Mecke!'s car t ilage is one of the ea rliest structu re to appear in a mandible derived from the lï rst branchi a l a rch and serves as the primorclium I"or the formation 01‘ mandible‘ mall eus. incus. and sphenomandibular li gament However. its direct role a nd the mechanism in mandibular clevelopment a re not well elucidated. 1'0 address t his Issue‘ we observed morphol ogical and histological changes and gene expression patterns in the Mecke!'s cart ilage 01" a cleveloping mouse. I"rom E13.5 to E18.5 embryos. using skeletal preparation samples a ncl routinely prepa red s lide secti ons for light mi croscopic observation in various sectional planes. The following methods were per |‘ ormecl : H&E staining I"or general hi st이 og i cal observation ‘ Von Kossafor detection of minerali zation. TRAP activ ity staining for locali zaLion 01’ osteoclastic cell s. immunohistochemistry for !Iα@-1 a ncl -9 forevaluati on of enzy matic activity 01" osteoclasLic cell s. a ncl in situ hybricli zation for detection of collagen type 1. Il. ancl X mRNA ex presslon‘ respecLively. AL E1 3.5 Mec kel's cartilage appeared as a V-shaped rod fused a t the micl line and thin minera li zed ma ndibular buccal plaLe was I"ormed lateral to. and at some clistance from. Meckel’s carti lage in an intramembranous ossi lïcation mocle. WiLh the progression of tooth development. t he Meckel’s in carti lage adjacent incisors revealecl hyperLrophi c chonclrocyte di ff"er entiation with minerali zation of the chondroid matrix. The Meckel’s car Li lage was replacecl with bone by o~ L eoc l asLs . showing strong immunoreact ivity for MMP- l ancl -9 from E16 5 Wi Lh ti me‘ Lhis bony replacement of Meckel's cartilage in an endochondral ossification mode was ex Lenclecl up Lo the mid-porLion of Lhe molar sockets til l EI8.5. The bony replacement of minera li zed hypertrophic chondrocyte zone expressing X collagen mHNA conLri buted to the formation of thick mandibular lingual plate . 1'hese f"i ndings suggesL LhaL mandibular formalion and development is closely relatecl with not only Mecke!'s carLi lage. buL also wiLh Lhc developing LooLh. and thaL C'erLai n in f"l uence from the developing tooth may play a role in detcrmin in g Lhe faLc of Meckel’s ca rLi lage cluring ma ndi bular development.

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.