The role of macrophages in the disappearance of Meckel's cartilage during mandibular development in mice.

Meckel's cartilage is a supporting tissue in the embryonic mandible that disappears during development; however, the precise mechanisms of this disappearance process are still undetermined. In this study, we observed morphological changes of Meckel's cartilage with development and analyzed the factors which might be related to this process. Meckel's cartilage of ICR strain mice from 14 to 19 days gestation (E14-19) were used in this study. Histological and immunohistochemical studies indicated the decrease in the amount of sulfated glycoconjugates and the localization of type I collagen in the Meckel's cartilage matrix during development. Chondrocytes also expressed high acid phosphatase activities at these stages. An organ culture study indicated that Meckel's cartilage at E17 disappeared during the cultivation period, while the cartilage at E14 did not disappear. Massive penetration of macrophages into the perichondrium was detected at E16. RT-PCR analysis of Meckel's cartilage indicated the expression of interleukin-1β, type I collagen, MMP-9 at E17, but not at E14. MIP-1α, the candidate molecule for macrophage chemoattractant factor, was expressed at E14. These results indicated the dynamic matrix changes of Meckel's cartilage during development and suggested that the functional changes of chondrocytes in synthesis of type I collagen might be induced by interleukin-1β secreted by the penetrating macrophages.