Early appearance of pancreatic hormone-expressing cells in the zebrafish embryo

Adult pancreatic islets comprise four cell types, alpha, beta, delta and PP , expressing glucagon, insulin, somatostatin and pancreatic-polypeptide, re spectively, arising from cell lineages whose relationships during endocrine pancreas differentiation are still uncertain [Edlund, 1998. Diabetes 47, 1 817-1823]. As zebrafish (Danio rerio) represents an attractive vertebrate m odel to study mutants affecting pancreatic organogenesis [Pack et al., 1996 . Development 123, 321-328], we have investigated the expression patterns o f islet hormones in zebrafish embryos, from the 16-somite (17 h) to 48-h st ages, by whole-mount in situ hybridization and immunofluorescence. Results showed that in the zebrafish pancreatic primordium (a) insulin is the first hormone gene to be expressed, and (b) somatostatin colocalizes with insuli n while glucagon-expressing cells, since their appearance, are distinct fro m insulin- or insulin/somatostatin-expressing cells. Notably, both somatost atin and glucagon, but not insulin, are first expressed in extrapancreatic regions. (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.