Fig. 4.

Failure of thymopoiesis in c-myb mutants. (A) Lack of thymus colonization in c-myb mutants transgenic for ikaros:eGFP. Still photographs were taken from Movies S9 and S10 at the indicated time points of the observation period (t₀ = 55 hpf), equivalent to ∼68 hpf. Note the cluster of green cells in the thymus (encircled), whereas the thymus of mutants lacks such cells. A progenitor cell approaching the thymus is seen at the bottom right corner of the image of wild-type fish. (B) No evidence for thymopoiesis in early larvae. WISH was done with probes indicated at 5 dpf; the thymic area (encircled) is shown. Note the presence of small numbers of cells expressing l-plastin in the c-myb mutants, a marker associated with the myelo-monocytic lineage. This most likely represents embryonic macrophages situated in the thymus (Movies S9 and S10).