Altered timing of the extracellular-matrix-mediated epithelial-mesenchymalinteraction that initiates mandibular skeletogenesis in three inbred strains of mice: Development, heterochrony, and evolutionary change in morphology

Subtle changes in embryonic development are a source of significant morphol ogical alterations during evolution. The mammalian mandibular skeleton, whi ch originates from the cranial neural crest, is a complex structure compris ing several components that interact late in embryogenesis to produce a sin gle functional unit. It provides a model system in which individual develop mental events at the basis of population-level evolutionary change can be i nvestigated experimentally. Inbred mouse strains exhibit obvious morphologi cal differences despite the relatively short time since their divergence fr om one another. Some of these differences can be traced to small changes in the timing of early developmental events such as the formation of the cell ular condensations that initiate skeletogenesis. This paper examines an eve n earlier event for changes in timing, the epithelial-mesenchymal interacti on(s) required to initiate chondrogenesis of Meckel's cartilage and osteoge nesis of the dentary bone. Using three inbred strains of mice (CBA, C3H and C57) we found that, within each strain, cartilage and bone are induced at the same time and by the same (mandibular) epithelium, that chondrogenesis and osteogenesis are initiated by a matrix-mediated epithelial-mesenchymal interaction, and that timing of the interactions differs among the three in bred strains. These results are discussed with respect to the possible mole cular basis of such temporal shifts in inductive interactions and how such studies can be used to shed light on heterochrony as a mechanism of evoluti onary change in morphology. (C) 2001 Wiley-Liss, Inc.