[Heredity and environment in the genesis, epigenesis and evolution of the orofacial area]

Genetic and evolutionary aspects of the dento-facial complex are described according to the concept of heredity as a force of preservation for the human species, though ruled by natural selection and by mutational changes. Ontogenesis and growth of the oral structures reflect mutual regulations between genes and environmental factors. Therefore, the authors carry out an analysis of environmental and constitutional factors affecting dental and facial development. Three levels are identified: individual morphogenesis and growth; actual heredity of parietal dento-facial traits; role and meaning of teeth, jaws and temporo-mandibular joint during evolution leading to Homo sapiens sapiens. As far as individual development is concerned, genes provide only the input for initial cell proliferation and/or differentiation. Further on, growth and morphogenesis of oro-facial structures takes place by means of cell-to-cell and cell-to-substrate interactions. The final, structural result is due to reciprocal interactions among developing structures: muscles modify bone, teeth alignment influences bony bases alignment, ecc. Each structure is genetically determined (teeth, bones, muscles) and each structure carries out epigenetic regulations on other structures: no structure is secluded from biological function of the organism. The inheritance of oro-facial "traits", then, is hardly valuable. First of all, "traits" actually do not exist: they are the product of complex, multifactorial biological mechanisms. Moreover, facial characteristics are affected by polygenic regulation, each gene often showing pleiotropic effects. It has been calculated that 85-90% of facial dimensions in due to epigenetic modifications. Evolution and phylogenesis give evidence about the deep environmental influence on oro-facial morphology. Apart from certain molecular features regarding tooth structure (several dental proteins are today the same as those of the first Vertebrates who lived 500 millions of years ago), tooth number, form, and size depend upon environmental factors. Therefore, dentistry has the important task to preserve fundamental characteristics of dentition, and to intercept environmental pathogenetic factors, as dentition represents a precious instrument for the survival of the human species.