Differences in embryonic growth hormone secretion between slow and fast growing chicken strains

This study was designed to test for differences in somatotroph ontogeny, abundance, growth hormone (GH) secretion rate and GH-releasing hormone (GHRH) responsiveness between pituitaries of slow and fast growing chicken strains during embryonic development. Day 10, 12, 14 and 16 embryonic anterior pituitary cells from slow and fast growing chickens were subjected to reverse hemolytic plaque assays (RHPA) for GH. No differences were found in the day on which GH-secreting cells were first detected; somatotrophs were first present on day 14 of embryonic development for both strains. Similarly, no differences were found in the proportions of pituitary cells that secreted GH between the two strains at any of the ages tested. In contrast, differences were observed in GH secretory characteristics of individual somatotrophs between slow and fast growing embryos on day 16 of development, when a substantial somatotroph cell population was first present. Somatotrophs of fast growing embryos released more GH per hour in the presence of GHRH and had a greater capacity for GH release than those of slow growing birds. Furthermore, the majority of GH-secreting cells of fast growing embryos were responsive to GHRH on embryonic day 16, while less than half of the somatotrophs found in slow growing embryos were GHRH responsive. In contrast to these enhanced GH secretory characteristics in the fast growing strain during embryonic growth, a greater percentage of GH-secreting cells was found in the slow growing strain 5 weeks after hatch. It is concluded that differences in GH secretion during embryonic development may contribute to the increased growth rate of chickens selected for greater body weight.