Insulin-like growth factor II mRNA is expressed in neurones of the brain of the bony fish Oreochromis mossambicus, the tilapia

The physiological meaning of insulin-like growth factor II (IGF-II) is still enigmatic. IGF-II occurs in the adult mammalian brain where it is expressed in the mesodermal portion of the choroid plexus and the meninges, but results on its presence in cells of neuroepithelial origin are controversial. However, IGF-II mRNA is transiently expressed in neurones during mammalian early development. In bony fish, IGF-II mRNA is also present in the adult brain but nothing is known about its synthesis sites. Thus, the present study using in situ hybridization with digoxigenin-labelled RNA species-specific probes investigates the cellular distribution of IGF-II mRNA in the adult brain of a bony fish, the tilapia (Oreochromis mossambicus ). As in mammals, IGF-II mRNA was strongly expressed in the choroid plexus and meninges. Thus, IGF-II synthesis by choroid plexus and meninges seems to have a long evolutionary history and may be common to all vertebrates. However, as shown by the detailed investigation of landmark nuclei and regions, IGF-II mRNA occurred also in numerous neurones at all levels of the tilapia brain. The distinct localization of IGF-II mRNA in neurones might indicate that neuronal IGF-II acts as transmitter or modulator. However, the widespread occurrence of the IGF-II-producing neurones argues against this assumption and most probably suggests that IGF-II plays a role in the differentiation, maintenance and regeneration of neurones. It is further assumed that the sustained neuronal IGF-II expression in the brain of the adult tilapia correlates with continued post-embryonic up to life-long brain growth as has been shown in many teleost fishes.