This lavishly illustrated, practical guide to endodontic treatment covers the latest developments in instrumentation and filling techniques. Ideal for all dental practitioners involved in endodontic therapy [root canal treatment], this new edition has been fully updated throughout and now includes a new author team from the Eastman Dental Institute.
Tooth organogenesis, morphology and physiology
K Gulabivala and Y-L Ng
Many readers approach human embryology with a view to satisfying academic test requirements and may even believe such academic knowledge to be far removed from clinical practice. Yet this book begins with this fascinating subject, not merely to lay an academic foundation for the knowledge of endodontics but because contemporary practice recognizes that these biological processes hold the key to future therapeutic strategies. Regenerative treatment approaches depend upon insight from developmental processes to engineer the growth of new tissues to replace those that are diseased or damaged. The ultimate may even be to grow whole replacement teeth on demand, in situ or for implantation. Among the clinicians involved should be endodontists in whose field of knowledge and practice these procedures should lie. Any clinician involved in delivering procedures that even border on regenerative techniques should have a basic understanding of tooth development and its associated structures.
The “intelligence” or “activating force” that directs the precise coordination of multiple cell line activity, growth, migration, induction, fusion and disintegration with such control and symphonic grace, still eludes us. In our current state of knowledge, we are left merely to describe the observable and timed changes gleaned through various biological studies. Experimental studies also give us some insight about the genomic and proteomic involvement in the process, even though the picture is far from complete. Yet there is already sufficient intuitive knowledge to enable the culture of tooth tissues and whole teeth in the laboratory, albeit in a neophytic way (Fig. 1.1).
Fig. 1.1 Something to chew on (courtesy of Takashi Tsuji, Tokyo University of Science)
Early development of teeth
The primitive mouth cavity is evident as a slit-like space lined by ectoderm in the 3–4-week-old human embryo. It is located under the surface of the brain capsule and above the pericardial sac where the heart forms. The mouth cavity is still separated from the primitive pharynx by the oropharyngeal membrane. The mandibular processes grow ventrally on each side of the head to meet gradually in the midline, where they form the lower border of the mouth opening. The maxillary processes arise from the upper surfaces of the origin of the mandibular process and likewise grow towards the midline, to form the upper border of the mouth below the brain capsule (Fig. 1.2). The maxillary and mandibular processes are essentially extensions of mesenchyme tissue covered by ectoderm. The ectoderm is a layer of low columnar epithelial cells, resting on a basal lamina which separates them from the mesenchymal tissue, which originates from the neural crest cell line. In some regions, such as the tooth-bearing part, the epithelium has a more superficial part, which consists of 2–3 layers of flattened cells. At this stage, the maxillary and mandibular processes do not show separate lip or gum regions; the development of the lips, cheeks and gum regions is closely associated with the development of the dental lamina, from which teeth arise.
Fig. 1.2 Maxillary and mandibular processes in the head of human embryo (approx. 5 weeks)
Primary epithelial band, vestibular band and dental lamina
The first indication of formation of tooth development structures becomes evident at 6 weeks of embryonic life when the oral epithelium in the lateral regions of the maxillary and mandibular processes proliferate and then spread towards the midline where they become continuous into horseshoe-shaped bands. These bands are not evident on the surface but project into the underlying mesenchyme and are called the primary epithelial bands.
During the seventh week of embryonic life, the primary epithelial band divides on its deep surface into two processes; the outer, thicker one becomes the vestibular lamina (responsible for the later separation of lips/cheeks from gums) and the inner, smaller one becomes the dental lamina (which later gives rise to the teeth) (Fig. 1.3). As the dental lamina grows in length, it penetrates deeper into the mesenchyme; at the front of the mouth in a lingual direction, to form a shelf-like projection and at the back of the mouth remaining more vertical (Fig. 1.4). It is not known whether this results from active invagination of the lamina or upward proliferation of the mesenchyme.
Fig. 1.3 The dental lamina
Fig. 1.4 The primary enamel organ
A short while after formation, the dental lamina thickens into small rounded swellings, involving the whole thickness from free edge to the base of attachment to the oral epithelium. These are the enamel organs of the deciduous teeth with four in each quadrant (2 incisors, canine and first deciduous molar) (see Fig. 1.4). The dental lamina continues to grow backwards, giving rise to further enamel organs for the second deciduous molar (10-week embryo), and the permanent molars (first permanent molar at 16-week embryo; second and third permanent molars after birth). At 10 weeks of embryonic life, the enamel organs and dental lamina conform to a catenary curve. As the tooth germs grow, the spacing between them is reduced. There is at this early stage no indication of the successional permanent teeth, which develop later by budding off from the lingual aspects of each deciduous enamel organ.
The mesenchymal tissue surrounding the developing enamel organ responds by proliferation to form a dense mass of cellular tissue. This gives rise to the dental papilla (primitive pulp) and the follicular sac for each tooth bud. The enamel organ in the “bud” stage appears as a simple, spherical to ovoid, epithelial condensation that is poorly morpho- and histo-differentiated. The epithelial component is separated from the adjacent mesenchyme by a basement membrane. The combination of enamel organ, dental papilla and follicular sac are collectively known as the tooth germ (Fig. 1.5). The enamel organ becomes concave on its papillary surface and begins to grow at the rims so as to encircle the dental papilla, which, at this stage, is partly capped by the enamel organ (hence “cap” stage) (Fig. 1.6) and progressively embraces a greater volume of it, to be called the “bell” stage (Fig. 1.7). At the cap stage, the centre of the concavity develops a projection of epithelium called the enamel knot (Fig. 1.6), which soon disappears by programmed cell death (apoptosis) and seems to contribute cells to the enamel cord. The enamel knot represents an important regulatory signalling centre during tooth development by producing bone morphogenetic proteins (BMP-2, BMP-7), fibroblast growth factor (FGF–p21 cyclin-dependent kinase inhibitor), sonic hedgehog (Shh), WNT and transcription factors. These signals regulate growth and development of the epithelial folds that correspond to the cusp pattern of the mature tooth. The primary enamel knot also determines the position of the secondary enamel knots corresponding to the site of the future cusps. The enamel cord is a strand of cells seen at the early bell stage of development. When present, it overlies the incisal margin of a tooth or the apex of the first cusp to develop. It has been suggested that the enamel cord may be involved in the process, by which the cap stage is transformed into the bell stage or that it is a focus for the origin of stellate reticulum cells.
Fig. 1.5 The tooth germ
Fig. 1.6 The enamel organ at “cap” stage
Fig. 1.7 The enamel organ at “bell” stage
Concurrent with the enamel organ development, the vestibular band growth continues apace. At around the time of the cap stage, a vertical cleft becomes established in the vestibular band, separating the formative lips and cheeks from the formative gums (Fig. 1.8). As for the dental lamina, the vestibular band development progresses backwards.
Fig. 1.8 The formative lips and cheeks separated from the formative gums at the advanced “bell” stage
Changes to and further development of dental lamina for permanent molars
As the enamel organ is reaching the cap stage, so the dental lamina lengthens and divides into buccal and lingual parts, though the function of this is unknown. By the time the enamel and dentine formation begins during early bell stage, the dental lamina connecting the tooth germs to the oral epithelium starts to degenerate leaving a network of strands and clumps of epithelial cells. At the same time, the dental lamina continues to grow backwards to give rise to the permanent molars but, by this stage, is separated from the oral epithelium.
Development of successional permanent teeth
The enamel organs for the successional teeth arise so differently from the permanent molars that it raises...