Early embryonic growth is independent of pituitary growth hormone (GH), since it occurs prior to the differentiation of pituitary somatotrophs. Embryogenesis is therefore thought to be regulated by local growth factors. As GH is now known to be produced in many extrapituitary sites, in which it acts in an autocrine or paracrine manner, the possibility that extra-pituitary GH may participate in embryogenesis and organogenesis was assessed by determining the immunocytochemical presence and location of GH- and GH-receptor (GHR)-like proteins in the peripheral tissues of chick embryos during their 21-day incubation period. Immunoreactive (IR)-GH, detectable by a monoclonal and two polyclonal antibodies for chicken GH, was specifically and ubiquitously present in tissues of 3-day-old embryos. At embryonic day (ED) 5, IR-GH was widespread in ectodermal, mesodermal and endodermal tissues, but it was not present in every cell of each tissue. IR-GH was particularly abundant i! n the neural tube, notochord, limb bud, somites, heart, stomach, liver, kidney, Wolffian duct and the amnion. By ED8, IR-GH was still widespread and was now present in limb bud cartilage, although the heart and liver were no longer GH immunoreactive. GH receptor immunoreactivity was also present in most tissues and cells of ED3-ED8 embryos. These results demonstrate that extrapituitary GH is abundantly present during early embryogenesis, prior to the differentiation of pituitary somatotrophs (at ED12). Since GH- and GHR-like proteins are present in most tissues of the chick embryo, it is proposed that extrapituitary GH may act as a local growth factor during embryonic development.

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