Book - Oral Histology and Embryology (1944) 3

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Orban B. Oral Histology and Embryology (1944) The C.V. Mosby Company, St. Louis.

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Chapter III Enamel

A. Histology

1. Physical Characteristics

Human enamel forms a protective covering of variable thickness over the entire surface of the crown. On the cusps of human molars and bicuspids it attains a maximum thickness of about 2 to 2.5 mm., thinning down to almost a knife edge at the cervix or neck of the tooth. The shape and contour of the cusps receive their final modeling in the enamel.

The enamel is the hardest calcified tissue in the human body. This is due to the high content of mineral salts and their crystalline arrangement. The specific function of the enamel is to form a resistant covering of the teeth. rendering them suitable for mastication.

The enamel varies in hardness from apatite, which is fifth in the scale of Mohsi used to determine this physical quality, to topaz, which is eighth. The specific structure and hardness of the enamel render it brittle, which is particularly apparent when the enamel loses its foundation of sound dentin. In cases of a fracture or in cavity preparation it breaks with a concoidal surface. The specific density of enamel is 2.8.

The color of the enamel—covered crown ranges from yellowish White to grayish-white. It has been suggested that the color is determined by differences in the transluc-ency of enamel, yellowish teeth having a thin translucent enamel through which the yellow color of the dentin is visible. grayish teeth having a more opaque enamel (Fig. 26).‘ The translucency may be due to variations in the degree of calcification and homogeneity of the enamel. Grayish teeth frequently show a slightly yellowish color at the cervical areas presumably because the thinness

First draft submitted by Charles F. Bodecker. Revised for 3rd Ed. by Reidar F. Sognnaes. tln this scale hardness is compared to that of 10 different minerals: (1) talc; (E) gypsum; (3) calcite; (4) fluorite; (5) apatite; (6) orthoclase (feldspar); (A) quartz; (8) topaz; (9) sapphire (corundum); (10) diamond.

50 The enamel consists mainly of inorg

2. Chemical Properties anic material (96 per cent) and only a small amount of organic substance and water (4 per cent)

Fig. 27. Influence of thickness and calcification 0!.’ enamel upon the color of the tooth. .-1. Thin, well-calcified translucent enamel giving the tooth a yellowish appearance (Y). 3. Thick, less calcifieyl opaque enamel givi

cervical area enamel thm, color yelloxv 1)’).

ng the tooth a. Wrayish appearance G). In s_Bodecker.‘) :5 (

of the enamel permits the light to stx-ik and be reflected. edge consists only

The inorganic material of the enamel is similar to apatite. Table II“ shows the most reliable data on the chemical contents of_ enamel. Some values for dentin and compact bone are added for comparlson.

The figures shown in the table represent dry weights. A comparison of the relative volume of the orgamc framework and m111eI'a1 Contents Of

the enamel shows that these are almost equal. Fig. 28 illustrates this by comparing a stone and a sponge of approximately equal volume: the former represents the mineral content, and the latter the organic framework of the enamel. Although their volume is almost equal their Weights are vastly different: the stone is more than one hundred times heavier than the sponge or, expressed in percentage, the weight of the sponge is less than one per cent of that of the stone.

TABLE II CHEMICAL Coxrsxrs or ENAMEL, DENTIN, CEMENTUM AND Bonn

CEMENTUM m“mEI‘ DENT“ (‘OMPACT BONE

Water 3.3 % 13-2 % 32 % Organic Matter 17 17.5 22

Ash 96.0 59-3 46

In 100 g. of Ash: _

Calcium 36.1 g 33.3 g. 35.5 g. Phosphorus 17.3 17.1 17-1 Carbon dioxide 3.0 4-0 4-4 Magnesium 0.5 1.2 0.9 Sodium 0.2 0.2 1.1 Potassium 0.3 0-07 0-1 Chloride 0.3 0-03 0-1 Fluorine 0.016 0.017 0.015 Sulfur 0.1 0.2 0.6 Copper 0.01

Silicon 0.003 0.04 Iron 0.0025 0-09 Zinc 0.016 0.018

WHOLE TEETH BONE Lead 0.0071 to 0.037 0.002 to 0.02

Small amounts of: Ce, La, Pr, Ne, Ag, Sr, Ba, 01-, Sn, Mn, Ti, N1, V, Al, B, Cu,

Li, Se,

The nature of the organic elements of enamel is incompletely understood. In development and histologic staining reactions, the enamel matrix resembles hornifying epidermis. Recently more specific methods have revealed sulfhydryl groups and other reactions suggestive of keratin.“' Sinlilarly. lrvdrolysates of mature enamel matrix have shown a ratio of aminoacids (histidine 1: lysine 3: arginine 10) indicative of an eul:eratin.2- 3" In addition, histochemical reactions have suggested that the enamel forming cells of developing teeth also contain a carbohydrate protein,“ and that an acid mucopolysaccharide enters the enamel itself at

The editor is indebted to Dr. Harold C. Hodge, University of Rochester, School of Medicine and Dentistry, Rochester, New York, for compiling this table. The chemical constituents of ash are here given as elements, while they are in reality present in difierent compound: e.g., phosphorus as phosphate. The neglect of these other elements, e.g., oxygen, hydrogen. nitrogen, accounts for the difference between 100 and the actual grams.

the time when calcification becomes a prominent feature:"" Tracer studies have indicated that the enamel of erupted teeth of rhesus monkeys can transmit and exchange radioactive isotopes originating from the saliva and the pulp.“ Considerable investigation is still required to determine the normal pltysiologécal characteristics and the age changes that occur in the enamel.

Fig. 2S. A sponge (.1) and a stone (B1 are comparable to the organic and mineral elements of enamel. Their volumes are approximately equal but their weights differ greatly. (Bodecke1-.4)

3. Structure

The enamel is composed of enamel rods or prisms. possibly rod sheaths, and a cementing inter-prismatic substance. The number of enamel rods has been estimated”' 13 as ranging between five millions in lower lateral incisors. and twelve millions in the upper first molars. From the dentinoenamel junction the rods proceed outward to the surface of the tooth. The length of most rods is greater than the thickness of the enamel, because of the oblique direction and wavy course of the rods. The rods located in the cusps, the thickest part of the enamel, are naturally much longer than those at the cervical areas of the teeth. It is generally stated that the diameter of the rods averages four microns, but this measurement, necessarily, varies since the outer surface of the enamel is greater than the dentin surface where the rods originate. It is claimed“ 33- *9 that the diameter of the rods increases from the dentinoenamel junction toward the surface of the enamel at a ratio of about 1:2.

The enamel rods were first described by Retzius“ in 1‘35. They are tall columns or prisms, passing through the entire thickness of the enamel. Normally, they have a clear crystalline appearance, permitting the light to pass through them freely. In cross section the enamel rods appear. occasionally. hexagonal: sometimes they are round or oval. Many rods resemble fish scales in cross sections of human enamel (Fig. 29.1). An explanation for this peculiar shape has been attempted by the following hypothesis: The manner in which calcification takes place seems to exert a marked influence upon the shape of the rods. The calcification of each rod begins close to its surface and proceeds toward the center. In human enamel calcification of the rods does not occur on the entire circumference of the red at the same time. but begins on one side. Consequently, one side of each rod hardens sooner than the other and, in the process of calcification which seems to be accompanied by increased pressure, the harder side presses into the softer side of the adjacent rods, compressing it and leaving a permanent impression.* The calcified portions of the enamel rods are lost in the preparation and appear as clear white spaces. The dark areas, located excentrically within the sheaths, are interpreted as uncalcified organic substances in the rods. This may indicate that the calcification of human enamel rods begins at. the periphery of each rod. and the calcification sets in earlier on one side than on the other.

Fig. 29. Decalified section of enamel of a human tooth germ. Rods cut transversely appear like fish scales.

A thin peripheral layer of each rod shows a different refractory index, stains darker than the rod, and is relatively acid resistant. It may be concluded that it is less calcified and contains more organic substance than the rod itself. This layer is the rod sheath‘! *“ (Fig. 29).

