Thymic microenvironmental factor: a possible chemoattractant for hemopoietic stem cells.

A thymic microenvironmental factor (TMF), distinct from thymic epithelial supernatant, was extracted from membranes of thymic stromal cells. Using a migration-under-agar assay, we showed that hematopoietic stem cells from fetal liver could migrate specifically towards TMF. Fetal thymic rudiments, which are known to attract such cells, can compete for TMF activity in this test. An antibody to TMF inhibited TMF activity. Bone marrow cells and peritoneal macrophages were not specifically attracted towards TMF. We also showed that TMF-attracted cells can colonize embryonic thymic rudiments.