Ontogeny of growth hormone (GH)-secreting cells during chicken embryonic development: initial somatotrophs are responsive to GH-releasing hormone.

Abstract

In the present study, a reverse hemolytic plaque assay (RHPA) for chicken GH was established and used to study the ontogeny of somatotroph differentiation and functional responsiveness to GH-releasing hormone (GHRH) during chicken embryonic development. Anterior pituitaries from embryos on days 10, 12, 14, and 16 of incubation were isolated and dissociated into single cells with trypsin. The resulting cells were then subjected to the GH plaque assay under basal and GHRH-stimulated conditions. No GH-releasing cells were detected on day 10 or 12 of embryonic development. In contrast, a few somatotrophs (< 2% of all cells) were consistently found on day 14, and a statistically significant population existed on day 16, when 6.3 +/- 1.4% of all anterior pituitary cells secreted GH. Thus, GH-secreting cells differentiated by embryonic day 16. Treatment of pituitary cells from day 16 embryos with GHRH was found to increase the proportion of GH plaque-forming cells during a shortened assay interval from 1.8 +/- 0.3% under basal conditions to 6.7 +/- 1.2% in the presence of GHRH. This nearly 4-fold increase in the proportion of plaque-forming cells indicates that at least 70% of the initial somatotrophs present on day 16 were responsive to the stimulatory effects of GHRH. To test whether the absence of GH cells on day 12 of embryonic development was due to the presence of cells that produced but did not release GH, pituitary cells from day 12 and day 16 embryos were subjected to immunocytochemistry for GH and to the GH RHPA in parallel. No significant differences were found in the percentage of cells that either contained or released GH on the two embryonic ages tested. On day 12, 1.1 +/- 0.8% of all cells contained GH, as determined by immunocytochemistry, whereas 0.5 +/- 0.5% released GH as determined by RHPA. By day 16, the proportions of cells that contained and released GH had increased to 9.5 +/- 0.6 and 11.2 +/- 2.5%, respectively. Taken together, these results indicate that GH-secreting cells differentiate by day 16 of chicken embryonic development and that these initial somatotrophs are responsive to GHRH. Given that growth and metabolism are regulated in part by GH in chick embryos, these findings suggest that these processes may be under hypothalamic control during late embryonic development in the chicken.

Independent differentiation of mammotropes and somatotropes in the chicken embryonic pituitary gland

“…It has been shown that murine somatotropes and mammotropes appeared in the dorsal part of the embryonic anterior pituitary gland (Thommes et al 1987;Porter et al 1995;Barabanov 1985;Woods et al 1998), and it is thought that, at least in rodents, some mammotropes are transdifferentiated from somatotropes (Frawley et al 1985). However, since these cells in the murine pituitary gland are not strictly confined in a region, it is difficult to evaluate the transdifferentiation of these cells only by their localization.…”

“…Previous studies have shown those growth hormone (GH)-producing cells, somatotropes, which are localized primarily in the caudal lobe, while prolactin (Prl)-producing cells, mammotropes, exist mainly in the cephalic lobe (Jozsa et al 1979;Barabanov 1991;Berghman et al 1988). Studies on somatotropes and mammotropes in the embryonic chicken pituitary gland have shown that somatotropes commence differentiation between embryonic day (E) 14 and E16 in the caudal lobe (Thommes et al 1987;Porter et al 1995), while mammotropes first appear on E17 in the cephalic lobe (Barabanov 1985;Woods et al 1998). It has been reported that the GH mRNA level in the caudal lobe increased on E16 and peaked on E20 (Bossis et al 2000) and that the Prl mRNA level in the cephalic lobe increased on E18 and peaked on E20 (Kansaku et al 1994).…”

Growth hormone as an early embryonic growth and differentiation factor

“…During avian development, the pituitary gland becomes fully functional with respect to GH mRNA expression (Kansaku et al 1994) and GH protein secretion (Porter et al 1995, Harvey et al 1998) from the somatotrophs, at approximately embryonic day (ED) 16, although GH immunoreactivity can be detected in the embryonic pituitary gland at ED 12 (Malamed et al 1993) and perhaps as early as ED 6 (Gasc and Sar 1981). However, extrapituitary GH mRNA is expressed as early as ED 2 in the chick embryo , and GH protein is detectable bound to cells in the germ layers of the ED 1 and 2 chick blastoderm (Wang 1989) and is widespread in the cytoplasm of most somatic cells from ED 3 onwards .…”

Profiles of mRNA expression of related genes in the duck hypothalamus–pituitary growth axis during embryonic and early post-hatch development

“…Similar to other faunas, the occurrence, development and perfection of the bird neuroendocrine system present a spatial-temporal specificity, which is controlled by genetic programming (Lu et al, 2008;Geris et al, 1998;Porter et al, 1995). Namely, profiles of relevant genetic information expression may change according to the given spatialtemporal order.…”