Morphogenesis of the Mouse Coronal Suture

The morphogenesis and maturation of the coronal suture in C57Bl-strain mice was studied histologically using light microscopy from day 15 postconception through day 71 postpartum. Early differentiation of the osteoid precursors proceeds centrally within the 7- to 8-cell-thick fibrous ectomeninx beginning in the regions of the ossification centers and extends peripherally toward the future sutural margin. Differentiation into osteogenic cellular elements is marked morphologically by an increased nuclear:cytoplasmic ratio, dark-staining nucleoli and a granular chromatin pattern. This zone of cells, designated the osteogenic induction front (OIF), extends in advance of any identifiable accumulation of osteoid matrix or biomineralization. As the OIFs of the parietal and frontal bones approach, but prior to any physical overlap, they lie in different planes dorsoventrally; the parietal OIF located dorsal to the frontal OIF, a pattern maintained throughout subsequent sutural morphogenesis. Upon physical overlapping of the parietal and frontal OIFs, the fibrous capsule thickens dorsoventrally assuming a 'fusiform' configuration. This morphological event, referred to as the 'fusiform blastema', has been suggested to presage sutural location and determine the relative vertical position of the bony plates comprising the suture. Our results suggest that this is actually a secondary event, the structure of the suture having already been established with the relative physical overlapping of the respective OIFs.