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Fibronectin (FN) is an important extracellular matrix (ECM) component that supports cell migration and deformation, as well as integrates cells into functional ensembles of tissues.Here, we demonstrate that FN fibrils are deposited as pillars between the somites and endoderm.Time-lapse imaging analysis of fluorescent-labeled embryos revealed that the basal surface of somitic epithelial cells have long, protruding filopodia-like pseudopods that penetrate the basement membrane to reach the FN pillars.Inhibition of filopodia formation by Ena/VASP depletion, and loss-of-function experiments of Integrin β1 and Talin1 resulted in disruption of the FN pillars,indicating that patterned deposition of FN occur in a reciprocally dependent manner by means of cytoskeletal molecules involved in cell-ECM interaction.Tissue-specific fluorescent labeling and knock-down experiments of FN revealed that the endoderm is responsible for the development of FN pillars.Basal filopodia in the somitic cells play special roles in the patterned deposition of endoderm-derived FN.The FN pillars physically bridge the somites and endoderm as well as mechanically support elongation of the somitic epithelial cells through filopodia-mediated fibrillar adhesions.We also examined the involvement of mechanical stretch in maintenance of the FN pillars by focusing on pulsatile constriction of the dorsal aortae, which are sandwiched between the somites and endoderm.FN pillars became scrambled after inhibition of vascular constriction, indicating that the FN pillars are maintained by the hemodynamic-associated mechanical stretch of adjacent tissues.We present in vivo evidence of site-specific FN pillar deposition and discuss a model for understanding a mechanism of tissue integration and adaptation to mechanical stress.