Scanning electron microscope views of (A) the enamel layer covering coronal dentin, (B) the complex distribution of enamel rods across the layer, (C and D) and perspectives of the rod-interrod relationship when rods are exposed (C) longitudinally or (D) in cross section. Interrod enamel surrounds each rod. R, Rod; IR, interrod; DEJ, dentinoenamel junction.

A and B, High-resolution scanning electron microscope images showing that crystals in rod and interrod enamel are similar in structure but diverge in orientation.

Transmission electron microscope images of a rod surrounded by interrod enamel from (A) young and (B) older forming enamel of a rodent. The crystals that make up the rod and interrod enamel are long, ribbonlike structures that become thicker as enamel matures. They are similar in structure and composition but appear in different planes of sections because they have different orientations.

Cross-sectional profiles of (A) recently formed, secretory stage enamel crystals and (B) older ones from the maturation stage. Initially the crystals are thin; as they grow in thickness and width, their hexagonal contour becomes apparent. B, The linear patterns seen in older crystals are a reflection of their crystalline lattice.

A to C, Interpretation of rod structure and orientation can be misleading in ground sections examined by light microscopy. When such sections are thinned down, what appears to be a longitudinal rod in some cases actually may be crosscut rods.

Fine structure of enamel. A, Crystal orientation along three faces of an enamel block. B to D, Transmission electron micrographs of the three faces. (Courtesy A.H. Meckel.)

A and B, Decalcified preparation of cat secretory stage enamel. The organic matrix near the ameloblasts is younger and shows a uniform texture. The distal portion of Tomes’ process (dpTP) penetrates into the enamel. In deeper areas, near dentin, matrix is older and partly removed. Matrix accumulates at the interface between rod (R) and interrod (IR) to form the rod sheath (arrowheads).

Representative micrographs of amelogenesis in the cat. A, Tooth formation shows an occlusal-to-cervical developmental gradient so that on some crowns finding most of the stages of the ameloblast life cycle is possible. The panels on the right (B corresponds with B1 and C with B2) are enlargements of the boxed areas: B, Secretory stage, initial enamel formation; C, secretory stage, inner enamel formation. D and E are from the incisal tip of the tooth (see Fig. 7-15). D, Midmaturation stage, smooth-ended ameloblasts; and E, late maturation stage, ruffle-ended ameloblasts. Am, Ameloblasts; D, dentin; E, enamel; N, nucleus; Od, odontoblasts; PL, papillary layer; RB, ruffled border; SB, smooth border; SI, stratum intermedium.

The various functional stages in the life cycle of the cells of the inner dental epithelium. 1, morphogenetic stage; 2, histodifferentiation stage; 3, initial secretory stage (no Tomes’ process); 4, secretory stage (Tomes’ process); 5, ruffle-ended ameloblast of the maturative stage; 6, smooth-ended ameloblast of the maturative stage; 7, protective stage.

Differentiating ameloblasts extend cytoplasmic projections (*) through the basement membrane (BM), separating them from the forming mantle predentin. The basement membrane is fragmented and removed before the active deposition of enamel matrix. mv, Matrix vesicle; sg, secretory granule.

Colloidal gold immunocytochemical preparations illustrating the expression of amelogenin by differentiating ameloblasts. A, Amelogenin molecules are immunodetected (black dots) extracellularly early during the presecretory stage, before the removal of the basement membrane (BM) separating ameloblasts from the developing predentin matrix. Thereafter, enamel proteins (B) accumulate as patches (arrowheads) at the interface with dentin and then (C) as a uniform layer of initial enamel. djc, Distal junctional complex; im, infolded membrane; mv, matrix vesicle; Odp, odontoblast process; ppTP, proximal portion of Tomes’ process; sg, secretory granule.

When initial enamel forms, the ameloblast only has a proximal portion of Tomes’ process (ppTP). The distal portion develops as an extension of the proximal one slightly later when enamel rods begin forming. dcw, Distal cell web; sg, secretory granules.

Scanning electron micrograph of the surface of a developing human tooth from which ameloblasts have been removed. The surface consists of a series of pits previously filled by Tomes’ processes the walls of which are formed by interrod enamel.

A and B, Scanning electron microscope illustrations showing the complex trajectory of rods in the inner two thirds of the enamel layer. B, The rods are organized in groups exhibiting different orientations; this illustration shows four adjacent groups.

The (A) first (initial) and (B) last (final) enamel layers are aprismatic, that is, they do not contain rods.

Scanning electron microscope views of the (A) ruffle-ended and (B) smooth-ended apices of maturation stage ameloblasts. m, Mitochondria.

The functional morphology of ruffle-ended and smooth-ended maturation stage ameloblasts.

Longitudinal section of enamel viewed by incident light. The series of alternating light and dark bands of Hunter and Schreger are apparent.

Section showing the hard and soft tissues of the tooth

High magnification of the dentinoenamel junction.

Enamel spindles represent odontoblast processes trapped in enamel.

Transmitted light image of cross-sectional ground section of a tooth showing a lamella and concentric lines/bands representing the striae of Retzius.

Ground section of a tooth showing the disposition of striae of Retzius and of enamel tufts at the dentinoenamel junction.

Enamel tufts resemble tufts of grass in ground section.

Ground sections permit ready visualization of the scalloped appearance of the dentinoenamel junction. Also note the complex trajectory of the enamel rods in the inner enamel.

Incisal tip of a tooth just before the start of the enamel layer formation.

Tooth bud at the stage when both enamel and dentin formation begins.

A and B, Early crown stage of tooth development. Dentin (D) and enamel (E) have begun to form at the crest of the folded inner dental epithelium (incisal tip). There is a reduction in the amount of stellate reticulum (SR) in the region where matrix deposition has occurred. Note the developmental gradient in cell differentiation from the tip toward the cervical portion of the tooth crown. Am, Ameloblasts; Od, odontoblasts; ODE, outer dental epithelium; SI, stratum intermedium; PD, predentin.

Secretory stage amelogenesis. Tomes’ processes jut into enamel and in certain species in a "picket fence" appearance. The line at the base of the ameloblasts represents the proximal cell web (pcw), and that at the apex, the distal cell web (dcw).

Histologic section of a decalcified tooth along the slope of the cusp showing an incisal to cervical gradient in enamel maturation. As maturation progresses, enamel matrix is lost and mineral content increases. Almost mature enamel (top right) appears whitish because mineral has been removed and there is very little matrix left in this area. Note the striae of Retzius and morphology of maturation stage ameloblasts (no Tomes’ process).

Maturation stage of amelogenesis. A, Smooth-ended ameloblasts. Note that the three other layers of enamel organ have amalgamated together to form a highly infolded and vascular layer, the papillary layer. Ameloblasts undergo modulation, a process by which their apexes alternate between a smooth-ended border (A) and a ruffle-ended border (B).

Once enamel is completely mature, the enamel organ forms the reduced dental epithelium. At this stage, ameloblasts are no longer distinguishable.

Striae of Retzius manifest on the surface of the tooth as a series of grooves called perikymata.

Transmitted light image of ground section showing the alternating orientation of a group of rods in the region of Hunter-Schreger bands.

Phase-contrast microscopic image of the longitudinal ground section of a calcified tooth.

Reference: Nanci A.: Ten Cate's Oral Histology: Development, Structure, and Function, 6th Edition, 2003, Mosby.