Each enamel rod is built up of segments, separated by dark lines which give it a striated appearance (Fig. 30). These transverse striations marl: the margins of the rod segment which become more visible by the action of mild acids. The striations are more marked in enamel which is insufficiently calcified. The rods are segmented because the enamel matrix is formed in a distinctly rhythmic manner. In man these segments seem to be of uniform length of about four microns.“ nterpfismatic Substance

Enamel rods are not in direct contact with each other hut 212-e c-emc-ntetl together by the intel-prismatic substance \\'l1l(‘l1 has :3 slightly l1ighe1' retractlve mclex than the rods.*’ Discussion is still active c-oiic-ex-niug the

o « Fig. 30.—Ground section through enamel. Rods cut longitudinally. Cross striation of rods.

structure of the iiitei-p1-ismzitic siibstanee «Fig. 31:. The interpi-ismatie substance appears to be at a minimum in human teeth. In some animals (dog, pig) the teeth show a amount of interprismatic substance in the enamel.

Lately, new methods have been devised to study ground sections of hard tissues. The principle is to take impressions of the surface after etching it with dilute acids?“ ‘*1 An improvement of this method has been achieved

bx‘ blowing vaporized metals onto the microcast at an acute angle, thus duplicating shadows thrown by projections of the cast.“

The study of shadowed replicas of cross sections of the enamel seems to indicate that the enamel rod is not homogeneous. The rod sheath seems to be the least completely calcified structure of the enamel. The 1nterpr1smatic substance appears to have a lower content of mineral salts than the rod itself (Figs. 32.4., 32B).

Fig. 31,-Decalcifled section of enamel. Rods, rod sheaths. and interprismatic substance are well difierentiated. (Photographed with ultra-violet light.) (Bodeckerfl)

Generally, the rods are oriented at right angles to the dentin surface. In the cervical and central parts of the crown of a deciduous tooth“ they are approximately horizontal (Fig. 33, A); near the incisal edge or tip of the cusps they change gradually to an increasingly oblique direction, until they are almost vertical in the region of the edge or tip of the cusps. The arrangement of the rods in permanent teeth is similar in the occlusal two-thirds of the crown. In the cervical region, however, the rods deviate from the horizontal in an apical direction (Fig. 33, B).

The rods are rarely, if ever, straight throughout; they follow a wavy course from the dentin to the enamel surface. The most significant deviations from a straight radial course can be described as follows: If the middle part of the crown is divided into thin horizontal discs, the rods in the adjacent discs bend in opposite directions. For instance, in one disc the rods start from the dentin in an oblique direction and bend more or less sharply to the left side (Fig. 34, A). In the outer third of the enamel they change often to an almost straight radial course. In the

Fig. 32.1.—Transverse section through enamel etched 5 seconds with 0.1 X I-IC1 (shadowed replicab. (X15t.*I).) (Courtesy Scott and \\’yckoff.“)

Fig. 32B.—Cross section of demineralized enamel of a. developing canine from a monkey fetus. Note rods. rod sheaths. and interprismatic substance. ()<T,200.) After Sog-nnaes, Scott, Ussing and \l.’yckoff.‘=

Fig. 33.—DiagnJ.ms indicating the general direction of enamel rods. .4. Deciduous tooth. B. Pemianent tooth.

Fig. 3~i.—-Horizontal ground section through enamel near dentino-enamel junction. 4 and 3 show change in the direction of rods in two adjacent layers of enamel,

adjacent disc the rods bend toward the right -Fig. 34. B ,. This alternating clockwise and counter-clockwise deviation of the rods from the radial direction can be observed at all levels of the crown if the discs are cut in the planes of the general rod direction I Fig.

If the discs are cut in an oblique plane. especially near the dentin in the region of the cusps or incisal edges. the rod arrangemeiit appears to be further complicated. the bundles of rods seem to intertwinc more irregularly; this appearance of enamel is called gnarled enamel.

The enamel rods forming the developmental grooves and pits, as on the occlusal surface of molars and premolars, converge in their outward course.

Fig. 35. Long'ituAlinel grcunri section through enamel pliotograpl:e-'1 by reflected light. Hunter-Schreger bands.

Fig. 36. DecaIcific-«I enamel. pliotogrnphed by reflectevl light showing HunterSchregei-'s bands. tsognnaes“ J. Dent. Research, 1949.)

The more or lcss regular change in the direction of rods may be regarded as a functional adaptation, minimizing the risk of cleavage in the axial direction under the influence of occlusal masticatory stresses. The change in the direction of rods is responsible for the appearance of the Htlnter-°a(-hreger bands. These are alternating dark and light stripes of varyiiig width (Figs. 35 and 369 which can best be seen in a. longitudinal ground section under oblique reflected light. They originate at the dentino~enamel border and P335 Ouf-Ward: ending at s_°me distance from the outer enamel surface. This Phenomenon is explamed as follows: In a longitudinal section the rods are, generally, cut obliquely. If the bundles of rods are traced from the surface of Such 3» Section into the depth, it will be observed that the)’ F1111 0b1iq11€1.V', in 0119 disc to the right, in the next disc to the left. If such a section is illuminated from the right side, the rays pass, without being reflected, through the rods bands. The dark band in A marked by a particle of dust (X) appears light In B.

Fig. 37.—Three photomicrographs or the same area. of a. of enamel. A and B by reflected light. The change in the direction of light (180') caused a reversal of the Hunter—schx-eg-er

C’, The same area photographed by transmitted light. (The particle of dust lies on the specimen under the coverglass.)

which the rods run in the opposite direction appear light because the rays which run in the same direction; such discs appear dark. The discs in are reflected from the lateral surfaces of the rods. This explanation is borne out by the fact that a 180 degree rotation of the slide reverses the phenomenon; the stripes which were dark in the first position appear light; those which were light appear dark Fig. 373. Some investigators" 9’ 37 claim that there are variations in calcification of the enamel which coincide with the distribution of the bands of Hunter-Sehreger. Careful decalcification and staining of the enamel have provided further evidence that these structures may not solely he the result of an optical phenomenon, but are composed of alternate zones hating a sIightly increased permeability and a higher content of organic material.

Fig. 38.—Inc1-emental lines of Retzlus in longitudinal ground sections.

A. Cuspal region. B. Cervical region (X).

The incremental lines of Retzius appear as brownish bands in ground sections of the enamel. They illustrate the successive apposition of layers of enamel matrix during formation of the crown (incremental pattern of the enamel). In longitudinal sections they surround the tip of the dentin

(Fig. 38, Al. In the cervical parts of the crown they run obliquely; from the dentiiio-enamel junction to the surface they deviate occlusally (Fig. 38, B L In transverse sections of a tooth the iiicremeiital lines of Retzius appear as concentric circles (Figs. 39.1, B). They may be compared to the growth rings in the cross section of a tree. The term “incremental lines” designates these structures appropriately, for they do, in fact, show the advance of growth of the enamel matrix. The incremental lines are an expression of the rhythmically recurrent variation in the formation of

Fig. 39.=l..—Increx-nental lines of Retzius in transverse ground section, arranged concentrically.

Fig. 39B.-—Decalc-ified paraffin section of enfoliated deciduous molar. (X20.) Heavy dark lamella. runs from darkly stained dentin to surface in an irregular course independent of developmental pattern. Roughly parallel to dentin surface are seen a. number it incremental lines. one of which, the neonatal line, is accentuated. (sognnaesfi J. Dent. Research. 1949.) Fig. -41.—Shadowed replica of the second molar showing the perik

surface of intact enamel (buccal surface of upper left ymata. (X15004 (Courtesy Scott and

the enamel matrix. The cross striation of the single rod (Fig. 30) is the result of an underlying shorter rhythm in the matrix formation (see section Development of Enamel). The variation in the formation of the enamel matrix causes secondary variations in the degree of calcification. The incremental lines and cross striations are areas of diminished cal cification.

Wherever the lines of Retzius reach the surface there is a shallow furrow. the imbrieation line of Pickerill; this is caused by an overlap of a younger layer of enamel over an older layer. The furrows are more numerous and closer together at the cervical part of the crown. The distances between adjacent furrows increase toward the occlusal part of the crown. They are missing entirely close to the iiicisal edge or tip of the cusps. The slight elevations between two furrows are known as periky— mata (Figs. 40 and 41).

