Studies on the phenotype of migrant thymic stem cells

Stem cells first enter the thymus around the 11^th to 12^th days of gestation in BALB/c mouse embryos. The phenotype of these stem cells has been difficult to determine because their entry occurs when the thymic primordium is very small and involves too few stem cells to allow studies by flow cytometry. We have been able to microdissect the thymus from embryos during this stage and immunophenotype cells in sections using a sensitive tyramide amplification system. Our results show that migrant stem cells express CD45, c‐kit, CD44, CD34 and α₄ integrin, but other markers such as CD62L, CD25, Thy‐1.2, CD3ϵ, α₅ integrin and RAG‐1 expression are detected only after stem cell entry. These results should help to improve the isolation and characterization of migrant thymic stem cells.