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Fig. 9. Model of fibronectin assembly in the chick PSM. Sagittal view of PSM
and first epithelial somite, showing ectoderm dorsally and endoderm ventrally.
Based on our data, we suggest that PSM fibronectin matrix assembly occurs
primarily in the posterior two-thirds of the PSM and that the fibronectin
matrix in the anterior-most PSM is (if left unperturbed) sufficiently
extensive to support somite formation. We propose that ectoderm is the major
source of fibronectin protein for this matrix, and that only a minor
contribution comes from the PSM itself. The PSM does however express
5ß1 integrin, which is essential for the assembly process.
Although Itga5 mRNA is not detected in the anterior PSM, we
hypothesise that the 5ß1 protein remains on its surface.
Itga5 mRNA is again detected in epithelial somites, whereas
Fn1 is expressed in their posterior half, suggesting a potential role
in the stabilisation of somite clefts
(Koshida et al., 2005).
According to our model, ectoderm ablation over the posterior two-thirds of the
PSM compromises fibronectin matrix assembly and the formation of somites by
the underlying PSM. By contrast, ectoderm ablation over the anterior-most PSM,
if performed without destroying the endogenous fibronectin matrix, does not
affect somite formation.
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