Fibronectins bind cell surfaces and various compounds including collagen, fibrin, heparin, DNA, and actin. Fibronectins are involved in cell adhesion, cell motility, opsonization, wound healing, and maintenance of cell shape (By similarity). Involved in osteoblast compaction through the fibronectin fibrillogenesis cell-mediated matrix assembly process, essential for osteoblast mineralization. Participates in the regulation of type I collagen deposition by osteoblasts (By similarity). Dimers or multimers of alternatively spliced variants, connected by 2 disulfide bonds near the carboxyl ends. A number of isoforms are produced. The diversity of isoforms depends on the V region and either of the two extra domain which can be either included or excluded (partially or completely for the V region). In early Xenopus embryo, cellular forms of fibronectin predominate which include both extra domains. In fibronectin of embryonic and adult liver the connecting strand 3 can be either completely excluded or included. Forms covalent cross-links mediated by a transglutaminase, such as F13A or TGM2, between a glutamine and the epsilon-amino group of a lysine residue, forming homopolymers and heteropolymers (e.g. fibrinogen-fibronectin, collagen-fibronectin heteropolymers).