Molecular patterning of the mammalian dentition

Abstract

Four conserved signaling pathways, including the bone morphogenetic proteins (Bmp), fibroblast growth factors (Fgf), sonic hedgehog (Shh), and wingless-related (Wnt) pathways, are each repeatedly used throughout tooth development. Inactivation of any of these resulted in early tooth developmental arrest in mice. The mutations identified thus far in human patients with tooth agenesis also affect these pathways. Recent studies show that these signaling pathways interact through positive and negative feedback loops to regulate not only morphogenesis of individual teeth but also tooth number, shape, and spatial pattern. Increased activity of each of the Fgf, Shh, and canonical Wnt signaling pathways revitalizes development of the physiologically arrested mouse diastemal tooth germs whereas constitutive activation of canonical Wnt signaling in the dental epithelium is able to induce supernumerary tooth formation even in the absence of Msx1 and Pax9, two transcription factors required for normal tooth development beyond the early bud stage. Bmp4 and Msx1 act in a positive feedback loop to drive sequential tooth formation whereas the Osr2 transcription factor restricts Msx1-mediated expansion of the mesenchymal odontogenic field along both the buccolingual and anteroposterior axes to pattern mouse molar teeth in a single row. Moreover, the ectodermal-specific ectodysplasin (EDA) signaling pathway controls tooth number and tooth shape through regulation of Fgf20 expression in the dental epithelium, whereas Shh suppresses Wnt signaling through a negative feedback loop to regulate spatial patterning of teeth. In this article, we attempt to integrate these exciting findings in the understanding of the molecular networks regulating tooth development and patterning.

Keywords: Dentition; Msx1; Osr2; Revitalization; Signaling network; Tooth development.

Figure 1

Histology of first molar tooth development in mice. (A – E) Selected coronal sections of developing first molar tooth germs at dental lamina (E11.5, A), bud (E13.5, B), cap (E14.5, C), early bell (E16.5, D), and late bell (P0, E) stages are shown. ab, ameloblast; de, dental epithelium, dm, dental mesenchyme, dp, dental pulp; ob, odontoblast; pek, primary enamel knot; sek, secondary enamel knot.

Figure 2

The Bmp, Fgf, Shh, Wnt, and Eda signaling pathways form an integrated network of positive and negative feedback loops to regulate early tooth development through the bud-to-cap transition. The tooth bud epithelium is represented in light cinnamon color whereas the primary enamel knot (not drawn to scale) is represented in blue color. The mesenchyme surround the tooth bud is not colored. Black arrows indicate positive input and red indicates repressive input. A question mark is placed next to Dkk1 because it has not been experimentally confirmed as the mediator of repression of Wnt signaling by Shh. The mesenchymal signal downstream of Shh is represented with a question mark because it has not been identified and it is not clear how increased Shh signaling in the tooth mesenchyme stimulates primary enamel knot function.

Figure 3

Genetic interactions between Bmp4, Msx1, and Osr2 regulate sequential molar development in mice. (A–J) Skeleton preparations showing the maxillary (A–E) and mandibular molar (F–J) regions of Bmp4^f/+;Wnt1Cre (A, F), Bmp4^f/f;Wnt1Cre (B, G), Msx1^+/− Bmp4^f/f;Wnt1Cre (C, H), Osr2^+/−Bmp4^f/f;Wnt1Cre (D, I) and Msx1^+/−OSR2^+/−Bmp4^f/f;Wnt1Cre (E, J) mice at 21 days after birth. M1, M2, and M3 indicate first, second, and third molars, respectively.

Figure 4

Differential gene expression along the buccolingual axis of the developing mouse tooth bud mesenchyme at E13.5. (A – E) Spatial patterns of Osr2 (A), Bmp4 (B), Msx1 (C), Dkk2 (D), and Sfrp2 (E) mRNAs are detected by in situ hybridization of coronal sections E13.5 mouse first molar tooth germs. The mRNA signals are shown in blue color. Lingual side is to the left for all panels. Red dashed lines mark the epithelial boundary between the tooth bud epithelium and mesenchyme. Each panel shows both the maxillary (top) and mandibular (bottom) first molar tooth germs.

Figure 5

Schematic diagram depicting antagonistic interactions between Msx1 and Osr2 in patterning the molar tooth developmental field. Msx1 and Bmp4 act in a positive feedback loop in the tooth mesenchyme. Osr2 restricts Msx1-mediated activation of Bmp4 and other mesenchymal odontogenic signals (represented by X) to the buccal side of the tooth mesenchyme. Expression of Dkk2 and Sfrp2 are negatively regulated by Msx1 and restricted to the lingual side, possibly resulting from Osr2-mediated repression of Msx1 function. Epi, epithelium; Mes, mesenchyme.