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Abstract

Somatolactin (Sl) is a pituitary hormone closely related to prolactin (Prl) and growth hormone that was until now only found in various fish species. We isolated the cDNA coding for zebrafish Slβ and we identified the gene encoding this hormone. We also obtained a 1kb genomic fragment corresponding to the slβ upstream promoter region. Furthermore, the slβ expression pattern was examined during zebrafish embryogenesis using whole-mount in situ hybridization. Slβ mRNA is first detected in a single cell at the anterior border of the neural plate starting at 23h post fertilization (hpf). Slβ-expressing cells also express the transcription factor pit1 and are located close to prl-expressing cells. Using combined fluorescent in situ hybridization, we show that slβ- and prl-expressing cells are clearly distinct at 29 hpf. Starting at 30 hpf, the number of slβ positive cells increases and their location becomes more clearly distinct from lactotrope cells, in a more posterior position. At later stages (48 hpf), slβ expression was observed posterior to growth hormone expression, again in a distinct cell type. We show that zebrafish mutants aal, as well as mutants in the pit1 gene, are deficient in slβ expression. In conclusion, slβ expression defines a new, additional cell type in zebrafish pituitary that depends on pit1 and aal for its differentiation.