The four skeletal components of the vertebrate skull are the cartilaginous neurocranium, cartilaginous viscerocranium, dermal skull roof and the sclerotomal occipital region [1
]. Osteoblasts can be produced from mesenchymal stem cells by two distinct processes during vertebrate embryogenesis: intramembraneous and endochondral ossification [2
]. The dermal skull roof, which is evolutionarily derived from the protective dermal plates of early jawless fishes, is formed from the more ancient process of intramembraneous ossification in which mesenchymal progenitors condense and subsequently differentiate directly into osteoblasts while endochondral ossification, which principally plays a role in the axial skeleton, occurs via the formation of a cartilaginous intermediate [4
]. Importantly, the five principle bones of the mammalian skull vault which includes the paired frontal bones, the paired parietal bones and the unpaired interparietal bones arise from two distinct embryonic origins; neural crest cells which are a mesenchymal cell type from the neural ectoderm unique to vertebrates [5
], and the paraxial mesoderm. Historically there has been considerable debate regarding the disparate embryonic origin of calvarial bones, specifically the frontal and parietal bones. Early studies extrapolated data from avian models because of difficulties at the time with cell and tissue lineage studies in mammalian embryos and drew different conclusions as to their embryonic origins [6
Quail-chick chimera studies performed by Noden et al.
, supported prior observations by Le Lievre that the cranial vault had a dual origin consisting of tissue derived from neural crest cells and mesoderm [6
]. Using the same model however Couly et al.
reported contradictory findings that the skeletal tissue of the cranial vault consisted solely of neural crest cells [8
]. These quail-chick chimera studies were, however, blighted by several constraints including the fact that the skull vault bones had only just begun to mineralize at the time the experiments were concluded at E14, and because of poor delineation of the calvarial sutures at this stage, the small size of the avian parietal bones, and the absence of postparietal bones. These deficiencies therefore, as Moriss-Kay noted, may have contributed to the disparities in the interpretation of the data gained through the study of this model, and the conclusions drawn thereafter [1
]. Extrapolations form avian data must also be drawn with some caution given the early evolutionary divergence of birds and mammals from reptilian lines, from which they arose, and the clear anatomical differences in the skull roof patterns in these groups [1
]. The real paradigm shift in our understanding of the embryonic origin of the mammalian skull vault dawned with the arrival of transgenic mouse technology. Jiang et al
., using mice with the Wnt1-Cre
transgene, which is expressed solely in neural crest cells, with the conditional LacZ
reporter, R26R, which is only expressed when activated by Cre, showed conclusively that the frontal and squamosal bones are neural crest derived, whereas the parietal bones are of mesodermal origin [9
]. More recently, Yoshida et al.
elegantly verified the same disparate dual embryonic origin of the frontal and parietal bones, by conducting the reciprocal study using the Mesp1-Cre
transgene combined with R26R, which specifically and permanently labelled mesodermal cells [10
]. In this way, they were able to validate their previous work, which relied solely on Wnt-1
, a permanent cell marker for neural crest cells [9
]. Given that the mixed developmental origin of the mammalian skull vault had therefore been established, we wanted to investigate the impact that this unique developmental and evolutionary history had on the molecular and genetic control of cell and tissue interactions and in particular, the effect on calvarial healing in neural crest derived frontal and paraxial mesoderm derived parietal bones. Encouraged by our early studies using both in vitro
osteogenic differentiation assays and a mouse calvarial defect model, which clearly demonstrated the superior osteogenic potential and healing capacity of neural crest-derived frontal bones and their derived osteoblasts [11
] (), we sought to use this calvarial model to elucidate, in a step-wise fashion, the molecular signaling pathways which confer superior osteogenic potential and regenerative capacity on frontal bones. We will describe how this approach has provided novel insights into the multiple signaling pathways that determine osteogenic potential and how they may communicate with each other to coordinate this process. Moreover, this work will be placed into the wider context of our current understanding of these signaling pathways as the scientific community moves closer to a deeper understanding of a core signaling network which orchestrates the osteoblast lineage.
Figure 1 (A) cartoon depicting neural crest and paraxial mesoderm origin of the cranial; (B) alizarin red staining of frontal (FOb) and parietal (POb) bone-derived osteoblast cells at osteogenic differentiation day 21 shows striking differences between FOb and (more ...)