Various Incremental Lines of Human Enamel Observed by
Scanning Electron Microscopy: Review

Tetsuo Kodaka and Shohei Higashi

Second Department of Oral Anatomy,
Showa University School of Dentistry,
1-5-8 Hatanodai, Shinagawa-ku, Tokyo, 142 Japan


Summary

We reviewed the various incremental lines of human enamel (I to VI) previously reported by adding our SE and BSE-SEM findings. It is suggested that their incremental lines, except for some perikymatas of the undulating pattern, are caused by the physiological and structural changes of ameloblasts as suggested by Boyde 2).
In this review, however, we reported only the structures and mineralization values of their incremental lines.

Introduction

Various incremental lines of human enamel have been observed by optical microscopy including transmitted light (LM), polarized light (POL), phase contrast (PHC) and differential interference contrast (DIC), microradiography (MRG), transmitted electron microscopy (TEM), and scanning electron microscopy (SEM) with secondary electron (SE) and backscattered electron (BSE) signals.
In this study, we review the previous morphological reports on incremental lines of the enamel including cross-striations of enamel prisms (I), the Retzius lines (II), the neonatal line of the enamel (III), the perikymatas (IV), and laminate striations within the prismless enamel (V), by adding our previous SEM findings. Moreover, we report fine incremental lines on the enamel surfaces with a prismless structure after phosphoric acid etching (VI), which we recently found.


I. Cross-Striations of Enamel Prisms

Under LM or DIC of the ground enamel sections, the cross-striations are recognized as the incremental lines of prisms showing an alternation of light and dark bands with a periodicity of about 3 to 5 micrometers 3, 5, 20, 23). The POL observations suggest that the cross-striations show the periodic changes of mineralization values and the rhythmical changes of enamel crystals 6). The BSE image in SEM reveals the former changes 2), while the TEM image suggests the later changes 9). Our SE-SEM observations of the ground enamel planes after lactic-acid etching 11) reveal that the cross-striations show a rhythmical fan-shaped arrangement of crystals (Fig. 1).
The SE-SEM images of the fractured enamel planes 2) and the reconstruction of prisms by LM 22) or by DIC 1) show an alternation of constrictions and varicosities of prisms (Fig. 2). In addition, the periodic undulation of risms is observed on the longitudinal-fractured plane across the prism-head and the regions of adjacent prisms by SE-SEM 3). The constricted regions of prisms or the bases of the fan-shaped arrangement of crystals are strongly erroded with acid as compared with the thicker regions 11, 21), because the thin regions are probably more poreous than the thicker. Similar structures are seen in BSE 2).
In this section, it is concluded that the cross-striations of prisms are formed with a rhythmical fan-shaped arrangement of crystals following the periodic constriction of prisms. The low mineralized bands of the cross-striations probably agree with the constricted regions of prisms or the bases of the fan-shaped arrangement of crystals. The periodic undulation of prisms may form well-marked cross-striations.

