During embryonic development, T cell precursors migrate to the thymus, where immunocompetency is acquired. Our previous studies have shown that avian hemopoietic precursor cells are recruited to the thymus by chemotactic peptides secreted by thymic epithelial cells (Champion, S., B. A. Imhof, P. Savagner, and J. P. Thiery, 1986, Cell, 44:781-790). In this study, we have characterized the homing of these precursor cells to the thymus in vivo by electron and light microscopy. Hemopoietic precursors could be seen to extravasate from blood or lymphatic vessels, migrate in the mesenchyme, traverse the perithymic basement membrane, and finally intercalate into the thymic epithelium. Labeled hemopoietic precursors injected into the blood circulation also followed the same pathway. Migrating hemopoietic precursor cells were found to express the fibronectin receptor complex. In the presence of thymic chemotactic peptides, hemopoietic precursors traverse a human amniotic basement membrane. This invasive process was inhibited by antibodies to laminin or to fibronectin, two major glycoproteins of the amniotic membrane, by monovalent Fab' fragments of antibodies to the fibronectin receptor, and, finally by synthetic peptides that contain the cell-binding sequence Arg-Gly-Asp-Ser of fibronectin. These results indicate that hemopoietic precursors respond to thymic chemotactic peptides by invasive behavior. Direct interactions between basement membrane components and fibronectin receptors appear to be required for this developmentally regulated invasion process.

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