Fig. 5. Initiation of lymphopoiesis is delayed in bls. Lymphoid precursors expressing rag1 can be detected in the thymus around 4 dpf. (A) Lymphoid precursors are present in the thymus of wild-type embryos at 4.5 dpf (arrowhead), but are absent from bls. However, if the bls embryos are raised to 7.5 dpf, rag1 expression begins to appear in the thymus, indicating the appearance of lymphoid precursors (arrowhead with asterisk). (B) Coronal section of 4.5 dpf embryos shows that lymphoid cells cluster in the thymus of wild-type embryos (arrowhead), but are absent in the bls thymus. Corresponding to the recovery of rag1 expression at 7.5 dpf, lymphocytes of wild-type morphology can be detected in the thymus of bls embryos (double arrowheads).