Expression of the somatolactin beta gene during zebrafish embryonic development

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 Sl beta and we identified the gene encoding this hormone. We also obtained a 1 kb genomic fragment corresponding to the sl beta upstream promoter region. Furthermore, the sl beta expression pattern was examined during zebrafish embryogenesis using whole-mount in situ hybridization. Sl beta mRNA is first detected in a single cell at the anterior border of the neural plate starting at 23 h post fertilization (hpf). Sl beta-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 sl beta- and prl-expressing cells are clearly distinct at 29 hpf. Starting at 30 hpf, the number of sl beta positive cells increases and their location becomes more clearly distinct from lactotrope cells, in a more posterior position. At later stages (48 hpf), sl beta 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 sl beta expression. In conclusion, sl beta expression defines a new, additional cell type in zebrafish pituitary that depends on pit1 and aal for its differentiation. (C) 2005 Elsevier B.V. All rights reserved.