II: Retzius Lines

The Retzius lines are roughly divided into regular and irregular or rhythmical and pathological incremental lines of the enamel 5, 6, 11). These Retzius lines have been observed by LM 2, 3, 5, 24, 28), PHC 24), DIC 28), POL and MRG 6), TEM 28), and SEM with SE 2, 4, 11, 14, 15,26, 29) and BSE 12). IIa. Rhythmical Retzius lines
The rhythmical Retzius lines are commonly visible in the surface to middle enamel of humal permanent teeth (Fig. 3). Though the LM and PHC observations suggest that the Retzius line is formed with either the row of well-marked cross-striations of prisms or the row of abrupt bent prisms. The POL, TEM, and SE-SEM observations show that the abrupt bent row forms the Retzius line (Fig. 4). In this zone, prisms bend towards the root and thereafter bend again to regain their
previous orientation 11). In the transverse-cut planes of prisms 11, 25), the prisms within the Retzius lines frequently show a stunted structure (Fig. 5). The constriction must have been induced by the prism bending; therefore, the bending causes the remarkable constriction or the well-marked cross-striations to follow without delay.
The rhythmical Retzius lines begin from the acid-susceptible cross-slits or bands of prisms (Figs. 3, 4, 6) when the longitudinal-cut planes of prisms are observed by SE-SEM after acid etching 2, 11, 15), although cross-slit structures have been reported by PHC with demineralized enamel sections 24) and by DIC 28). The cross-bands indicate the beginning of prism bending. In the longitudinal-cut planes across the prism-head and the regions of adjacent prisms 11), the cross-band follows the Y-shaped fine slits giving three subdivisions to the crystal arrangement in each prism along the long axis (Fig. 6). The triangular space under the TEM observation 28) is
probably the same region surrounded with the cross-band and the V-slit of the Y-shaped slit, although the longitudinal-cut planes of prisms are in the prism-head to tail direction. This relationship between the prisms in Pattern 3 enamel and ameloblasts with the Tomes' process 2) strongly suggested that such Y-shaped fine slits are the boundary lines of ameloblasts, and that they are formed with the double-bending of the prisms followed by the cross-slits 11, 26). The bent regions show a low mineralization in MRG 6) and BSE 12), because the prism bending probably causes the low density of enamel crystals 11, 28) and acid susceptivity (Figs. 3-6).
In this sub-section, it is concluded that the rhythmical Retzius line is formed with the row of double-bending prisms following the well-marked cross-striations showing a lower mineralization than the usual cross-striations.

IIb. Pathological Retzius Lines

The pathological Retzius lines are recognized as either the row of several to many well-marked cross-striations of prisms or the row of the remarkarble bending under LM, PHC, and POL. Our SE-SEM observations also show the same results 11); in addition, the remarkable bending does not always restore itself. The Retzius lines show either a low or a higher mineralization in MRG 6). In a particular case, the Retzius line appears between prismless layers 17). That is, the main crystal orientation of the Retzius line (the prism-head crystals) differs from that of the prismless layers (the parallel crystals transferred from prism-tail crystals 15)) in POL and SE-SEM, and the mineralization is lower than that of the prismless layers in MRG.
In this sub-section, it is concluded that the pathological Retzius lines come in several types caused by some abnormal factors, although the Retzius lines are based on the remarkable changes of prisms.

III. Neonatal Line of Deciduous Enamel

The neonatal line of the enamel in deciduous teeth, which are divided into the pre- and the post-natal enamel, is commonly observed as well as that in the 1st permanent molars 23, 30). The neonatal line is recognized as one of the pathological Retzius lines of hypo-mineralization in MRG 14, 27) or in SE-SEM after acid etching 27, 30). As shown in Figure 7, most neonatal lines are formed with the prism bending towards the root following the disorder of prism shapes and crystal arrangemet, and the prisms bend again to regain their previous orientation 14). However, in some teeth, the prisms run straight ahead after bending 14). The three-dimensional graphic image of the neonatal line in a deciduous incisor shows that some prisms cause disorder to the arrangement 1).
In this section, it is concluded that the neonatal lines of the enamel are more or less formed with the prism bending and show hypo-mineralization.

IV. Perikymatas

As is well known, perikymatas are the incremental lines of grooves running transversely on the natural enamel surface in macrophotography 3, 5, 20, 23). The perikymata patterns are divided into coronally undulating and cervically overlapping types by SE-SEM 10, 16). It has been reported that the regular perikymatas agree with the termination of the rhythmical Retzius lines 3, 6, 22), while the pathological Retzius lines form the abnormal perikymatas of deep grooves 17). When the natural enamel surface and the exposed outer enamel are observed in the
same field by SE-SEM 16), the undulating pattern is not always equal to the termination of the Retzius lines, although the overlapping pattern certainly agrees with it (Fig. 3). In the cuspal or incisal regions where the Retzius lines are parallel with the natural enamel surface, perikymatas may not appear 3); when perikymatas are present, they will not be related to the Retzius lines.
In this section, it is concluded that most perikymatas are formed with the termination of the Retzius lines, while the formation of some perikymatas of the undulating pattern are based on other unknown factors.

