In vivo and in vitro assays of thymic organogenesis.

We describe two complementary methods for the study of early thymus organogenesis in the mouse. The first is an in vitro technique for lineage analysis, where a chosen population of cells within the mouse embryo is labeled with a fluorescent cell tracker dye. The embryos are then transferred to whole embryo culture for a defined period, after which time the location of the labeled cells is determined with respect to the developing thymus. In the second method, an in vivo assay is used to determine the ability of a specific tissue type to form a structurally and functionally normal thymus. This method uses an ectopic grafting technique where the embryonic pharyngeal endoderm containing the prospective thymus tissue is carefully isolated and transplanted under the kidney capsule of an adult mouse. Together, these techniques have allowed the cell types that make a physical contribution to the formation of the thymic epithelium to be identified.