Abstract
The vertebrate skull varies widely in shape, accommodating diverse strategies of feeding and predation. The braincase is composed of several flat bones that meet at flexible joints called sutures. Nearly all vertebrates have a prominent ‘coronal’ suture that separates the front and back of the skull. This suture can develop entirely within mesoderm-derived tissue, neural crest-derived tissue, or at the boundary of the two. Recent paleontological findings and genetic insights in non-mammalian model organisms serve to revise fundamental knowledge on the development and evolution of this suture. Growing evidence supports a decoupling of the germ layer origins of the mesenchyme that forms the calvarial bones from inductive signaling that establishes discrete bone centers. Changes in these relationships facilitate skull evolution and may create susceptibility to disease. These concepts provide a general framework for approaching issues of homology in cases where germ layer origins have shifted during evolution.
Highlights
At the beginning of skull vault development, mesenchymal cells of either neural crest or mesoderm origin condense into nascent bone fields
Embryonic calvarial bones grow through addition of new osteoblasts at their leading edges, until bone covers the majority of the roof of the head
The general pattern of the major sutures is largely conserved across vertebrates, gain or loss of specific sutures has been linked with evolutionary changes in skull morphology
Summary
At the beginning of skull vault development, mesenchymal cells of either neural crest or mesoderm origin condense into nascent bone fields. Embryological analysis of germ layer contributions to skull bone boundaries Whereas the pineal foramen and to a lesser extent the cartilage epiphyseal bar are potentially useful landmarks for inferring the relationship of the neural crest-mesoderm boundary to the coronal suture, lineage tracing in several model organisms provides more direct evidence (Figure 3).
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