Formation of the vertebrate face epigenetic and functional influences

Abstract

Broadly speaking, "epigenetics" refers to the entire series of interactions among cells and cell products which leads to morphogenesis and differentiation. Thus all cranial development is epigenetic, by definition. Among the numerous epigenetic factors influencing the vertebrate face is mechanical loading Loading seems to be particularly significant for formation and growth of skeletal tissues. As a first approximation, bone grows in the direction of tensile loading, while the shapes of growing cartilages are modulated by axial compression. One of the many sources of loading is function (defined, before birth or hatching, as muscle-controlled movements). For example, muscle activity is a source of tension for bones of attachment, and differences in muscle anatomy may thus be responsible for differences in bony anatomy. A possible instance involves a new addition to the masseter muscle in pigs and an associated unusual process of the zygomatic bone. Changes in the mammalian mandibular condyle in paralyzed fetuses provide a dramatic illustration of how function regulates cartilage growth. Such ontogenetic processes may account for species-specific variations in mandibular shape that are conventionally regarded as the direct adaptive results of natural selection. In general, mechamcal loading is a relatively late-acting epigenetic factor and affects primarily quantitative features of facial structure However, loading appears to be involved in at least one more fundamental ontogenetic event, the transformation of the primary jaw joint into the mammalian middle ear. © 1993 by the American Society of Zoologists.