Localization of insulin-like growth factor-I immunoreactivity in larval and juvenile barramundi (Lates calcarifer).

Antisera to mammalian IGFs cross-react with several fish species, suggesting a long phylogenetic history of IGF-like peptides and their functional importance to all vertebrates. In this study, the tissue distribution of IGF-I-immunoreactivity (IGF-I-IR) was studied at various larval and juvenile stages of the life cycle of barramundi (Lates calcarifer). It was shown that the distribution of IGF-I-IR in this species was tissue-specific and age-dependent. In newly hatched larvae, presumptive musculature in the trunk and the pectoral fin rudiments reacted positively for IGF-I. As specimen age increased, however, IGF-I-IR in these tissues became less evident. In the retinas of barramundi 42 hr and older, a distinct band of IGF-I-IR was consistently detected between the presumptive outer nuclear and bacillary layers. Examination of sections from specimens 9 days and older revealed strong reactivity for IGF-I in a large proportion of renal tubule epithelial cells. In sections from larvae 13 to 28 days old, diffuse cytoplasmic IGF-I-IR was identified in cells lining the gill structures. At these same developmental stages, IGF-I reactive cells were also observed in the islets of Langerhans. In young juveniles (22 to 28 days posthatching), sparsely scattered clusters of neurons in the lower brain were observed which exhibited granular IGF-I-IR in perikarya. In all instances reported, IGF-I-IR in barramundi tissue was abolished by replacing antisera with normal rabbit serum or by preabsorption of antisera with purified IGF-I peptide, indicating the specificity of the reactions obtained. The distribution patterns of IGF-I-IR in barramundi tissues were broadly consistent with the reported distributions of IGF-I-like peptides and transcripts in other teleost species. The findings of this study are in general agreement with the hypothesis that IGF-I-like peptides may be involved in the regulation of tissue growth, differentiation, and function during early barramundi development.