Studies on the phenotype of migrant thymic stem cells

Stem cells first enter the thymus around the 11(th) to 12(th) days of gesta tion in BALB/c mouse embryos. The phenotype of these stem cells has been di fficult to determine because their entry occurs when the thymic primordium is very small and involves too few stem cells to allow studies by flow cyto metry. We have been able to microdissect the thymus from embryos during thi s stage and immunophenotype cells in sections using a sensitive tyramide am plification system. Our results show that migrant stem cells express CD45, c-kit, CD44, CD34 and alpha(4) integrin, but other markers such as CD62L, C D25, Thy-1.2, CD3 epsilon, alpha(5), integrin and RAG-1 expression are dete cted only after stem cell entry. These results should help to improve the i solation and characterization of migrant thymic stem cells.