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.