Fig. 4‘_’.—Ca.refu1ly decalcifled section tl Ii 1. Thick ‘ ' - stance (say in p

The incremental lines of Retzius, if present in moderate intensity, are not considered pathologic. However, the rhythmic alternation of periods of enamel matrix formation and of rest can be upset by metabolic disturbances, causing the rest periods to be unduly prolonged and close together. Such an abnormal condition is responsible for the broadening of the incremental lines of Retzius, rendering them more prominent. At the mcreinental lines of Retzius the iiiterprismatic substance seems to be thickened at the expense of the rods (Figs. 39B, 42).

The enamel of the deciduous teeth develops partly before, and partly after birth. The boundary between the two portions of ename1 in the deciduous teeth is marked by an accentuated incremental line of Retzins, mum. 65

the neonatal line or neonatal ring.“ This appears to be the result of the abrupt change in the enfironment and nutrition of the newborn. The prenatal enamel is, usually, better developed than the postnatal (Fig. 43). This is explained by the fact that the fetus develops in a Wellprotected environment, with an adequate supply of all the essential materials, even at the expense of the mother. Because of the undisturbed and even development of the enamel prior to birth, perikymata are absent in the occlusal parts of the deciduous teeth, whereas they are present in the postnatal cervical parts. The diagram in Fig. -14 shows the amount of enamel formed during prenatal and postnatal periods.

Fig. 43. Neonatal line in the enamel. Longitudinal ground section of a deciduous cuspid. (Schom-.3’)

A delicate membrane covers the entire crown of the newly erupted tooth. This membrane was long described as Nasmyth’s membrane," after its first investigator. When the ameloblasts have produced the enamel rods they produce a thin continuous pellicle termed the primary enamel cuticle which covers the entire surface of the enamel (Fig. 5). This cuticle is largely organic and, being more resistant to acid than the enamel itself, can be floated off in acid. It is worn ofl early from all exposed surfaces.

During the emergence of the tooth, the reduced enamel epithelium covering the crown, produces a keratinous secondary cuticle on the surface of the primary. If a thin ground section of enamel is decalcified in acid cel C:r.tra.l Lateral Deciduous First Second First zciducus deciduous cuspid deciduous deciduous permanent 1nCi50!' incisor molar molar molar

Semidiagrammatic tracin 5 showing the enamel and dentin

of the deciduous teet and first Permanent molar at ond after birth

Prenatal enamel Prenatal dentin

Fig. 44. Enamel and dentin of deciduous teeth and flrst permanent molar at and after birth. (Schoui-37)

loidin*- 7 the outer or secondary cuticle will resist acid and show marked birefringence in polarized light. This indicates a structurally oriented fibrous protein, presumably keratin“! 2‘ In specimens stained with hematoxylin and eosin the secondary cuticle stains bright yellowish-red. It varies in thickness from 2 to 10 microns, is homogenous in character, and seems to be brittle (see section on Epithelial Attachment).

Mastication wears away the enamel cuticles on the incisal edges, occlusal surfaces, and contact areas of the teeth. On other exposed surfaces, they may be worn oif by mechanical influences, e.g., brushing of teeth. In protected areas (proximal surfaces and gingival sulcus) they may remain intact throughout life. i:.\'.u1EL 67

I-lnamel lamellae are thin leaflike structures which extend ironi the enamel surface toward the dentino enamel junction ll-‘i,qs_ 46. .1, B». They may extend to, and sometimes penetrate into. the dentin Hlentinal part of lamellal. They consist of organic material. with but little mineral content. In ground sections these structures may be confused with cracks caused by grinding of the specimen (Fig. 39, -14. Careful decalcification of the enamel makes possible the distinction between cracks and enamel lamellae: the former disappear while the latter persist (Figs. 39, B, 47).

Fig. -15.-—Decalcifled section through the crown of an unerupted human tooth. Enamel lost in decalciflcation. Primary enamel cuticle in connection with the united

enamel epithelium. At (X) a cell of the epithelium is lost thus making the cuticle more visible.

Lamellae develop in planes of tension. Where rods cross such a plane. a short segment of the rod may not fully ealcify. If the disturbance is more severe a crack may develop which is filled either by surrounding cells it the crack occurred in the unerupted tooth, or by organic substances from the oral cavity if the crack developed after eruption. Three types of lamellae can thus be differentiated. Type A» 131391139 composed of 1,001.1‘. calcified rod segments; Type B, lamellae consisting of degenerated cells- Type C those arising in erupted teeth Where the cracks are filled with organic matter, presumably originating from s.al1vZ.“‘ last type (Fig. -17) may be more common than formerly believe . B E lamellae of Type A are restricted to the enamel, those of Types an

C may reach into the dentin. If cells from the enamel Organ fin 3 Crack in the enamel, those in the depth degenerate, whereas those close ‘to the surface may remain vital for a time and produce a hormfied secondary cuticle in the cleft.“ In such cases (Fifi 49) the greater inner Parts of the lamella consist of an organic cell detritus, the outer parts of a double

Fig. 46A.-——Decalcifled incisor afiected with moderately severe mottled enamel (from

material obtained in Texas). Numerous lamellae can be observed. (x8.) (Sognnaesfl J. Dent Research, 1950.)

Fig. 46B,—Ma.xiIlary first permanent molar of caries-free, two-year-old rhesus monkey._ Numerous cracks revealed themselves as bands of organic matter (lamellae) once specimens had been decalcified. (x8.) (Sognnaes“ J. Dent. Research, 1950.)

H-3:-an-9.

layer of the secondary cuticle. If connective tissue invades a crack in the enamel, cementum may be formed. In such cases lamellae consist entirely or partly of cementum.3“

Lamellae extend in the longitudinal and radial direction of the tooth, from the tip of the crown toward the cervical region (Figs. 46, A, B). This arrangement explains why they can be observed better in horizontal sections. Enamel lamellae may be a source of weakness in a tooth inasmuch as they may form a road of entry for bacteria which initiate caries." 1“ ‘3 On the other hand, it has been suggested“ that the organic matter which fills in enamel cracks occurring during fimction of the teeth, may serve a crude “reparative” function, possibly as a nucleus for secondary mineral deposition. nxannz. 69

Enamel tufts (Fig. 50) arise at the dentino-enamel junction and reach into the enamel to about one—fifth to one—third of its thickness. They were so termed because they resemble tufts of grass when viewed in ground sections. It has been proved“ 32 that this conception is erroneous. An enamel tuft does not spring from a single small area but is a narrow, ribbon-like structure the inner end of which arises at the dentin. The impression of a tuft of grass is created by examining such structures in

Fig. 4T.—Parafiin section of decalcifled enamel of human molar showing the relation between Iamella. and surrounding organic framework between the enamel prisms. H. & E. stain. (X10004 tsognnaes“ J. Dent. Research, 1950.)

thick sections under low magnification. Under these circumstances the imperfections, lying in different planes and curving in different directions (Fig. 3-1), are projected into one plane (Fig. 50).

Tufts consist of hypocalcified enamel rods and interprismatic substance. Like the lamellae they extend in the direction of the long axis of the crown; therefore, they are abundantly seen in horizontal, and rarely in longitudinal sections. Their presence and their development is a consequence of, or an adaptation to, the spatial conditions in the enamel.

In microscopic sections the dentino-e11an1el junction is not 21 straight line but appears scalloped 1 Figs. 50 and 51). The convexities of the scallops are directed toward the dentin. This line is already pre-formed in the arrangement of the aiueloblasts and the basement membrane of the dental papilla, prior to the development of hard substances. This arrangement contributes to the firm attaelnnent of the enamel to the dentin and presumably to the structural pattern of the enamel as refleeted in the arrangement of the tufts and the Hunter-Schreger bands.

Fig. 48.-—'l‘ransverse ground section through a lamella reaclfng from the Surface into the dentin. The dentinal part of the lamella is surroundedlby transparent

Occasionally odontoblast processes pass across the dentino-enamel junction into the enamel. Some terminate there as finely pointed fibers; others are thickened at their end (Fig. 52‘) and are termed enamel spindles. They ENAMEL 71

seem to originate from processes of odontoblasts which extended into the enamel epithelium before hard substances were formed. The direction of the odontoblastic processes and spindles in the enamel corresponds to the original direction of the ameloblasts. i.e., at right angles to the surface of the dentin. Since the enamel rods are formed at an angle to the axis of the

3 Dentinal part of lamella Dentin

. Fig. 49.-Decalcifled transverse section through a tooth. Enamel is lost. in decaiciflcatron; lamella of Type B collapsed. Diagram showing the relationship prior to decalciflwr tion. Secondary enamel cuticle is hornified. Horniflcation extends into the outer part or the Iamella. torban.-”=)

arneloblasts, the direction of spindles and rods is divergent. In ground sections of dried teeth the organic contents of the spindles disintegrate and are replaced by air; then-etore, the spaces appear dark.

