Current knowledge of tooth development: patterning and mineralization of the murine dentition

Abstract

The integument forms a number of different types of mineralized element, including dermal denticles, scutes, ganoid scales, elasmoid scales, fin rays and osteoderms found in certain fish, reptiles, amphibians and xenarthran mammals. To this list can be added teeth, which are far more widely represented and studied than any of the other mineralized elements mentioned above, and as such can be thought of as a model mineralized system. In recent years the focus for studies on tooth development has been the mouse, with a wealth of genetic information accrued and the availability of cutting edge techniques. It is the mouse dentition that this review will concentrate on. The development of the tooth will be followed, looking at what controls the shape of the tooth and how signals from the mesenchyme and epithelium interact to lead to formation of a molar or incisor. The number of teeth generated will then be investigated, looking at how tooth germ number can be reduced or increased by apoptosis, fusion of tooth germs, creation of new tooth germs, and the generation of additional teeth from existing tooth germs. The development of mineralized tissue will then be detailed, looking at how the asymmetrical deposition of enamel is controlled in the mouse incisor. The continued importance of epithelial-mesenchymal interactions at these later stages of tooth development will also be discussed. Tooth anomalies and human disorders have been well covered by recent reviews, therefore in this paper we wish to present a classical review of current knowledge of tooth development, fitting together data from a large number of recent research papers to draw general conclusions about tooth development.

Fig. 1

Molar tooth development schematic. (A) At E12.5 an obvious invagination of the dental epithelium is visible. Light pink (epithelium). Dark pink (mesenchyme). (B) By E13.5 the invagination has formed a bud and the underlying neural crest-derived mesenchyme starts to condense (brown spots). (C) By E14.5 the epithelium starts to fold, forming the cap stage tooth germ, with the primary enamel knot (green) visible as a bulge in the dental epithelium, surrounded by the condensing mesenchyme (brown). (D) By E16.5 the epithelium has extended further into the mesenchyme, forming a bell. The inner enamel epithelium (IEE) encloses the dental papilla (blue), and the dental follicle (purple) surrounds the outer dental epithelium. The primary enamel knot has disappeared, to be replaced by the secondary enamel knots (light blue). (E) In the newborn mouse (E20) the adjacent ameloblasts and odontoblasts have differentiated and start producing enamel and dentin. Purple (Enamel). White (Dentin). Grey (Odontoblasts). Green (Ameloblasts).

Fig. 2

Division of the mandible mesenchyme into modules by the nested expression of transcription factors. (A) Schematic of E9.5–10.0 mouse mandible, showing areas of mesenchymal gene expression. For simplicity, expression in the epithelium is not shown. (B) Schematic of E13.5 mouse mandible (flat mounted). Arrows link the earlier expression patterns to the developing structures: teeth (green), salivary glands (submandibular/sublingual) (purple), Meckel's cartilage (orange), and the developing middle ear (red).

Fig. 3

Loss of gene expression disrupts different regions of the developing mandible. (A) Schematic showing wild-type expression of transcription factors at E10 in the mouse mandible and the structures that form from these regions at E16.5. *Position of presumptive tooth development. At E16.5 the bones of the mandible have formed around Meckel's cartilage and the middle ear is distinct at the end of Meckel's cartilage (malleus, incus and tympanic ring). Bones are shown in red. (B–E) Mutant phenotypes. Area in white indicates region affected by loss of gene. (B) Pitx1knockout. Loss of Pitx1 leads to a reduction of the cusps in the mandibular molars. The proximal mandible and tympanic ring of the middle ear are also affected. (C) Hand1/2 double knockout. Loss of Hand1/2 expression in the midline leads to loss of the midline structures, such as the rostral symphysis, and fusion of the incisors. (D) Satb2 knockout. Loss of Satb2 leads to loss of intermediate regions of the mandible, resulting in loss of the incisors. (E) Ectodinknockout. Additional teeth develop in the diastema region (+). The forming teeth develop a shape analogous to that of a premolar.

Fig. 4

Changing molar tooth number. The size of the molar field affects the number of teeth that form. (A) A small molar field at E13.5, as generated by recombination or in an Edamutant, leads to the formation of a reduced number of teeth. (B) A wild-type molar field at E13.5. Inhibitory signals from the intermolar region lead to the formation of three molars of diminishing size. (C) If the anterior part of the molar field is cut off at E13.5, the posterior part is released from inhibition by M1 and up to four molars can form, with M2 reaching the normal size of M1. M2 and M3 when isolated from M1 have an accelerated initiation compared to that of whole cultured explants or in vivo (compare to B). (D) If the molar field is large in size, as after recombination with large numbers of mesenchymal cells, four molars can form, with M2 reaching the normal size of M1. Downward arrows indicate development of the molar field from E13.5 to formation of distinct teeth.

Fig. 5

Cross-section of a mineralizing tooth. (A) Incisor P21. Ameloblasts are columnar epithelium polarized cells secreting to the extracellular matrix and forming the enamel. The enamel lies next to the dentin secreted by the underlying odontoblast layer of mesenchyme-derived cells.

Fig. 6

Mouse incisor cervical loops: regulation of ameloblast differentiation. (A) Sagittal section of a P0 mouse incisor showing the cervical loops, stained with Haematoxylin and Eosin. Black arrowhead points to the labial cervical loop and blue arrowhead to the lingual cervical loop. These correspond to the same regions arrowed in (B). (B) Schematic representation of the regulation of ameloblast differentiation in the incisor leading to the asymmetric deposition of enamel. Follistatin (green), induced by activin (brown stars), is expressed mainly in the lingual cervical loop. Follistatin inhibits activin in the dental papilla. The absence of activin in this area allows for the inhibition of Fgf (red) by Bmp4 (orange stars). At the same time, Sprouty 2 and 4 (blue dots) inhibit an Fgf regulatory loop between epithelial and mesenchymally expressed Fgfs. This decreases the concentration of Fgfs, inhibiting ameloblast differentiation in the labial cervical loop.