The vertebrate skull develops within a layer of mesenchyme between the embryonic brain and surface ectoderm. This mesenchymal layer gives rise to an outer skeletogenic membrane in which the skull bones form, and an inner meningeal layer from which the dura, arachnoid and pia mater layers form. The bones of the skull vault, like those of the face, are formed directly in mesenchyme (intramembranous ossification), in contrast to the cartilaginous origin of the bones of the skull base and occipital region (endochondral ossification).
The sutures between the membrane bones ofthe skull vault are growth centres in which proliferating osteogenic stem cells provide the source material for incremental growth of the calvarial bones. Osteogenic stem cells are located at the periphery of each membrane bone and also, in smaller numbers, on their inner and outer surfaces. Continued growth of the skull vault depends on maintenance of a balance between proliferation of the osteogenic stem cells and their differentiation to form new bone.
The developmental origin of the coronal suture is of particular interest in understanding mechanisms of skull growth. This bilateral, vertical suture is formed between the frontal and parietal bones; it is responsible for most of the growth of the skull in the fronto-occipital (front-to-back) plane. Interspecific grafting experiments in avian embryos suggested that both frontal and parietal bones have the same origin, either from mesoderm (Noden et al 1988) or neural crest (Couly et al 1993). However, investigation of a transgenic mouse with a permanent neural crest cell lineage marker suggests that the coronal suture is formed at the interface of a neural crest-derived frontal bone and a mesoderm-derived parietal bone (X. Jiang, H. Sucov & G. Morriss-Kay, unpublished work). This juxtaposition of tissues of different origins is likely to be functionally significant.
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