Extrapituitary βTSH and GH in early chick embryos
Introduction
The ontogenic appearance of pituitary somatotrophs occurs during mid-late gestation in most mammalian fetuses and towards the end of the first trimester in human pregnancy (Harvey et al., 1997). Early embryonic and fetal growth is therefore independent of pituitary growth hormone (GH). However, while fetal development is thought to reflect a growth-without-GH syndrome (Geffner 1996), GH and GH mRNA are present in extrapituitary tissues of murine (Pantaleon et al., 1997), piscine (Yang et al., 1999) and avian (Harvey et al., 2000a) embryos. It is therefore possible that extrapituitary GH acts as a paracrine or autocrine growth factor during early embryogenesis (Harvey et al., 1997), especially as the distribution of GH overlaps the distribution of the GH receptor (Pantaleon et al., 1997, Harvey et al., 2000a) and as GH antibodies block the stimulatory effect of GH on the uptake of amino acids by murine embryos (Pantaleon et al., 1997). The widespread distribution of GH in peripheral tissues of early chick embryos (Harvey et al., 2000a) is also consistent with the presence of GH in neural, immune, reproductive, digestive and respiratory systems of neonates and adults (Wu et al., 1996, Harvey and Hull, 1997, Tresguerres et al., 1999, Allen et al., 2000). It is therefore possible that other pituitary hormones in extrapituitary sites may similarly be expressed prior to the differentiation of the pituitary gland and its cell-types. Indeed, luteinizing hormone (LH)-immunoreactivity has been detected in the trachea, lung, esophagus and stomach of ED3–ED7 chick embryos, prior to the differentiation of pituitary gonadotropes on ED8 (Shirasawa et al., 1996).
Thyrotropes appear ontogenically prior to somatotroph differentiation in mammals (Burrows et al., 1999) and birds (Porter, 1997). The expression of thyrotropin (βTSH) is not, however, confined to these cells in neonatal or adult mammals, since it also occurs in the brain (e.g. Hojvat et al., 1982a, Hojvat et al., 1982b, Hojvat et al., 1985, DeVito et al., 1985), in immune tissues (Smith et al., 1983, Harbour et al., 1989, Peele et al., 1993, Bodey et al., 2000) and in the gut (Wang et al., 1997) and placenta (Harada and Hershman, 1978). The possibility that βTSH may be produced in extrapituitary tissues of the chick during early embryogenesis was therefore investigated in the present study, especially as βTSH-and GH- secreting cells are thought to be derived from the same cellular linage and as both βTSH and GH gene expression is thought to be dependent on the Pit-1 transcription factor (Burrows et al., 1999).
Section snippets
Tissues
Fertile White Leghorn eggs from the University of Alberta Poultry Unit were incubated at 37.5 °C in humidified air (Hamburger and Hamilton, 1951). The eggs were turned one quarter of a revolution each day during incubation. At embryonic day (ED) 7 (stage 31) the embryos were collected into phosphate-buffered saline (PBS, pH 7.4). Rathke's pouch is present at this stage of the 21 days incubation period, and extrapituitary GH-immunoreactivity is widespread in central and peripheral tissues of
Immunoreactivity in the head
Within the head, GH-immunoreactivity was widespread, but most intense in neural tissue. Strong GH labeling was found in the anterior (Fig. 1c) and posterior (Fig. 2e) diencephalon. Within the diencephalon, the infundibulum was stained (Fig. 1c), as were ependymal and subependymal cells lining the diocoele (Fig. 1c, Fig. 2e). The GH staining in the diencephalon was in discrete layers of cells that were separated by concentric layers of unstained cells. GH-immunoreactivity was also present in
Discussion
These results clearly show, for the first time, the presence of βTSH-immunoreactivity in central and peripheral tissues of ED7 chick embryos, prior to the differentiation of pituitary thyrotropes. They also show that βTSH- and GH-immunoreactive cells are differentially located within embryonic tissues.
In the developing brain, βTSH-immunoreactivity was located in ependymal layers of the mesencephalon, in scattered cells within the trigeminal ganglia and in the otic vesicle. Although βTSH
Acknowledgements
The authors would like to thank Dr A.F. Parlow (NIDDK, Bethesda, MD) for providing the βTSH- antisera. This work was supported by the NSERC of Canada. A.E. Murphy is in receipt of studentship support from NSERC and the Alberta Heritage Foundation for Medical Research.
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