Thrombospondin (TSP) 2, and its close relative TSP1, are extracellular proteins whose functions are complex, poorly understood, and controversial. In an attempt to determine the function of TSP2, we disrupted the Thbs2 gene by homologous recombination in embryonic stem cells, and generated TSP2-null mice by blastocyst injection and appropriate breeding of mutant animals. Thbs2−/− mice were produced with the expected Mendelian frequency, appeared overtly normal, and were fertile. However, on closer examination, these mice displayed a wide variety of abnormalities. Collagen fiber patterns in skin were disordered, and abnormally large fibrils with irregular contours were observed by electron microscopy in both skin and tendon. As a functional correlate of these findings, the skin was fragile and had reduced tensile strength, and the tail was unusually flexible. Mutant skin fibroblasts were defective in attachment to a substratum. An increase in total density and in cortical thickness of long bones was documented by histology and quantitative computer tomography. Mutant mice also manifested an abnormal bleeding time, and histologic surveys of mouse tissues, stained with an antibody to von Willebrand factor, showed a significant increase in blood vessels. The basis for the unusual phenotype of the TSP2-null mouse could derive from the structural role that TSP2 might play in collagen fibrillogenesis in skin and tendon. However, it seems likely that some of the diverse manifestations of this genetic disorder result from the ability of TSP2 to modulate the cell surface properties of mesenchymal cells, and thus, to affect cell functions such as adhesion and migration.

SUMMARYMice that lack the matricellular protein thrombospondin 2 (TSP2) develop a pleiotropic phenotype characterized by morphological changes in connective tissues, an increase in vascular density, and a propensity for bleeding. Furthermore, dermal cells derived from TSP2-null mice display adhesion defects, a finding that implicates TSP2 in cell-matrix interactions. To gain a better understanding of the participation of TSP2 in the development and maturation of the mouse, we examined its distribution in embryonic and adult tissues. Special attention was paid to the presence of TSP2 in collagen fibers, because collagen fibrils in the TSP2-null mouse appear to be irregular in size and contour by electron microscopy. Immunohistochemical analysis of Day 15 and Day 18 embryos revealed TSP2 in areas of chondrogenesis, osteogenesis, and vasculogenesis, and in dermal and other connective tissue-forming cells. Distinctly different patterns of deposition of TSP2 were observed in areas of developing cartilage and bone at Days 15 and 18 of embryonic development. A survey of adult tissues revealed TSP2 in dermal fibroblasts, articular chondrocytes, Purkinje cells in the cerebellum, Leidig cells in the testis, and in the adrenal cortex. Dermal fibroblasts were also shown to synthesize TSP2 in vitro. The distribution of TSP2 during development is in keeping with its participation in the formation of a variety of connective tissues. In adult tissues, TSP2 is located in the pericellular environment, where it can potentially influence the cell-matrix interactions associated with cell movement and tissue repair. (J Histochem Cytochem 46:1007-1015, 1998)