Tooth Enamel Microstructure

Proceedings of the enamel microstructure workshop, University of Bonn, Andernach, Rhine, 24-28 July 1994

W. von Koenigswald, P.M. Sander

• Publisher: CRC Press
• Year: 2020
• eBook ISBN: 9781000150605
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Civil Engineering
• Index: Technology & Engineering

Enamel, the shiny material covering the teeth of vertebrates is the hardest tissue the vertebrate body can produce and one of the most impressive products of biomineralization. This hard tissue is closely related to feeding, the first part in the energy intake process so basic to vertebrate life. Enamel has a complex internal microstructure full of phylogenetic and biomechanic information.
Topics covered: Ontogeny; Crystallite level; Prism level; Enamel type level; Schmelzmuster level; Dentition level; Evolution; Biomechanical level; Glossary.

CHAPTER 1

The ontogeny of mammalian enamel

LETTY MOSS-SALENTIJN, MEL VIN L. MOSS & MICHAEL SHENG-TIEN YUAN
Department of Anatomy and Cell Biology, Columbia University, New York, USA

SUMMARY: The ontogeny of mammalian enamel is reviewed briefly in this chapter. Ameloblast differentiation and function, as they are currently understood, are presented. The significance of Tomes’ process during the secretory phase of amelogenesis is emphasized. Tomes’ process morphology controls the orientation of the matrix proteins, the alignment of the hydroxyapatite crystallites and ultimately the prismatic or non-prismatic patterns of mature enamel. During the maturation phase of amelogenesis mineralization is completed and as much as 90% of the enamel proteins degraded and removed from the enamel by maturation ameloblasts. The mature enamel structure reflects several aspects of the process of amelogenesis. Cyclic patterns of enamel deposition in primate teeth are represented by cross-striations, striae of Retzius and perikymata. The motions of the secretory ameloblasts, or more precisely, of their Tomes’ processes, produce the curvilinear course of the enamel prisms. The latter are, in part, responsible for such phenomena as Hunter-Schreger bands. Any explanation of the genesis of these bands must take into account the developmental constraints presented by cellular movements of an intact sheet of secretory ameloblasts.

ZUSAMMENFASSUNG: Dieses Kapitel gibt eine Übersicht über die Ontogenie des Säugetierschmelzes.

Dabei wird das derzeitige Bild von der Differenzierung und Funktion der Ameloblasten sowie die Bedeutung des Tomes’ schen Fortsatzes während der Schmelzbildung dargestellt. Die Morphologie des Tomes’ schen Fortsatzes ist für die Orientierung der Proteinmatrix, der Ausrichtung der Hydroxyapatit- Kristallite und letztlich der prismatischen bzw. aprismatischen Struktur des reifen Schmelzes verantwortlich. Während des Ausreifens des Schmelzes wird die Mineralisierung abgeschlossen und etwa 90% der Proteine werden während dieser Reifephase vom Ameloblasten degradiert und entfernt. Die reife Schmelzstruktur spiegelt verschiedene Aspekte des Prozesses der Schmelzbildung wieder. Eine zyklische Schmelzbildung wird bei den Primaten als ‘cross-striations’ beziehungsweise als Retzius-Linien und als Perikymata abgebildet. Die Bewegung der sezernierenden Ameloblasten, oder genauer ihrer Tomes’ schen Fortsätze, formen den undulierenden Weg der Schmelzprismen. Diese Biegungen sind verantwortlich für Phänomene wie die Hunter-Schreger-Bänder. Jede Erklärung zur Entstehung dieser Hunter-Schreger-Bänder müssen dem Umstand Rechnung tragen, daß die dazugehörige Bewegung der Ameloblasten als festgefügtes Epitel erfolgt.

1 INTRODUCTION

Improvements in microscopy and tissue preparation, as well as significant advances made in cell biological techniques, have reinvigorated the study of ontogenetic development of enamel (amelogenesis) during the past three decades (see Allan 1967, Fearnhead & Suga 1984, Frank & Nalbandian 1967, Nylen & Termine 1979, for general overviews). The field is now provided with fairly specific information on the cellular events in the process of amelogenesis, from the time of early differentiation of the enamel epithelium through completion of the process, as monographically described by Sasaki (1990).

Most studies focus on mammalian amelogenesis, with descriptive studies of human teeth taking a leading role for obvious clinical reasons. Other descriptive and experimental work has been done on rodents. In addition, but to a lesser extent, amelogenesis has been studied in other mammals (Sasaki 1990). Data on amelogenesis in non-mammalian tetrapods are considerably more limited (Peyer 1968, Roux & Chibon 1973, Smith & Miles 1971, Spinelli & Chibon 1973, Wakita 1993, Weill & Tassin 1969, Zaki et al. 1970, see also Suga 1983 for reviews). However, the presently available descriptions suggest that a broad pattern of similarities exists.

