March 15, 2009;
The physical state of fibronectin matrix differentially regulates morphogenetic movements in vivo.
This study demonstrates that proper spatiotemporal expression and the physical assembly state of fibronectin
(FN) matrix play key roles in the regulation of morphogenetic cell movements in vivo. We examine the progressive assembly and 3D fibrillar organization of FN and its role in regulating cell and tissue
movements in Xenopus embryos. Expression of the 70 kD N-terminal fragment of FN blocks FN fibril assembly at gastrulation but not initial FN binding to integrins at the cell surface. We find that fibrillar FN is necessary to maintain cell polarity through oriented cell division and to promote epiboly, possibly through maintenance of tissue
-surface tension. In contrast, FN fibrils are dispensable for convergence and extension movements required for axis elongation. Closure of the migratory mesendodermal mantle was accelerated in the absence of a fibrillar matrix. Thus, the macromolecular assembly of FN matrices may constitute a general regulatory mechanism for coordination of distinct morphogenetic movements.
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Fig. 4. Blastopore closure and epiboly but not axial extension are defective in embryos expressing 70 kD FN. (A, B) Time-matched frames from movies of blastopore closure in representative sibling embryos expressing control (Con) or 70 kD FN (70 kD) constructs. Yellow arrowheads mark the approximate diameters of the blastopores. (C) A representative 70 kD FN gastrula with delayed blastopore closure initiates a putative second blastopore lip (red arrowhead). (D–F) Tailbud stage embryos; (D) uninjected (UN), (E) injected with 70 kD FN into both blastomeres at 2-cell stage, or (F) into the marginal region of two presumptive dorsal blastomeres at the 4-cell stage (DMZ: dorsal marginal zone). (G) Quantification of 70 kD FN tailbud embryo lengths normalized to the mean length of mock injected (dextran only) tailbud embryos from each batch. The horizontal lines represent the mean tailbud lengths. Numbers of all tailbud embryos analyzed are listed above each group. (H) Quantification of the mean width of the axial marker Chordin from in situ hybridizations. Error bars are ± SD (n = 9–10). (I–L) In situ hybridizations after completion of gastrulation (stg. 13) showing localization of Chordin mRNAs. Axial elongation in embryos expressing (L) 70 kD FN in the DMZ are comparable to (I) uninjected embryos. Broad expression of 70 kD FN results in two phenotypes: (J) embryos that fail to complete blastopore closure show a wide pattern of Chordin expression, and (K) embryos that complete blastopore closure express Chordin in a similar pattern to uninjected controls.
Self-assembly of fibronectin into fibrillar networks underneath dipalmitoyl phosphatidylcholine monolayers: role of lipid matrix and tensile forces.