RNA in situ hybridization analysis of 72 hpf zebrafish embyros. (A) Lack of whna expression in thymic rudiment; note expression in developing eye and brain structures. (B) Expression of whnb in third branchial arch; arches are indicated by Roman numbers. (C) Transverse section through thymic primordium indicating the location of whnb-expressing cells (purple). ov, otic vesicle; ph, pharynx; no, notochord; y, yolk (with non-specific staining); op, operculum. Only one half of the section is shown. (D–F) Expression of whnb in developmental mutants. D, wild-type embryo. E, cloche mutant; F, casanova mutant.

Characterization of adult zebrafish thymus tissue. (A) Haematoxylin–eosin staining of a zebrafish thymus 6 weeks after fertilization. Note the darkly stained thymus (th) adjacent to the gill chamber (gi). la, larynx; hb, hindbrain. (B) Binding pattern of ConA lectin in zebrafish thymus; inset, adult mouse thymus. The presumptive cortical (co) and medullary (me) regions are indicated. (C) RNA in situ hybridization with radio-labelled rag-1 probe; silver grains are displayed in false colour (green). (D) Analysis of whnb in thymus section at the cortico-medullary junction. Note the equal distribution of silver grains (black dots). (E) Analysis of whna in thymus section at the cortico-medullary junction. Note that expression is confined to a few cells.