4. Age Changes

The organic nuitrix of the enamel and the enamel surface appear to undergo changes with age, but this change is not well understood. It has been suggested that the surface change is due to accretion of salivary or bacterial products. As a result of these age changes in the organic portion of enamel, the teeth may become darker and their resistance to

Fig. 5D.—-Transverse ground section through a tooth under low magnification Numerous tufts extending from the dentino-enamel junction into the enamel.

i Dentino-enamel junction

Fig‘ 51"I‘°"3it“dim1 87011115 39¢fi0n- Swlloped deutino-enamel junction. ENAMEI: 73

decay may be increased? Suggestive of the aging change is the greatly reduced permeability of older teeth to fluids.“ There is insufiicient evidence to show that enamel becomes harder with age.“

The most evident age change in enamel is attrition or wear of the occlusal surfaces and proximal contact points as a result of mastication. Histologically, the results of attrition are most prominent in the tissues below the enamel, the dentin. pulp, and periodontium.

Fig. 52.—Ground section. Odontoblastic process extending into the enamel. and an enamel spindle.

Dentino-enamel junction

5. Submicroscopic Structure

By means of studies in polarized light“ 9" it has been shown that completely calcified enamel consists of submicroscopic units, hexagonal in shape and arranged with their long axes approximately parallel with the long dimensions of the rods. There may be a deviation of as much as twenty degrees from parallel in this relationship in human enamel (Fig. 53.4). In dog enamel the parallel relationship is common.

Fig. 53B is a eelloidin model of the submicroscopic crystal which is the calcification unit of enamel and dentin. The two different axial planes are represented by sheets of celloidin placed inside the hollow hexagonal form. On these planes the velocity of the passage of light rays in each is indicated by wave-like lines. A line with few waves symbolizes a more rapid rate of travel and lower index of refraction, while one with many waves shows a slower rate and a higher index of refraction. The light ray vibrating in the plane parallel to the long axis is known as the extraordinary, while the ray vibrating in the plane at right angles to the long axis is the ordinary ray. In the case of this particular crystal the birefringence is of a negative type because the so-called ordinary ray is the one with the higher index of refraction.

This difference in indices of refraction causes a double refraction, known as birefringence, when the enamel is viewed with crossed nicols in any aspect except that of looking down on the ends of enamel rods. The greatest birefringence occurs when viewing the rods at right angles to their long axis.

Fig. 53.-1.-—Submicroscoplc, hexagonal crystals (highly magnified) in their relation to the longitudinal axis or a. human enamel rod.

The use of the electron microscope has made it possible to photograph the submicroscopic crystals of the enamel“ (Fig. 54). Recent advances in electron microscopy of ultrathin sections of deealcified enamel“ have revealed that a submicroscopie organic network permeates both between

and within the enamel prisms, presumably enveloping the crystallites (Fig. 55). E.\‘A.\IEL 75

6. Clinical Considerations

To know the course of the enamel rods is of importance in cavity preparations. Straight enamel cleaves more readily than bundles of enamel prisms which take a wavy course. The cement or interprismatie substance is apparently weaker than the body of the rods, so that the line of cleavage usually follows this substance. It can 1-eaclily be understood that, in enamel where the bundles of rods do not lie parallel to each other, cleavage does not occur so easily. for the stronger bodies of the

Fig. 53b‘.—Cel1oidin model of the submicroscopic crystal in the enamel. The _ordinar,\' ray (horizontal plane) has a slower rate 0.‘. travel and higher index of refraction than the extraordinary ray (vertical plane).

intertwined rods make a clean, straight fracture impossible. Inter-twining rods present a greater resistance to dental instruments. The operator-‘s choice of instruments depends upon the location of the cavity in the tooth. Genei-all_v, the rods run at a right angle to the underlying dentin or tooth surface. Close to the cemento-enamel junction the rods run in a. more horizontal direction «Ficr. 33. B1. In preparing cavities it is important. that unsupported enamel rods do not remain at the cavity margins. These would soon break and produce a leakage. Bacteria would lodge in these spaces. inducing early dental caries. Enamel is brittle and does not Withstand forces in thin layers. nor Where it is not supported by the underlying dentin (Fig. 56:).

Fig. 54.—Submicroscopic crystals of guinea pig enamel,‘ photographed ‘th the electron microscope. (X23000.) (Boyle. Hillier. and Davidson. E.\'A1IEIi 77

Deep enamel fissures are predisposing to caries. Although these deep clefts between adjoining cusps cannot be regarded as pathologic, they afford areas for retention of caries—producing agents. Caries penetrates the floor of fissures rapidly because the enamel is very thin in these areas“ (Fig. 56B). As the destructive process reaches the dentin. it mushrooms out along the dentino-enamel junction undermining the enamel, leaving only a small opening to the cavity. An extensive area of dentin becomes carious without giving any warning to the patient because the entrance to the cavity is minute. A most careful examination by the dentist is necessary to discover this condition. Even so, the base of most enamel fissures is more minute than a single toothbrush bristle and cannot be detected with the dental probe.

Fig. 55.-—Electi-on-micrograph ()<10.000) 0! cross section of clegnineraliaed enamel of an adult human molar. showing one prism and part or two adjoining prisinsflwith the submicroscopic organic framework within and between the prisms. (Scott et al J. Dent. Research. 1952.)

Enamel lamellae may also be predisposing locations for caries. The abundant organic material in the enamel lamellae may present an excellent medium for bacterial growth. If protein tends to fill cracks in the enamel of erupted teeth, then the resulting lamellae may Well be preferable to the open cracks. The bacteria may penetrate, along cracks and lamellae, from the surface to the dentino-enamel junction, and into the dentin. In some instances caries in the dentin may occur without gross clinical destruction of the enamel surface. thereby undermining the enamel itself. Hornification of the enamel cuticle. at the entrance of the laxnellae s.Ficr. 49), may prevent the bacteria from penetrating. It has been suggested that a proper impregnation of the organic matter in the enamel may he a prophylactic measure against this type of caries.“

Fig. 56.-1.——Diagrammatic illustratioii of the course of en2_:.meI rods in a molgr in relation to cavity preparation. I and 3 indicate wrong preparation of cavity margms; 3 and 4 indicate correct preparation.

Fig. 56B.—Diagramma.tic illustration of development of a deep enamel fissure. Note the thin enamel layer forming the floor of the fissure. (K1-on1'eld.=') E.\'A.\IEL 79

The surface of the enamel in the cervical region should be kept smooth a11d well polished by proper home care and by regular prophylactic treatment by the dentist. If the surface of the cervical enamel becomes decalcified, or otherwise roughened. food debris. bacterial plaques, etc.. accumulate on this surface. The gingival tissues in contact with this roughened, debris-covered enamel surface undergo inflammatory changes

(gingivitis) which, unless pronlptly treated. lead to more serious periodontal disease.

References

(Histology of Enamel)

1. Beust, T.: Morphology and Biology of the Enamel Tufts With Remarks on Their Relation to Caries. J. A. 1). A. 19: 455, 1932.

2. Block, R. J., Hornitt, M. K.. and Bolling, D.: Comparative Protein Chemistry. The Composition of the Proteins of Human Enamel and Fish Scales, J. Dent. Research 28: 513, 1949.

3. Bibby, B. G., and Van Huysen. G.: Changes in the Enamel Surfaces; A Possible Defense Against Caries, J. A. D. A. 20: S28, 1933.

