The Insulin-Like Growth Factor I (IGF-I) Within the Bony Fish Pituitary: New Morphofunctional and Phylogenetic Aspects

ABSTRACT

IGF-I is a major hormonal regulator of differentiation, growth, proliferation, and development. It is mainly produced in the liver, the principal source of endocrine IGF-I. The main stimulus for synthesis and release of liver IGF-I is growth hormone (GH) from the anterior pituitary, and IGF-I specifically inhibits GH gene transcription and secretion via a negative feedback mechanism. As shown for species throughout phylogeny, IGF-I is also produced in extrahepatic sites, including the pituitary, and very recently, new insights have been achieved into the distinct localization of IGF-I. Bony fish pituitary preserves the embryonic compartmentalization throughout life, thus, each endocrine cell type is situated in a distinct region. This makes fish pituitary an excellent tool for morphologic investigations. IGF-I mRNA and/or peptide has been located to subtypes of endocrine cells with similar distribution patterns in lower and higher vertebrates suggesting highly conserved and, thus, important physiological roles of intrapituitary IGF-I. Since a major task of IGF-I is to prevent apoptosis and promote cell proliferation, IGF-I released from the endocrine adenohypophyseal cells may have protective and proliferative autocrine and/or paracrine effects. This is supported by the presence of the type 1 IGF receptor (IGF-1R) at all endocrine subpopulations as has been shown in rat and by the constitutive presence of IGF-I in ACTH cells in bony fish and mammals. ACTH cells probably are challenged in stressful situations by pro-apoptotic cytokines and hormones, and might, thus, have a special demand for IGF-I. The increased transient expression of IGF-I in gonadotrophs during puberty, and in subordinate males of tilapia suggests an impact in sexual differentiation and maturation, a question which has been recently underlined to be of major importance. The pronounced inter-individual differences in the IGF-I content of the GH cells may indicate that synthesis and release of IGF-I from GH cells depend on the physiological status, most likely the serum IGF-I level. Further studies should be performed to elucidate these morphofunctional observations.

Keywords:

ACTH, GH, FSH, LH, IGF-I, prolactin, TSH, α-MSH, somatotroph, gonadotroph, feed-back mechanism, intrapituitary loop, adenohypophysis, neurohypophysis, phylogeny, ontogeny.