MECHANISMS INVOLVED IN THE AVIAN PATTERNS OF GROWTH-HORMONE SECRETION DURING GROWTH AND DEVELOPMENT

The pattern of plasma concentrations of growth hormone (GH) during growth and development is very similar in all species of birds studied. Plasma concentrations of GH rise during late embryonic and early post hatching development. This reflects increased GH -secretion. There are considerable increases in the number of somatotrophs, in the pituitary content of GH and in the sensitivity to GH secretagogues. Later in development, plasma concentrations of GH decline due to an increase in clearance and a reduction in secretion. This ontogenic pattern of circulating concentrations of GH is remarkably similar in four avian orders suggesting that this was found in the common ancestor and that strong evolutionary pressure existed to retain this pattern. The ontogenic pattern of liver GH receptors is inversely related to the concentration of GH. During development, plasma concentrations of GH are elevated by the same environmental factors (e.g. nutrition, toxicants) which attenuate growth. This may be part of the "catch-up" growth mechanisms to ensure growth to an optimal size once the temporary constraint has been removed.