4. Bodecker, C. F.: Enamel of the Teeth Decalcified by the Celloidin DeCal(.‘if_\'ing Method and Examined by Ultraviolet Light, Dental Review 20: 317,

1906. 5. Bodecker, C. F.: Nutrition of the Dental Tissues, Am. J. Dis. Child. 43: -£16,

1932. 6. Bodecker, C. F.: The Color of the Teeth as an Index of Their Resistance to I

. Bodecker, C. F .: The Cake-Kitchin Modification of the Celloidin Decaleifying Decay, Int. J. Orthodontia 19: 356, 1933. Method for Dental Enamel, J. Dent. Research 16: 143, 1937. . Bodecker, C. F.: Concerning the "\’italit_x"' of the Calcified Dental Tissues. I\'é Vital Staining of Human Dental Enamel, J. Dent. Research 20: 3773S . 1941. Bodecker, C. F'., and Lefkowitz, ‘\\‘.: Concerning the "Vitality" of the Calcilled Dental Tissues, J. Dent. Research 16: -L63, 1937.

10. Boyle, P. E., Hillier, J., and Davidson. )7. B.: Preliminary Observations of the Enamel of Human and Guinea Pig Teeth Using the Electron Microscope, J. Dent. Research 25: 156. 19-16.

11. Cape. A. ’1'., and Kitchin. P. C.: Histologic Phenomena of Tooth Tissues as Observed Under Polarized Light; With a Note on the Roentgen Ray Spectra of Enamel and Dentin, J. A. D. A. 17: 193, 1931).

1:}. Chase, S. Y\'.: The Absence of Supplementary Prisms in Human Enamel, Aunt. Rec. 28: 79. 19:24.

13. Chase, S. W.: The Number of Enamel Prisms in Human Teeth, J. A. D. A. 1-1: 1921. 1927.

1-1. Engel, 11. B.: Glycogen and Carboh_\'drat&Protein Complex in Developing Teeth of the Rat. J. D. Res. 27: 4581. 19-18.

15. Fish, E. \V.: An Experimental Investigation of Enamel. Dentin and the Dental Pulp, London, 1932. John Bale Sons 8: Dauielsson. Ltd.

16. Gottlieb, B.: lftersuehungcn iiher die organische Substanz im Schmelz menschlicher Ziihne (Investigation of Organic Substances in the Enamel 1, Oesterr.ungar. Vrtljschr. f. Zahnh. 31: 19, 1915. _

17. Gottlieb, B.: Aetiologie und Prophylaxe der Zahnkaries (Etiology and Prophylaxis of Dental Caries), Ztschr. f. Stomatol. 19: 129, 1921.

IS. Gottlieb, B., and Hinds, E.: some New Aspects in Pathology of Dental Caries, J. Dent. Research 21: 317. 1942.

19. Gottlieb, B.: Dental Caries, Philadelphia, 1947, Lea 8: Febiger.

-I:

. _q 30. 31.

32. 33.

34. 35. 36. 37. 38.

46. 47. 48. 49. 50. Wislocki, G. B., and Sognn 51. Wolf, J.:

. Gruner,

. Gurney, B. F., and Rapp, G. W.:

. Gustaphson,

. Kitchin, . Klein, H., and Palmer, C. E.:

. Scott, D. B., and Wyckofi, R. W. G.: . Scott, D. B., and Wyckoif, R. W. G.: . Scott, D. B., ‘Cssing, . Skillen,

. Smreker,

J. W'., McConnell, D., and Armstrong, W. D.: The Relationship Be tween the Crystal Structure and Chemical Composition of Enamel and

Dentin, J. Biol. Chem. 121: 771, 1937.

Technic for Observing Minute Changes in the

Tooth Surfaces, J. Dent. Research 25: 367, 1946. '

G.: The Structure of Human Dental Enamel, Odont. Tidskr. (Supplement) 53: Elanders Boktryckeri, Griiteberg, Sweden.

Hodge, H., and McKay, H.: The Microhardness of Teeth, J. A. D. A. 20: 227, 1933.

Hollander, F., Bodecker, C. F., Applebaum, E., and Saper, E.: A Study of the Bands of Schreger by Histological and Grenz-Ray Methods, Dental Cosmos 77: 12, 1935.

Karlstroem, S.: Physical, Physiologic and Pathologic Studies of Dental Enamel With Special Reference to the Question of Its Vitality, Stockholm, 1931, A. B. Fahlcrantz.

P. C.: Some Observations on Enamel Development as Shown in the

Mandibular Incisor of the White Rat, J. Dent. Research 13: 25, 1933.

The Relationship Between Post-Eruption Tooth

Age and Caries Attack Rate of the Lower First Permanent Molar, J‘. Dent.

Research 18: 283, 1939. Kronfeld, R.: First Permanent Molar. Its Condition at Birth and Its Postthe “Vitality” of the Cal natal Development, J. A. D. A. 22: 1131, 1935.

Lefkowitz, W., and Bodecker, C. F.: Concerning cified Dental Tissues. II. Permeability of the Enamel, J. Dent. Research 17: 453, 1938.

Losee, F. L., and Hesse, W. C.: The Chemical Nature of the Proteins From Human Enamel, J. Dent. Research 28: 512, 1949.

Nasmyth, A.: Researches on the Development, Structures and Diseases of the Teeth, London, 1839, John Churchill.

Orban, B.: Histology of Enamel Lamellae and Tufts, J. A. D. A. 15: 305, 1928.

Pickerill, H. P.: The Prevention of Dental Caries and Oral Sepsis, ed. 3, New York, 1924, Paul B. Hoeber, Inc., p. 340.

Retzius, A.: Microscopic Investigation of the Structure of the Teeth, Arch.

Anat. 87 Physiol. 486, 1837. Robinson, H. B. G., Boling, L. R., and Lischer, B.: in Cowdry’s Problems of (Manual of Bio Ageing, Baltimore, 1942, Williams & Wilkins, Chapter 13.

Schmidt, W. .T.: Handbuch der biologischen Arbeits Methoden logic Working Methods), Abderhalden, Abt. 5, Teil 10, 1934, p. 435.

Schour, I.: The Neonatal Line in the Enamel and Dentin of the Human Deciduous Teeth and First Permanent Molar, J. A. D. A. 23: 1946, 1936.

Schour, I., and Hoffman, M. 1312.: Studies in Tooth Development. I. The 16 Microns Rhythm in the Enamel and Dentin From Fish to Man, J. Dent. Research 18: 91, 1939.

Schour, I.: Recent Advances in Oral Histology, Int. Dent. J. 2: 10, 1951.

Typical Structures on Replicas of Ap Intact Tooth Surfaces, Pub. Health Rep. 61: 1397, 1946.

shadowed Replicas of Ground Sections

Through Teeth, Pub. Health Rep. 62: 422, 1947.

M. J., Sognnaes, R. F., and Wyckofi, R. W. G.: Electron

Microscopy of Mature Human Enamel, J. Dent. Research 31: 74, 1952.

Eli’. C.: The Permeability of Enamel in Relation to Stain, J. A. D. A.

11: 402, 1924. Skinner, E. W.: Science of Dental Materials, Philadelphia, 1937, W. B. Saunders

Co. E.: Ueber die Form der Schmelzprisme 111‘ h Z"h die Kittsubstanz des Schmelzes (On the Form 0? Enrlrfilrldgl :l].£.'iSe1:].S oat 1g&:ii Teeth, and the Cement Substance of the Enamel), Arch. 1?. mikr. Anat 66' 312, 1905. ' ' Sognnaes, R. F.: The Organic Elements of th E 1. II, 111 Dent. Research 28: 549, 1949; 28: 55s,1949;%9:n2%31,e195o. ’ ’ and IV" J’ Sognnaes, R. F., and Shaw, H.: Salivary and Pulpal Contributions to the Radiophosphorus Uptake 111 Enamel and Dentin, J. A. D. A. 44: 489 1952. Stiller, A. E.: A Study of the Direction of the Enamel Rods in the Deciduous Molar-s Thesis, Northwestern University Dental School, 1937. Williams, J. Leon: Disputed Points and Unsolved Problems in the Normal and Pathological Histology of Enamel, J . Dent. Research 5: 27, 1923. Am J.éAnat' 87: 239, f3;,lR. F.: Histochemical Reactions of Normal Teeth, lastische Histologie der Zahn eweb Pl t‘ His 1 Tissues), Deutsche Zahn-, Mund- und Kgieferlfeil(kuiifidc7: 26g(,)1)9gy4i).0f Dental

parently

B. Development

1. Enamel Organ

The early development of the enamel organ and its differentiation have been discussed in the chapter on Tooth Development. At the stage preceding the formation of hard structures (dentin and enamel) the enamel organ, originating from the stratified epithelium of the primitive oral cavity, consists of four distinct layers: the outer enamel epithelium,

Fig. 5'.'.——'1'ooth gem: (lower incisor) or human embryo (105 mm., 4th month). Four Iayers of the enamel organ. X. See Fig. 59.

stellate reticulum, stratum intermedium, and inner enamel epithelium (ameloblastic layer) (Fig. 57). The borderline between the inner enamel epithelium and the connective tissue of the dental papilla is the subsequent dentino-enamel junction; thus, its outline determines the pattern of the occlusal or ineisal part of the crown. At the border of the wide basal opening of the enamel organ the inner enamel epithelium reflects into the outer enamel epithelium; this is the cervical loop.“ The inner and outer enamel epithelium are separated from each other by a large mass of cells differentiated into two distinct layers. One, which is close to the inner enamel epithelium and consists of two to three rows of flat polyhedral cells, is the stratum intermedium; the other layer, which is more loosely arranged, constitutes the stellate reticulum.

