UNSW Embryology

Integumentary Development Stage 22 Embryo

© Dr Mark Hill (2008)

Acknowledgements

Introduction

An overview of the Stage 22 Embryo integumentary system.

Stage 22 Embryo (Human)
A1	A2	A3	A4	A5	A6	A7
B1	B2	B3	B4	B5	B6	B7
C1	C2	C3	C4	C5	C6	C7
D1	D2	D3	D4	D5	D6	D7
E1	E2	E3	E4	E5	E6	E7
F1	F2	F3	F4	F5	F6	F7
G1	G2	G3	G4	G5	G6	G7

Page Links: Introduction | Trachea | Lungs | Embryo Sections | Glossary

Introduction

• week 6 odontogenesis begins
• tooth bud
• 2 sets of teeth
• deciduous and permanent
  • 20 deciduous teeth
  • 32 permanent teeth
• differential rates of growth
• shed at different times over 20 year period
• ectoderm, mesoderm and neural crest mesenchyme contribute
• inductive influence of neural crest with overlying ectoderm
• see attached summary

Tooth Development

• odontoblasts
  • form predentin
  • calcifies to form dentin
• ameloblasts
  • produce enamal
  • tooth growth occurs in ossifying jaws
• periodontal ligament holds tooth in bone socket

Teeth Postnatal

• Deciduous teeth
  • 6-24 months erupt from gums
  • by pushing toward surface
  • year 2 all deciduous teeth present
• Permanent teeth
  • year 6 until early adult
  • tooth bud lie in gums beneath deciduous teeth
  • osteoclasts resorbe deciduous teeth roots
  • growth affects face shape

Molecular Tooth Development 1

Bone Morphogenic Protein (BMP) / Fibroblast Growth Factor (FGF)

Growth factors in the BMP- and FGF-families are expressed in dental epithelium during initiation of tooth development and their effects on the underlying mesenchyme mimic those of the epithelium. They upregulate the expression of many genes, including the homeobox-containing Msx-1 and Msx-2, and stimulate cell proliferation suggesting that they may act as epithelial signals transmitting epithelial-mesenchymal interactions. During subsequent morphogenesis, when the characteristic shapes of individual teeth develop as a result from folding of the dental epithelium, several signal molecules including Sonic hedgehog, Bmps-2, 4, 7 and Fgf-4 are expressed specifically in restricted and transient epithelial cell clusters, called enamel knots.

Text: Irma Thesleff and Carin Sahlberg Seminars in Cell & Developmental Biology, v 7, n 2, April, 1996, p185-193

Molecular Tooth Development 2

Epithelial-mesenchymal interactions

govern the development of epidermal organs such as teeth. During the early stages of tooth development, a local ectodermal thickening which expresses several signaling molecules appears. It is believed that these in turn signal to the underlying mesenchyme triggering mesenchymal condensation and tooth development. For example, epithelially expressed Bmp4 induces Msx1 and Lef1 as well as itself in the underlying mesenchyme. In this paper we have investigated the role of four epithelial signaling molecules, Bmp2, Shh, Wnt10a, and Wnt10b, in the early inductive cascades that govern tooth development. We show that all four genes are specifically expressed in the epithelium between E11.0 and E12.0 when tooth morphogenesis is first apparent. Although Shh, Bmp2, and Wnt10b have similar, if not identical, expression patterns, each signal has a distinct molecular action on the jaw mesenchyme. Whereas Shh and Wnt10b can induce general Hedgehog and Wnt targets, Ptc and Gli for Shh and Lef1 for Wnt10b, only Bmp2 is able to induce tooth-specific expression of Msx1. Thus, there are distinct targets for all three pathways. Interestingly, both Bmp and Wnt signaling activate Lef1, making it a candidate for integrating the two distinct signaling pathways.

Text: Hélène R. Dassule and Andrew P. McMahon Developmental Biology, v 202, n 2, October 15, 1998, p215-227

Molecular Tooth Development 3

Delta/Notch

The expression pattern of Delta 1 in ameloblasts and odontoblasts is complementary to Notch1, Notch2, and Notch3 expression in adjacent epithelial and mesenchymal cells. Notch1 and Notch2 are upregulated in explants of dental mesenchyme adjacent to implanted cells expressing Delta1, suggesting that feedback regulation by Delta-Notch signaling ensures the spatial segregation of Notch receptors and ligands. TGF1 and BMPs induce Delta1 expression in dental mesenchyme explants at the stage at which Delta1 is upregulated in vivo, but not at earlier stages. In contrast to the Notch family receptors and their ligand Jagged1, expression of Delta1 in the tooth germ is not affected by epithelial-mesenchymal interactions, showing that the Notch receptors and their two ligands Jagged1 and Delta1 are subject to different regulations.

Text: Mitsiadis etal Developmental Biology,v 204, n 2, December 15, 1998, p420-431

Glossary of Terms

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