Formation of the sphenomandibular ligament by Meckel's cartilage in the mouse: possible involvement of epidermal growth factor as revealed by studiesin vivo and in vitro

In mammals, the midportion of the soft tissue of Meckel's cartilage at the degenerating stage forms a ligament known as the sphenomandibular ligament. To clarify the mechanism of formation of this ligament by Meckel's cartila ge in mouse, we examined the effects of epidermal growth factor (EGF) on th e chondrocytes in terms of the proliferation and differentiation of cells a nd calcification of the matrix in vivo and in vitro. The effects of EGF wer e examined by immunohistochemical staining, with EGF-soaked beads, by elect ron microscopy, and by general histochemical analysis of proteoglycans and calcification. Analysis of labeling with bromodeoxyuridine (BrdU) and the r ate of cell growth revealed that EGF enhanced DNA synthesis acid the prolif eration of Meckel's chondrocytes. Histological findings in organ culture an d in cell culture, with and without the application of EGF-soaked beads, re vealed that EGF inhibited the differentiation of cells to chondrocytes and induced phenotypic changes in fibroblastic cells. The inhibition of alkalin e phosphatase activity that resulted from exposure to EGF was accompanied b y prolonged calcification of the matrix. Whole-mount staining revealed that subcutaneous injection of EGF enhanced the disappearance of Meckel's carti lage. Our results suggest a possible mechanism whereby the midportion of Me ckel's cartilage remains uncalcified and is rapidly transformed into the sp henomandibular ligament.