The different layers of epithelial cells of the enamel organ are named according to their morphology, function, or anatomic location. Of the four layers only the stellate reticulum derives its term from the morphology of its cells; the outer enamel epithelium and stratum intermedium are so named because of their location; the fourth, on the basis of anatomic relation, is called inner enamel epithelium or, on the basis of function, ameloblastic layer.

Fig‘. 58. Capillaries in contact with the outer enamel epithelium. Basement membrane separates outer enamel epithelium from connective tissue.

In the early stages of development of the enamel organ the outer enamel epithehum consists of a single layer of cuhoiclal cells, separated from the surrounding connective tissue of the dental sac by a delicate base ment membrane (Fig. 58). Prior to the formation of hard structures this regular arrangement of the outer enamel epithelium is more prominent in the cervical parts of the enamel organ. At the highest convexity of the organ (Fig. .37) the cells of the outer enamel epithelium hccome irregular in shape and cannot be easily distinguished front the outer portion of the stellate reticulum. The vascularized connective tissue surrounding the enamel organ on its convexity is in close contact with the outer enamel epi thelium. The capillaries are prolific in this area and protrude toward the enamel organ (Fig. 58). Immediately before enamel formation com mences, capillaries may even invade the stellate reticulum.‘-‘° This increased vascularity insures a rich metabolism of the avascular enamel organ during the formation of hard structures when a rich influx of substances from the blood stream to the inner enamel epithelium is required.

Fig. 59.—Region of the cervical loop (higher magnification of X in Fig. 5?). Transition of the outer into the inner enamel epithelium.

The stellate reticulum, which forms the middle part of the enamel organ, corresponds to the middle layer of the surface epithelium. Here, the neighboring cells are connected by intercellular bridges spanning the minute intercellular spaces. The features which characterize the stellate reticulum are primarily due to the great increase of the gelatinous intercellular substance. It separates the cells without breaking the intercellular connections, and causes each cell to become stellate, or starshaped, with long processes reaching in all directions from a central body and anastomosing with similar processes of neighboring cells (Figs. 58 and 59). The origin of the stellate reticulum, from the central portion of a stratified epithelium, explains further the fact that the cells are connected, by inter-cellular bridges, with the cells of the outer enamel epithelium and the stratum intermedium.

F18. 60.——Tooth germ (lower incisor) or a. human fetus (5th month). Beginning of

t<_iheil(1:;‘i1!l1e;’a.sI.1d §_na.rsneeel Ifggriggon. The stellate reticulum at the tip or the crown reduced in

The structure of the stellate reticulum renders it resistant and elastic; theretore, it seems probable that it has a supporting and protecting funcfmn {11 Preserving the shape of the inner enamel epithelium, as well as msurmg undisturbed development until the time when the hard structures have acquired adequate resistance. It seems to permit only a axannn 85

limited flow of nutritional elements from the outlying blood vessels to the formative cells. Indicative of this is the fact that the stellate reticulum is noticeably reduced in thickness when the first layers of dentin are laid down and the inner enamel epithelium is thereby cut ofi from the dental papilla, its original source of supply tFig. 60 ,1.

The cells of the stratum intermedium are situated between the stellate reticulum and inner enamel epithelium. They are flat to cuboid in shape, and are arranged in one to three layers. They are connected with each other, and with the neighboring cells of the stellate reticulum and inner enamel epithelium, by intercellular bridges. They may play an important role in the development of the enamel.“ It is possible that they take an active part in the calcium metabolism of the inner enamel epithelium. That they are rich in phosphatase, would tend to support the theory that they are actively involved in the process of calcification.“-'= *5 The cells of the stratum intermedium show mitotic division. and are active in this regard even after the cells of the inner enamel epithelium cease to divide.

The cells of the inner enamel epithelium which lie in contact with the dental papilla assume a columnar form before enamel formation begins and come to be known as ameloblasts. Like the outer enamel epithelium. the cells of the inner enamel epithelium are derived from the basal cell layer of the oral epithelium. Their basal end is in contact with the connective tissue; the peripheral end is in contact with the stratum intermedium. The cells are separated by narrow intcrcellular spaces which are crossed by intercellular bridges and contain a cementing substance. Terminal bars, which are condensations of the intercellular substance sealing the intercellular spaces, are found on both the basal and peripheral ends of the cells. The ameloblasts undergo changes in shape and structure which will be described as the life cycle of the ameloblasts.

At the free border of the enamel organ, where the outer and inner enamel epithelial layers are continuous and reflected into one another, is formed the portion known as the cervical loop” (Figs. 57 and 59l. Here is a zone of transition between the cuboidal cells of the outer enamel epithelium and columnar cells of the inner enamel epithelium in which the cuboidal cells gradually gain in length. This zone of transition is found in the cervical parts of the outer enamel epithelium. When the enamel organ of the crown is fomied the cells of this portion give rise to Hertwig’s epithelial root sheath (see chapter on Tooth Development).

2. Life cycle of the Ameloblasts

The cells of the inner enamel epithelium differentiate into ameloblasts. which produce the enamel matrix. However, the cells of the inner enamel epithelium may be termed ameloblasts even before they actually begin to produce enamel.

According to its function the life span of an ameloblast can be divided into several stages. The differentiation of ameloblasts is most advanced in the region of the incisal edge or tips of the cusps; least advanced in

Fig. 6L—-(For legend see opposite page.) E.\'.\.\il-IL E7 the region of the cervical loop. Thus. all at‘ some stages of the developing ameloblast can be observed in one tooth germ. Because these cells enter into this ditferentiation process successively the manner in which enamel formation takes place maybe referred to as a stagger system.

Before the ameloblasts reach their full differentiation. and produce the enamel, they play an important part in fixiiig the morphologic shape of the crown (dentino-enamel junction) (Fig. 60?, During this morphogenetic stage the cells are short columnar. with a large oval nucleus which almost fills the cell body. The ameloblastic layer is separated from the connective tissue of the dental papilla by a delicate basement membrane. The adjacent pulpal layer is a cell-free, narrow, light zone containing fine argyrophile fibers and the cytoplasmic processes of the superficial cells of the pulp (Fig. 61).”

In the organizing stage of development the ameloblasts seem to exert an influence upon the adjacent connective tissue cells which causes them to differentiate into odontoblastsf-’ This stage is characterized by a change in the appearance of the ameloblasts whereby they become longer and the nucleus-free zone. at the basal end of the cells. becomes almost as long as the peripheral part containing the nucleus (Fig. 613. In preparation for this development a reversal of functional polarity of these cells takes place becoming apparent by the migration of the central bodies" and the Golgi apparatus.‘ from the periphery of the cell into the basal end (Fig. 62). Moreover. the cytoplasm shows difierences in staining reaction, in the region peripherally and basally to the nucleus. The narrow peripheral part stains red in hematoxylin eosin preparations. and the wide basal part slightly pink.” Special staining methods reveal the presence of fine acidophile granules in the peripheral part of the cell.” At the same time, the clear cell-free zone between the ameloblast layer and dental papilla disappears (Fig. 61), probably due to elongation of the ameloblasts toward the papilla.” By this process the ameloblasts come into close contact with the connective tissue cells of the pulp, which are stimulated to differentiate into odontoblasts. During the terminal phase of the organizing stage of the ameloblasts the formation of the dentin by the dental pulp begins, and this is accompanied by a slight shortening of the elongated ameloblasts (Fig. 61).

