Abstract
The migration of murine fetal liver cells to thymus rudiments was studied in vitro using a migration under agar technique. There appeared to be a minor population that migrated specifically to the thymus from the age of 10 to 14 days of gestation. The specificity of migration was demonstrated in 12βday fetal liver cells by a series of competition studies. The ability of these cells to colonize a thymus rudiment was shown by further incubation after invasion of the epithelial thymus rudiments: small colonies of lymphoid cells were present in invaded tissue but absent from uninvaded control tissue. At 13 to 14 days of gestation, there appeared an additional population that migrated specifically to the spleen, as demonstrated, again, with a competition protocol. Studies with avian and human tissue as attractants in the same system showed that migration was specific to the thymus and did cross species barriers. This observation was used to demonstrate a similar attractant activity in cellβfree conditioned medium from human thymus epithelial cultures, and to demonstrate the absence of such a cellβattractant factor in the conditioned medium from the thymus of a child with previously documented severe combined immunodeficiency disease.