1971 BAENZIGER and co-workers’ identified a 190,000-dalton polypeptide that was selectively released from human platelets as a result of exposure to thrombin. The next ten years brought a gradual accumulation of information, mostly on the structure and subcellular location of this protein, designated thrombospondin, that has paved the way for a rapid expansion of information during the past four years. Recent advances include (1) the detection of thrombospondin in several cell types;(2) the concept of thrombospondin as a multifunctional protein with binding sites for heparin, calcium, fibrinogen, fibronectin, type V collagen, plasminogen, histidine-rich glycoprotein, and sulfated glycolipids;(3) the identification of potential functions for thrombospondin in cell physiology; and (4) a concept of native calcium-dependent structure. Thrombospondin can become associated with cell surfaces and has been shown to have lectin-like activity in hemagglutination assays. Collectively, these data have led to a concept of thrombospondin as an adhesive protein that is involved in cell-to-cell and cellto-matrix interactions. Such interactions may be specifically involved in the association of many cell types with extracellular matrices. These interactions may be especially important at the surface of the platelet and at the site of vascular injury.

Thrombospondin has been identified by several names in the literature. Baenziger et aP2 designated the 190,000-dalton polypeptide as the thrombin-sensitive protein (TSP). When it was realized that TSP was actually a subunit of a larger protein, the name thrombospondin was proposed for the native protein to distinguish it from its subunits that, although equivalent in molecular weight (mol wt), may not be identical and to emphasize that the protein was secreted in response to thrombin, rather than released as a result of direct proteolysis. 3 Other investigators have chosen to emphasize the fact that thrombospondin is localized in the a-granules of platelets and have designated it glycoprotein Ig or glycoprotein G (GPG). 7 The glycoprotein characteristic is also emphasized in some of the literature on several types of cells in culture that identifies the subunit of thrombospon-din as GP-l60 or