The first appearance of dentin seems to be a critical phase in the life cycle of the ameloblasts. As long as they are in contact with the connective tissue of the dental papilla. they are nourished by the blood vessels of this tissue. When dentin forms. however, it cuts ofi the ameloblasts from their original source of nourishment and, from then on, they have to be supplied by the capillaries which surround and may penetrate

Fig. 61.——High magnification of ameloblasts, from (X) in Fig. 60. In the cervical region the ameloblasts are short and the outermost layer at the pulp is cell-tree. Occlusally the ameloblasts are long and the cell-tree zone of the pulp has disappeared. aye amelolggasts are again shorter where dentin formation has set in. (Diamond and einmann.

the outer enamel epithelium. This reversal of nutritional source is characterized by proliferation of capillaries of the dental sac, and by reduction and gradual disappearance of the stellate reticulum (Fig. 60). Thus, the distance between the capillaries and ameloblast layer is shortened. Experiments with vital stains demonstrate this reversal of the nutritional stream.“

The ameloblasts enter their formative stage only when the first layer of dentin has already been formed. The presence of dentin seems to be necessary to induce the beginning of enamel matrix formation just as it was necessary for the ameloblasts to come into close contact with the connective tissue of the pulp to induce dilferentiation of the odontoblasts and the beginning of dentin formation. This mutual action of one group of cells upon another is one of the fundamental laws of organogenesis and histodiiferentiation

Fig. 62.—Migx-ation of the centrioles from the peripheral (A) into the basal part (B) of the ameloblasts indicating reversed functional polarity. D : Dentin. (Renyifl)

During formation of the enamel matrix the ameloblasts retain, approximately, the same length and arrangement. The minute changes in the cell bodies are related to the formation of enamel matrix.

Enamel maturation occurs after the entire thickness of the enamel matrix has been formed in the occlusal or incisalarea.’ In the cervical parts of the crown, enamel matrix formation is, at this time. still progressing. During enamel maturation the ameloblasts are slightly reduced in length and are closely attached to the enamel matrix. The cells of the stratum intermedium lose their cuboidal shape and regular arrangement and assume spindle-shape. It is probable that the ameloblasts also play a part in the maturation of the enamel: ultimately they produce the primary cuticle.

When the enamel has completely developed and matured {calcified} the ameloblasts cease to be arranged in a well-defined layer, and can no longer be dificrentiated from the cells of the stratum intermedium and outer enamel epithelium. These cell layers then form a stratified epithelial covering of the enamel, the so-called reduced enamel epithelium. The function of the reduced enamel epithelium is that of protecting the mature enamel by separating it from the connective tissue until the tooth erupts. If connective tissue comes in contact with the enamel. anomalies may develop. Under such conditions the enamel may be either resorbed or covered by a layer of cementum.“ '

The reduced enamel epithelium seems also to induce atrophy of the connective tissue separating it from the oral epithelium. so that fusion of the two epithelia can occur (see chapter on Oral Mucous Membrane). It is probable that the epithelial cells elaborate an enzyme that is able to destro_v connective tissue fibers by desmolysis. Premature degeneration of the reduced enamel epithelium may prevent the eruption of a tooth?’

3. Amelogenesis

Development of enamel takes place in two distinct phases, i.e., formation of enamel matrix and maturation of enamel matrix. The fully developed enamel matrix is structurally identical to the mature enamel in that it is formed by enamel rods and interprismatic substance. Chemically and physically, however, it differs from the mature enamel. The fully developed matrix contains approximately 25 to 30 per cent mineral salts in solution, the rest is organic material and water.“ The process by which the matrix is transformed into the finished enamel, containing 96 per cent mineral salts and 4 per cent organic substance and water, is called maturation of the enamel. In the process of maturation more mineral salts are deposited and cr_vstall.ize in the matrix, and water is eliminated.

The chemical and physical differences between enamel matrix and mature enamel can be summarized as follows: {1} the enamel matrix has the consistency of cartilage whereas mature enamel is the hardest substance of the body: {'2} the enamel matrix is less radiopaque than the mature enamel; and (3" the enamel matrix is not birefringent; the mature enamel is birefringent when viewed in polarized light at right angles to the long axis of the rods?‘ ‘" A. Formation of the Enamel Matrix. The formation of the enamel matrix is a very intricate process in its morphogenesis as well as in its chemistry. In analyzing this process the following stages can be distinguished:

(a) Formation of dentino-enamel membrane

(b) Development of Tomes’ processes

(c) Horuogenization of Tomes’ processes

((1) Formation of pre-enamel rods

(e) Influx of mineral salts in solution into the matrix

Dgnuno. It has been shown that, prior to the formation of dentin, the connective ‘figfiglmne tissue of the dental papilla is separated from the inner enamel epithelium by a basement membrane (Fig. 63). On the connective tissue side fibers

Fig. 63.—Basement membrane of the dental papilla can be followed on the outer surface of the dentin, forming the dentino-enamel membrane. (Orban, Sicher and Weinmannfi‘)

of the pulp are attached to this membrane fomuing the fibrous precursor of the dentin. When a thin layer of dentin has been laid down the anteloblasts begin their amelogenetie activity by forming a. continuous thin menlbrnne on the enamel side of the basement membrane;“’ it has been termed dentinoenamel membrane.” In later stages of amelogenesis it is found to be continuous with the interprismatic substance. Its presence acE.\‘A.\:EL 91

counts for the fact that the dentinal ends of the rods are not in direct contact with the dentin «Fig. 64?. The dentinna:-name} membrane calcifies soon after its formation. similar to the interprismatic stthstanne. After formation of the dentino-enamel membrane the ameloblasts produce short proeesses at their basal end which are known as Tomes’ processes (Fig. 65). These are hexagonal prismatic in shape and are a continuation of the ameloblasts. Synchronized with the appearance of Tomes’ processes the terminal bars appear at the basal end of the anteloblasts. They denote the boundary between the cell body and Tomes’ I it i ‘ (Z1 Enamel rods

Fig. 64.-—Dentino-enanzel membrane separates the rods from dentin.

processes. Structurally, they are condensations of the intereellular substance and appear, in a surface view, as more or less regular hexagons which can be compared to a honeycomb -.'_Fig. 66'. The Tomes’ processes are separated from each other by thin extensions of the terminal bars. They retain their approximate length throughout the entire formation of the enamel rods. The Tomes’ processes are continuously transfomied into enamel rod substance at their dentinal end, and rebuilt at their 3.111810blastic end.”

The portion of the ameloblast designated as Tomes’ process is granular during amelogenesis. The first indication of formation of the enamel rod is a homogenization in the dentinal end of the Tomes’ process; the chemical nature of this change is unknown; the homogenized Tomes’ process is slightly basophil in reaction (Fig. 67:1). rounation or At the time this change is occurring, the lateral parts of the homogegfggnamel nized processes are transformed into a diiferent chemical substance, denser in structure and strongly basophil in character (Fig. 67A). This substance does not contain calcium salts; it can be regarded as preenamel. The transformation of the homogenized Tomes’ processes into pre-enamel proceeds rhythmically by the formation of the so-called globules or segments (Fig. 6713). The transformation of each segment proceeds excentrically, starting from one lateral surface, thus giving a picket-fence-like appearance to the pre-enamel. The developing rods are at an angle to the axis of the ameloblast and Tomes’ processes”! 2“ The primary segmentation of the rods remains visible as a cross-striation of the mature rods (Fig. 30). The outer layer of each rod shows a slightly different staining reaction and is known as the rod sheath.

Fig. 65.—For-matlon of Tomes’ processes and terminal bars, as the first step in enamel rod formation Rat incisor. (Orban. sicher and Weinmannfi)

The interprismatic substance, which is continuous with the extensions of the terminal bars between the Tomes’ processes, can be distinguished between the forming rods. The thickening of the terminal bars at the basal end of the ameloblasts can be explained, therefore, by their role in the production of the interred substance.

