Embryonic development of the coronal suture in man and rat

Histological examination of skulls of 3- to 7-month-old human fetuses reveals in the fourth month of fetal life the development of a morphologically distinctive tissue in the area between the frontal and parietal bones. In this particular area, the pericranium tends to bulge outward while the dura mater shows the same in an inward direction. Consequently, a spindle-shaped blastema is enclosed. This spindle-like structure is only temporarily present; by about the sixth month of fetal life it has disappeared. Secondly, it was observed that prior to the development of this blastema mainly undifferentiated mesenchymal cells and only few fibroblasts are present in the area between the approaching ossification centres. In the full-grown blastema both types of cells are present in about equal numbers. However, once the blastema has been formed, fibroblasts predominate over mesenchymal cells. This tissue structure is also clearly distinguishable in the developing fetal rat skull. In this species, however, the spindle-shaped blastema appears at a later phase of fetal development, i.e. at about the 20th day. In both species, man and rat, the temporary presence of this blastema is not confined to the presumptive coronal suture only, but is demonstrable as well in other cranial sutures.