General term describing enamel which lacks ® enamel prisms. Prismless enamel is the opposite of prismatic enamel, i.e. enamel with prisms.

Radial enamel: Enamel type with prisms directed away radially from the EDJ. In transverse section, the prisms are perpendicular to the EDJ. In vertical section, the prisms exhibit inclinations from zero to nearly vertical.

Orientation of the IPM crystallites relative to prism direction defines different subtypes of radial enamel: primitive radial enamel, radial enamel with anastomosing IPM, radial enamel with ® interrow sheets, and ® modified radial enamel.

Retzius lines see Striae of Retzius

Rodent enamel: Term sometimes used for ® uniserial HSB in the portio interna of sciuromorph and myomorph rodent incisors. Other synonyms of this enamel type characterized by its great symmetry are "lamellar enamel" or Boyde's "pattern 2B" (Fig. 2).

Seam: A ribbon-shaped planar crystallite convergence bisecting the basal opening of the prism sheath in plesiomorphic prisms (see also Lester & Koenigswald 1989, Wood & Stern 1997, Sander 1997).

Schmelzmuster: In prismatic enamel, the schmelzmuster describes the spatial distribution of enamel types in the enamel cover of a tooth. ® Enamel types may be arranged in layers from the EDJ to the OES but also may vary along the side and between the top and the bottom of an enamel cap. The distribution of the different enamel types in the enamel cover of a tooth is controlled by biomechanical and phylogenetic constraints.

In ® prismless enamel, the spatial distribution of ® major structural discontinuities should be described in addition to that of the enamel type.

In contrast to the limited number of enamel types, the possible combinations of enamel types into schmelzmusters are too numerous to be described here. The evolutionary differentiation of the schmelzmuster can be observed in detail in the fossil record and can provide important characters for phylogenetic reconstructions (Koenigswald 1980).

Striae of Retzius: Histological landmarks of the mineralization front during the formation of enamel. They are formed by hypomineralized bands. The neonatal line is the most prominent marker (Moss-Salentijn et al. 1997 and ® growth lines).

Synapsid columnar enamel (SCE): SCE is a subtype of columnar enamel. The columnar units of SCE are small, 5 - 10 μm in diameter, and have irregular polygonal cross sections. The angle of crystallite divergence in the columnar units is commonly small and lack a central line of divergence. The columnar units are usually short and discontinuous but can also be fairly continuous. The degree of continuity is, of course, linked to the relative completeness of the unit boundary planes (Sander 1997).

Synchronous prism deviation: Thin zone parallel to the EDJ between different enamel types with parallel prisms such as radial enamel and tangential enamel. All prisms change orientation at the same plane in the same direction and do not decussate. Observations show that at synchronous prism deviations the IPM always changes orientation in the direction opposite to the prisms. Synchronous prism deviations occur frequently in marsupials but should not be confused with HSB (Koenigswald 1994) which they may resemble in some transverse sections.

Tangential enamel: Enamel type with prisms that have strong lateral deviation and almost no ® inclination. All prisms are parallel to one another but the IPM anastomoses at nearly right angles. Tangential enamel evolved from radial enamel and frequently occurs in marsupials but only occasionally in placentals (Koenigswald 1977, 1994).

Tomes' process: Process of the enamel forming ameloblast. The shape and form of the Tomes' process is significant for the shape of the prisms and the orientation of the IPM (Boyde, 1964, Moss-Salentijn et al. 1997).

Transition zone: Descriptive term for detail in HSB, the zone between two bands in which prisms change regularly from one band to the next (Wahlert 1989). This transition zone may be wide or narrow. In uniserial HSB individual prisms change from one band to the next but do not form a transitional zone.

Tufts: Poorly mineralized segments of enamel prisms originating from the EDJ in vertical orientation in human enamel (Moss-Salentijn et al. 1997).

Uniserial HSB (= lamellar enamel): Term only used in rodent incisors and molars. Uniserial HSB consist of layers one prism thick. Short distances with two prisms in one layer may occur. In the primitive state, the IPM crystallites run parallel to prism direction. In derived states, the IPM crystallites are at an angle of nearly 90° to both sets of prisms. The term "lamellar enamel" is synonymous to uniserial HSB (Wahlert 1989, Martin 1992).

Wavy enamel: Wavy enamel is an enamel type in prismless enamel. Wavy enamel consists of parallel crystallites in a staggered arrangement. In addition, the crystallites are not perpendicular to the EDJ but constantly set at a specific angle with a gradual change of direction towards the enamel surface. In combination with their staggered orientation this results in a helical or whorled arrangement of the crystallites. The indistinct growth lines in wavy enamel are presumably due to the staggered origination of crystallites (Sander 1996).

Whorled enamel: Descriptive term for irregular enamel found in the marsupial wolf Thylacinus (Young et al. 1990).

Zigzag HSB: Subtype of transversely oriented HSB in which the undulation is exaggerated to such a degree that vertical elements are introduced. This enamel subtype evolved independently in several bone-feeding carnivores such as hyenas and borophagines as well as in a few large mammals such as Entelodon (Stefen 1995, 1997).

Zipper enamel: Narrow zone of prism decussation where two fields of radial enamel meet at a prominent crest or ridge. Zipper enamel is found in the enamel ridges of most diprotodont marsupials (Koenigswald 1994).


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Last updated 8 June 1999 (JHW)