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pp.93-102
Meckel’s cartilage is one of the earliest structure to appear in a mandible derived from the first branchial arch and serves as the primordium for the formation of mandible, malleus, incus, and sphenomandibular ligament. However, its direct role and the mechanism in mandibular development are not well elucidated. To address this issue, we observed morphological and histological changes and gene expression patterns in the Meckel's cartilage of a developng mouse, from E13.5 to E18.5 embryos, using skeletal preparation samples and routinely prepared slide sections for light rnicro scopic observation in various sectional planes. The following methods were performed: H&E staining for general histo logical observation, Von Kossa staining for detection of rnineralization, TRAP actiγity staining for localization of osteoclastic cells, immunohistochernistry for MMP- 1 and - 9 for evaluation of enzymatic activity of osteoclastic cells, and in situ hybridization for detection of collagen type 1. II, and X mRNA expression, respectively. At E13.5 Meckel's cartilage appeared as a V- shaped rod fused at the rnidline and thin rnineralized mandibular buccal plate was formed lateral to, and at some distance from, Meckel's cartilage in an intramembranous ossification mode With the progression of tooth development, the Meckel’s in cartilage adjacent incisors revealed hypertrophic chondrocyte differentiation with rnineralization of the chondroid matrix The Meckel’s cartilage was replaced with bone by - osteoclasts, showing strong immunoreactivity for MMP-1 and - 9 from E16.5. With time, this bony replacement of Meckel’s cartilage in an endochondral ossification mode was extended up to the rnid- portion of the molar sockets till E18 5 The bony replacement of rnineralized hypertrophic chondrocyte zone expressing X collagen mRNA contributed to the formation of thick mandibular lingual plate. These findings suggest that mandibular formation and development is closely related with not only Meckel's cartilage, but also with the developing tooth, and that certain influence from the developing tooth may play a role in deterrnining the fate of Meckel’s cartilage during mandibular development.