V. Laminate Striations within Prismless Enamel

Laminate striations within the surface prismless enamel have been observed by PHC 25), POL 7, 8), DIC 18, 19), and SEM 12, 15, 16, 18, 19, 30). The intervals are about 1 to 4 micrometers. Our SEM observations reveal that the striations run in parallel with the Retzius line in the surface enamel, and finally finish with the Retzius line on the perikymata (Fig. 3) or stay in the enamel along the natural surface. The laminate striations are also seen in the middle prismless enamel although the prismless enamel is in rare cases 17). Under the BSE images 12, 17), the striations show a low mineralization. Such laminate striations probably succeed to the cross-striations of prisms during the change from the prismatic to the prismless enamel as shown in Figure 3 15).
In this section, it is concluded that the laminate striations of hypo-mineralization are the so-called cross-striations within prismless enamel.

VI. Fine Incremental Lines on the Enamel Surfaces

Recently 13), we have found fine incremental lines with a periodicity of about 0.5 to 2 micrometers on the enamel surfaces with a prismless structure in two caries-free human teeth by SE-SEM after 30% phosphoric acid etching for 60 sec (Fig. 8). These surface regions were the shallow depression of the upper-coronal enamel in an exfoliated deciduous molar and the inner occlusal enamel of a young permanent premolar extracted for orthodontic purposes. At a glance, the fine incremental lines are similar to the laminate striations within prismless enamel. However, it is suggested that such incremental lines are not the termination of the laminate striations within the prismless enamel 13). The fine incremental lines go across or surround the transverse-cut abnormal prisms, but not the longitudinal-cut prisms.
In this section, it is expected that the detailed observations of the fine incremental lines on the enamel surfaces with a prismless structure will be necessary.

Acknowledgment

We thank Mr. Masayuki Yamada, from our laboratory, for his kind co-operation.


References

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(Explanation of Figures)

{　Sorry, Figures will be installed soon.　}

(Plate 1)


Figs. 1, 2
The cross-striations of prisms in permanent teeth (SE-SEM). Fig. 1: the longitudinal-ground plane etched with 0.01-mol lactic acid for 10 min. The rhythmical fan-shaped arrangement of crystals are clearly visible in a prism (P). The arrowheads show the basis of the fan-shaped arrangement. Fig. 2: the longitudinal-fractured plane. The alternation of constrictions (arrowheads) and varicosities of a prism (P) is observed. The right sides in Figs. 1 and 2 indicate the natural enamel surface.

Figs. 3, 4
The rhythmical Retzius lines in permanent teeth (SE-SEM). Fig. 3: the fractured plane etched with 2% EDTA for 15 min. P: prism. Large arrows: Retzius lines. Smaller arrows: laminate striations.PL: prismless enamel. PK: perikymata. NS: natural enamel surface. Fig. 4: the longitudinal-ground plane of the surface enamel etched with 0.01-mol phosphoric acid for 10 min. The arrow 1 and 2 show the beginning of the 1st and the 2nd bending of a prism (P), respectively. Theright side in Fig. 4 indicates the natural enamel surface.


(Plate 2)


Fig. 5
The tangential-ground plane of the surface enamel in a permanent premolar (SE-SEM). Etched with 0.01-mol lactic acid for 10 min. P: prisms within therhythmical Retzius line. The upper side indicates a cusp of the crown.

Fig. 6
An enamel prism on the rhythmical Retzius line in a permanent tooth (SE-SEM). The longitudinal-cut plane of the enamel across the prism-head to the regions of adjacent prisms was etched with 0.01-mol lactic acid for 5 min. The arrow and the 3 arrowheads show the cross-band and the Y-shaped fine slits in a prism (P).The right side indicates the natural enamel surface.

Fig. 7
The neonatal line of deciduous enamel (SE-SEM). The transverse ground plane was treated with 0.5% chromium sulfate for 17 hours. The prisms in the neonatal line (P) are remarkably bent. The prisms in the pre- (Pre) and the post-natal enamel(Post) show a normal orientation.

Fig. 8
Fine incremental lines on the inner occlusal surface in a permanent premolar (SE-SEM). The surface was etched with 30% phosphoric acid for 60 sec. Arrows: fine incremental lines. PL: prismless enamel. Arrowheads: abnormal prisms. The upperside indicates a cusp of the crown.