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 (PubMed:21826198, PubMed:19595790). Promotes endothelial cell survival, migration and differentiation into network structures in an AKT-dependent manner. Also promotes survival of cardiac myocytes (PubMed:20054002). Initiates various signaling cascades by activating different receptors on the cell surface such as DIP2A, TLR4 or BMP receptors (By similarity). Homodimer (PubMed:31351024). Interacts with SCN10A (By similarity). Interacts with DIP2A; DIP2A may act as a cell surface receptor for FSTL1 (PubMed:20054002). Interacts with BMP4 (By similarity). Interacts with CD14; this interaction promotes TL4-mediated signaling cascade (By similarity). During central nervous system development, strongly expressed in the telencephalon, diencephalon, brainstem, limbic system and spinal cord (PubMed:19595790). Widely expressed in all organs (PubMed:21826198). Deletion mice die at birth from respiratory distress and show multiple defects in lung development. In addition, skeletal development is strongly impaired.