GROWTH-HORMONE AND GROWTH-FACTORS IN AVIAN DEVELOPMENT

Avian models have proved to be powerful in the dissection of the hypot halami-pituitary growth hormone (GH) - insulin-like growth/factor (IGF ) axis. In contrast to the situation in mammals, somatotrophs develop late in embryonic development and do not appear to be the ancestral li ne for lactotrophs. The synthesis and release of GH begins in the late quarter of embryonic development and increase during the peri-hatch p eriod. Peak rates of GH synthesis and release are evident during early post-hatch growth with marked decreases in both occurring in the mid- phase of growth well prior to sexual maturation. This pattern of GH re lease is found in all avian species examined irrespective of altricial or precoccial and contrasts with the predominant pattern of GH secret ion in mammals with the very high circulating concentrations of GH in the fetus. Changes in the proportion of GH variants in the chicken pit uitary gland are also observed perhaps indicating differences in GH sy nthesis, release and/or degradation. In mammals the hypothalami-pituit ary GH-IGF axis is not fully functional until sometime post-natally du e to delayed expression of GH receptors (GHR). Similarly in birds, the axis is not functional in the embryo due to the absence of significan t quantities of GH and is coupled, albeit poorly in the young rapidly growing stage. The functioning of GH together with IGF-I and II will b e affected by the ontogenic changes in GHBPs and IGF-BPs.