Secreted glycoprotein that is involved in various physiological processes, such as angiogenesis, regulation of the immune response, cell proliferation and differentiation (By similarity). Plays a role in the development of the central nervous system, skeletal system, lungs, and ureter. Promotes endothelial cell survival, migration and differentiation into network structures in an AKT-dependent manner. Also promotes survival of cardiac myocytes (By similarity). Initiates various signaling cascades by activating different receptors on the cell surface such as DIP2A, TLR4 or BMP receptors (By similarity). Homodimer (By similarity). Interacts with SCN10A (By similarity). Interacts with DIP2A; DIP2A may act as a cell surface receptor for FSTL1. Interacts with BMP4. Interacts with CD14; this interaction promotes TL4-mediated signaling cascade (By similarity).