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Periodontology 2000, Vol. 13, 1997, 9-19 Printed in Denmark All rights reserved

Copyright 0 Munksgaard 1997

PERIODONTOLOGY 2000
ISSN 0906-6713

The development of the periodontiurn - a largely ectornesenchpdy derived unit
A. RICHARD TENCATE
The periodontium is simply defined as the tissues supporting and investing the tooth. The tissues supporting the tooth are developmentally derivedfrom the dental follicle proper, whereas those investing the tooth, that is the gingiva, are an adaptation of the oral mucosa. Apart from the gingival epithelium, which includes the dentogingival junction, and the epithelial cell rests of Malassez formed from the fragmented root sheath, the functioning periodontium is almost entirely derived from ectomesenchyme, embryonic connective tissue derivedfrom neuroectoderm, if neural and vascular elements are ignored. Epithelial tissue is, however, involved in directing ectomesenchyme to form some of the connective tissue components of the tooth. To explain how this takes place, some basic tenets of development must be described. munocytochemical techniques, which enable precise identification of gene expression and localization of varioussignaling molecules, a much clearer understanding has been gained of the mechanisms involved in induction, competence and differentiation (44). Thus regulatory homeobox genes are now recognized (31). These genes are associated with patterning and generate transcription proteins that bind to downstream genes to regulate their expression. One set of homeobox genes (the Hox genes) are associated withanterior-posterior patterning in embryos and are highly conserved, that is, they occur in the most primitive organisms through to humans. This set of genes is, however absent from the most anterior part of the developing mammalian embryo, where they are replaced by a different set of homeobox genes (which include Msx and Dlx genes) developed later in evolution that are associated with the developmentof the head (32). Gene expression is also regulated by two groups of regulatory molecules: growth factors and the steroid/thyroid/retinoic acid superfamily. Growth factors are polypeptides and belong to a number of families (Table 1). Three such families, transforming growth factor, nerve growth factor and platelet-derived growth factor are similar in structure, indicating their derivation from thesame ancestral gene. Furthermore, many growth factors are homologous to growth factors involved in signaling between tissue layers in the developing Drosophila embryo, indicating that signaling mechanisms during embryogenesis are also highly conserved. For growth factors to exert an effect, cells must express membrane receptors to capture them (making the cell competent) and, once captured, thereceptor must also be able to interact with both membrane and cytoplasmic bound compounds to bring about, after a complex set of intracellular events,

Induction, competence and differentiation
Induction, competence and differentiation are important concepts in embryology. All the cells of an individual stem from the zygote and somehow differentiate into populations that assume particularfunctions, shapes and rates of turnover. Such populations are compartmentalized; successive generations of cells in a compartment may remain constant or differentiate, that is, change their characteristics and establish a new population of cells. The process that initiates differentiation is termed induction and an inducer is the agent that persuades cells to differentiate. Furthermore, each compartmentof cells must be competent to respond to the induction process. With the advent of recombinant DNA and im-

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Table 1. Growth factor familiesa
lhmsfonnfng growth factors-beta transforming growth factors-beta 1-5 bone morphogenetic proteins 2-8 growth and differentiation factors 1-7 Epidermal growth factors epidermal growth factor transforming growth factor-alpha amphiregulin...