MORPHOLOGY OF THE DEVELOPMENT OF THE SAGITTAL SUTURE OF MICE

Syndesmotic sutures of the skull are formed by dense connective tissue and called ''open''; they are ''closed'' by formation of a synostosis between adjacent bones. Open sutures are considered as areas of growt h and as hinges. The importance of open sutures during the period of s kull growth is reflected by pathological situations in which premature closure of the sutures occurs. As alterations of the FGF receptor hav e been reported in genetical disorders accompanied by premature suture closure (Bellus et al. 1996), the role of fibroblasts and connective tissue in the development of the sagittal suture of mice has been inve stigated by light and electron microscopy. Morphological; changes of t he sagittal suture at the following stages are reported: at embryonic day 18, days 1, 5, 9, 14, 20, 26, 28 after birth and in adult mice. Tw o skulls per stage were investigated. Early osteogenesis appeared in a thin plate, followed by a second plate underneath the first one. Both were separated by blood vessels. In general, vascularization preceded desmoid mineralization; the space around blood vessels was occupied b y non-bone-forming cells leaving cavities for the presumptive bone mar row. Mineralization of the collagen-rich osteoid at the mineralizing r im of the bone plates was accompanied by apoptoses and cell disintegra tion. Newly formed bone was immediately covered by osteoblasts forming a sheet of bone-lining cells. At day 9, the double-layered bone plate s of both sides reached the median area of the skull but were separate d by non-mineralizing, collagen-rich connective tissue. From day 14 on wards, the bone plates thickened. Bone apposition, recognizable by the formation of collagen-rich osteoid and proceeding from day 14 pp onwa rds, occurred mainly at the outer and inner surfaces of the calvariae, but neither at bone marrow surfaces nor at the medial edges of the pa rietal bones. These opposite bone faces showed fewer osteoblasts and b one-lining cells, but an increased number of fibroblasts. Tendon-Like collagen bundles connected both bone plates of the open suture of day 26 pp as well as in the adult mice, whereby synostotically closed area s alternated. Formation of an open, syndesmotic suture can, therefore, be described as a transition of bone-forming tissue into a bone-tendo n junction. The results indicate the importance of the replacement of osteoblasts by fibroblasts at the sutural front of the bone plates in order to prevent a premature suture closure.