2 TOOTH DEVELOPMENT PRIOR TO THE ONSET OF AMELOGENESIS

It is well established that a tooth is the composite product of two adjacent tissues, epithelium and neural crest-derived mesenchyme. Dental enamel is produced by epithelial cells, which may be ectodermal or endodermal in origin (Graveson 1993, Smith & Hall 1993, Lumsden 1987). Among the odontogenic derivatives of the mesenchyme are dentine-forming odontoblasts, cementoblasts and fibroblasts of the pulp. There is substantial evidence for the important role of cranial neural crest in the development of teeth (Hall & Hörstadius 1988, Le Douarin 1982, Lumsden 1987). However, some experimental evidence would suggest that trunk neural crest, under appropriate conditions, may be induced to become committed dental mesenchyme as well (Graveson 1993, Lumsden 1987).

A series of inductive interactions between epithelium and mesenchyme determines the sequence of events in tooth development: The initial establishment of dental primordia, sequential stages of morphogenesis, cytodifferentiation of odontoblasts, deposition of dentine matrix, cytodifferentiation of ameloblasts and deposition of enamel matrix. Each successive stage requires conditions and interactions that have been determined in the preceding stage (see Kollar 1986, Ruch et al. 1983, Slavkin 1990, Thesleff & Hurmerinta 1981, Thesleff et al. 1988, for reviews of the experimental literature).

More recently, attention has been focused on the molecular controls of the epithelial-mesenchymal interactions that characterize tooth development. Positional information and patterning during the early morphogenetic stages of tooth development appear to be specified by selective expression in the dental primordia of homeobox genes, which in turn may be controlled by gradients of morphogens, such as retinoids, in the embryonic tissues (Mackenzie et al. 1991, 1992). Subsequently, the products of the homeobox genes, possibly in addition to the retinoids, appear to control the expression of specific regulatory peptides or growth factors. The transient and patterned expression of transforming growth factor (TGF-beta), epidermal growth factor (EGF) and nerve growth factor (NGF), as well as the selective expression of the appropriate cell surface receptors for these peptides in dental mesenchyme and epithelium, suggest that these peptides may serve as some of the molecular signals in the epithelial-mesenchymal interactions (Partanen 1990, Slavkin 1990, 1991). In this context the recent finding of a possible influence on cusp morphogenesis by the production of fibroblast growth factor (FGF-4) in the enamel knot (a cluster of closely packed, non-dividing epithelial cells in the stellate reticulum of the enamel organ) is particularly noteworthy (Jernvall et al. 1994). Finally, there is evidence that extracellular matrix molecules, such as tenascin and fibronectin, which bind to the cell adhesion molecule syndecan in a patterned fashion, similarly may have a role in the transmission of interactive odontogenetic signals (Thesleff et al. 1990).

It is fair to state that while the several separate lines of investigation in this rapidly evolving field have yielded data that have given us some fascinating insights, they do not as yet allow either a synthesis or a concise summary of the molecular controls of odontogenesis.

3 AMELOGENESIS

While it is possible to distinguish several discrete stages in the life cycle of an individual mammalian ameloblast, in this review only three stages will be considered: Cytodifferentiation, matrix secretion and enamel maturation. It should be kept in mind that the epithelial cells enter each of these stages in an occluso-apical gradient and that it is possible to observe all...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• List of contributors
• Introduction &
• 1 The ontogeny of mammalian enamel
• 2 A short review of studies on chemical and physical properties of enamel crystallites
• 3 Non-mammalian synapsid enamel and the origin of mammalian enamel prisms: The bottom-up perspective
• 4 The earliest prisms in mammalian and reptilian enamel
• 5 Characterization of enamel microstructure terminology and application of the origins of prismatic structures in systematic analyses
• 6 Tubules in Australian marsupials
• 7 Differentiations in Hunter-Schreger bands of carnivores
• 8 Brief survey of enamel diversity at the schmelzmuster level in Cenozoic placental mammals
• 9 Incisor enamel microstructure and systematics in rodents
• 10 The enamel structure of some fossil and recent whales from the Indian subcontinent
• 11 The variability of enamel structure at the dentition level
• 12 Evolutionary trends in the differentiation of mammalian enamel ultrastructure
• 13 Mechanical adaptation in enamel
• 14 Schmelzmuster differentiation in leading and trailing edges, a specific biomechanical adaptation in rodents &
• 15 Glossary of terms used for enamel microstructures &