Loss of zebrafish FOG-1 results in defective lymphopoiesis in the thymic organs. Lateral views of 4-dpf control (wild-type [wt], A,E,I,M,O) and FOG-1 morphant embryos (B-C,F-G,J-K,N,P) labeled for the following markers: lck (A-C), rag-1 (E-G), ikaros (I-K,M-N), and pax9 (O-P). In control embryos, the thymic cells express normal levels of lck (A), rag-1 (E), and ikaros (I). In contrast, the morphant embryos show strong reduction or absence of lck (B-C), rag-1 (F-G), and ikaros (J-K) expression in the thymi. The expression of ikaros is maintained in the CHT, indicating normal formation of lymphoid progenitors (M,N, brackets and insets). In the lateral views of 4 dpf embryos, the expression of pax9 is normal in the FOG-1 MO-injected embryos, indicating that the initial phase of pharyngeal endoderm formation is not affected (O-Q). In addition, the embryos were processed with Alcian blue staining to delineate the morphologic architecture of the pharyngeal arch cartilages in control (wt; R) and morphant (S) embryos. The morphant embryos display dysplastic development of the pharyngeal arches, indicating a disruption of late endodermal derivatives: pharyngeal arch cartilage and thymic anlage (S). The bars represent quantification of the normalized phenotypes (mean ± SD) with the numbers of embryos analyzed in each category indicated above the bars. White, black, and gray bars indicate embryos with normal, reduced, and absent expression for lymphoid markers, respectively (D,H,L,Q). Results were derived from 3 independent experiments. The statistical significance, marked by lines for each paired conditions, was analyzed using analysis of variance; *P < .05.