Mull! gi11é11l- When the pre-enamel rod attains a length of about 20 microns, calcium salts in solution are deposited into its substance. The calcification begins at the dentinal end of each rod and involves first the outer layers of each rod, its core being the last part to calcify. However, because more preenamel is forming all the while, the layer of pre-enamel remains approxiENAJIEL 93

mately of equal width. The calcium salts are transported into the preenamel from the blood vessels surrounding the enamel organ, by way of the stratum intermedium, ameloblasts and Tomes’ processes. This influx of mineral salts is accompanied by a chemical change in the pre—ename1. It becomes more acidophilic.‘*"’ This acidophil layer might be termed young enamel matrix. It forms a layer about 30 microns thick and remains visible as a distinctly stained zone of the enamel matrix, until maturation starts. The last stage of matrix formation is characterized by a gradual reversal of the acidophilic nature of the young matrix into a slightly basophilic state (Fig. 68]. The formation of the matrix follows an incremental pattern (bands of Retzius).

Fig. 66.—Terminal bar apparatus of the ameloblass in surface view. (Orban. Sicher and W'einmann.")

B. Maturation of Enamel Matrix (Calcification and Crystallization). The maturation of the enamel matrix is characterized by the gradual influx of almost three quarters of the ultimate contents of mineral salts 94 om.

Fig. 67A.—Homogenization of the dentinal ends of Tomes’ processes and their trar formation into pre-enamel matrix in a picket fence arrangement. The rods are at angle to the ameloblasts and Tomes’ processes. (orban. Sicher and We1nmann.")

Ins. $7B.—Devdopment of rod segments during formation of pre-enamel matrix, The alternating appearance or segmented and n ted rods is due to the honeycomb an-anzernmt of the hexagonal prismatic rods. ( rba . Sicher and Weinmannfi)

present in the mature enamel, by crystallization of the mineral salts, and by the simultaneous disappearance of water. The protein content of the enamel matrix remains, in all probability, unchanged. It begins after the enamel matrix has reached its final thicknes in the occlusal parts of the crown. It can be assumed that the ameloblasts play an important part in this transformation. The chemical changes in maturation are gradual.

Fig. 68.——Diagramma.tic illustration of enamel matrix formation. Tomes’ processes remain approximately the same in length during enamel matrix formation. Their dentinal end is homogenized and then transformed into pre-enamel. Pre-enamel changes into young enamel matrix and later into fully developed enamel matrix. The interred substance is a continuation of the terminal bar apparatus. (Orban. Sicher and Wein mann:-")

The protein of the enamel before and after maturation is acid soluble.“ The proteins lose their solubility if they are denatured, for instance by formalin fixation.“ Before maturation the enamel matrix is easily penetrated by the fixing fluid, while the density of the maturing and matured Fig. is.—Dlagra.mmatlc illu.stra.fion of enamel matrix tormatiqn and maturation. Formation follows an incremental pattern. maturation begins at

proceeds oerrkally in cross relation to the incremental pattern. and Weinmann!)

Fig. 70.—-Bucco-lingual section through a deciduous molar. Maturation of the enamel has started in the lingual cusp—-while it has fairly well 1) in the bu cal cusp. Note the gradual transltian between the enamel matrix an the fully matured enamel renders it almost impermeable. Routine fixation of specimens will therefore cause denaturing of the proteins of the enamel matrix only. Thus, the enamel matrix is preserved despite decalcification, while the maturing enamel disappears after its mineral contents have reached a critical value.

Fig. 71.-—Dlag:-ammatic illustration of the crystal (black) and space (white) relation in developing enamel as observed by polarized light. Compare with Fig. 53A to note the subsequent elimination of space during the last stages of enamel maturation.

The process of maturation starts in the incisal region of the crown, or at the heights of the cusps, and proceeds toward the cervical region‘

(Figs. 69, 70). It does not follow the incremental pattern but proceeds in planes at right angles to the long axis of the tooth. The pattern of maturation is correlated with that of tooth eruption.

During maturation the highest content of mineral salts is found at the tip of the cusps or on the incisal edge; the lowest content of mineral

References

Dean, H. T., and Kitchin, P. 0.: Fluorine and Dental Health, Washington, D. G., 1942, American Association for Advances of Science.

Diamond, M., and Weinmann, J. P.: The Enamel of Human Teeth, New York, 1940, Columbia University Press. _ _ Diamond, M., and Weinmann, J. P.: Morphogenesis of the Amelob_lasts in Re lation to the Establishment of the Fixed Dentino-Enamel Junction, J. Dent.

Research 21: 403, 1942. _ _ Diamond, M., and Applebaum, E.: The Epithehal Sheath:

Function, J. Dent. Research 21: 403 1942.

Histogenesis and

Engel, M. B.: Glycogen and Carbohydrate—-Protein Complexes in Developing Teeth of the Albino Rat, J. Dent. Research 27: 681, 1948. _ _ Engel, M. B., and Furuta, W.: Histochemical Studies of Phosphates: Distribu tion in Developing Teeth of Albino Rat, Proc. Soc. Exper. Biol. £5 Med. 50: 5 1942.

Frisbie, H. E., Nuckolls, J., and Saunders, J. B. de C. M.: Distribution of Organic Matrix of Enamel in the Human Tooth and Its Relation to Histopathology of Caries, J. Am. Coll. Dent. 11: 243, 1944.

Glasstonc, S.: Development of Toothgerms in Vitro, J. Anat. 70: 260, 1936.

Gomori, G.: Calcification and Phosphatase, Am. J. Path. 19: 197, 1943.

Gottlieb, B.: Calcium Deposition and Enamel Hypoplasia, J. Dent. Research 20: 549 1941. Hahn, E.: The Capacity of Developing Tooth Germ Elements for Self Diflferentiation When Transplanted, J. Dent. Research 20: 5, 1941.

Hampp, E. G.: Mineral Distribution in the Developing Tooth, Anat. Rec. 77: 273 1940. Held, H.,: Ueber die Bildung des Schmelzgewebes (On the Formation of Enamel),

Ztschr. f. mikr.-anat. For-sch. 5: 668, 1926. Jump, E. B.: Vascularity of the Human Enamel Organ, J. Dent. Research 17: 505 1938. Kitchin,’ P. 0.: Some Observations on Enamel Development as Shown on the Mandibular Incisor of the White Rat, J. Dent. Research 13: 25, 1933. Kotanyi, E.: Histologische Befunde an retinierten Zahnen (Histologic Findings on Embedded Teeth), Ztschr. f. Stomatol. 22: 747, 1924.

Logan, W. H. G., and Kronfeld, R.: Development of the Human Jaws and Surrounding Structures From Birth to the Age of Fifteen Years, J. A. D. A. 20: 379 1933.

Orban, Zur Entwicklung und feinei-en Struktur des Schmelzes (On the Development and Finer Structure of the Enamel), Ztschr. f. Stomatol. 23: 599 1925.

Orban, Zur Histologie des Schmelzes und der Schmelzdentingrenze (Histology of Enamel and Dentino-Enamel Junction), Vrtljschr. f. Zahnheilk. 42: 336, 1926.

Orban, B., Sicher, H., and Weinmann, J. P.: Amelogenesis (A Critique and a. New Concept), J. Am. Coll. Dentists 10: 13, 1943.

Renyi, G. S. de: Central Bodies in the Cells of the Inner Enamel Epithelium, Am. J. Anat. 53: 413, 1933.

Sarnat, B. G., and Schour, I.: Enamel Hypoplasia (Ghronologic Enamel Aplaaia) in Relation to Systemic Disease, J. A. D. A. 28: 1989, 1941; 29: 67, 1942.

Saunders, J. B. de G. M., Nuckolls, J., and Frisbie, H. E.: Amelogenesis, J. Am. Coll. Dentists 9: 107, 1942. Waserman, F.: Enamel Production and Calcification: Normal and Experi mental, J. Dent. Research 20: 254, 1941. Weinniann, J. P., Wessinger, G. D., and Reed, (3.: Correlation of Chemical and Histological Investigations on Developing Enamel, J. Dent. Res. 21: 171,

1942. Weinmann, J . P., Svoboda, J . F., and Woods, R. W.: Hereditary Enamel Formation and Calcification, J . A. D. A. 32: 397, 1945. Weinmann, J. P.: Developmental Disturbances of the Enamel, The Bur 43: 20,

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