The extracellular matrix (ECM) plays crucial roles during morphogenesis but how it is assembled and patterned in vivo is poorly understood. Here, Yuki Sato, Rusty Lansford and colleagues investigate this by examining the distribution of the ECM component fibronectin (FN) in quail embryos (p. 281). They reveal that FN fibrils form pillars that span the gap between somites and the endoderm. The tissue-specific depletion of FN reveals that both the somites and endoderm provide FN that contributes to these pillars. The authors also observe filopodia-like structures that extend from the basal surface of somatic epithelial cells and are oriented along FN pillars. The formation of these filopodia influences the formation of FN pillars, while the polymerisation of FN is shown to modulate both pillar formation and filopodial elongation. Importantly, both structures are required for proper somite morphogenesis. Finally, the researchers report that blood flow in the nascent dorsal aorta (DA), which is located between the somites and endoderm, controls FN pillar distribution; disruption of DA formation, or occlusion of the DA, leads to a scattered distribution of FN pillars. Together, these findings suggest that pulsations from the DA help establish FN pillars that bridge the somite-endoderm gap and potentially aid communication between these tissues.
