Development of Thyroid-Stimulating Hormone Beta Subunit-Producing Cells in the Chicken Embryonic Pituitary Gland

Abstract

In previous studies, the distribution of thyrotropes in the chicken pituitary gland has been analyzed by immunohistochemistry using heterologous antibodies. In this study, we examined the distribution of thyroid-stimulating hormone beta subunit-immunopositive (TSHbeta-ip) cells and the expression of TSHbeta mRNA in the pituitary glands of chicken embryos by immunohistochemistry using a specific antiserum to the chicken TSHbeta, in situ hybridization and RT-PCR. Immunohistochemical and morphometric analyses revealed that the TSHbeta-ip cells first appeared on embryonic day 10 (E10) in the pituitary gland and were mainly distributed in the cephalic lobe and that the cell density on E20 was almost 4 times greater than that on E10. The chicken TSHbeta-ip cells could be classified into two types based on morphological characteristics: round-shaped cells and club-shaped cells, which have long cytoplasmic processes. In situ hybridization analysis revealed that TSHbeta mRNA-expressing cells were expressed from E9 in the cephalic lobe and that the extent of TSHbeta mRNA-expressing cells coincided with that of TSHbeta-ip cells. RT-PCR also showed that TSHbeta mRNA was expressed from E9 and that Pit-1 mRNA was expressed from E5. These results clearly demonstrated that the expression of chicken TSHbeta mRNA starts on E9, that TSHbeta-ip cells appear on E10, mainly in the cephalic lobe, and that TSHbeta-ip cells can be classified into two cell types (round- and club-shaped cells).

Increasing plasma thyroxine levels during late embryogenesis and hatching in the chicken are not caused by an increased sensitivity of the thyrotropes to hypothalamic stimulation

“…Serum T 4 levels were similar, but temporally lagging behind those of pituitary TSHmRNA, supporting a role for TSH in stimulating the increase in T 4 production and secretion towards hatching. This assumption is further supported by the ontogenic changes of chicken thyrotrope number, which increases gradually to reach a maximum around E18-E20 (Nakamura et al 2004, Muchow et al 2005.…”

“…The decreasing CRH content of the median eminence towards E20 is suggestive of such an increased CRH secretion (Vandenborne et al 2005). Other factors worth considering are the increasing number of thyrotropes in the pituitary (Nakamura et al 2004, Muchow et al 2005) and the increasing size of the thyroid gland. Finally, it is highly probable that the observed changes are also related to changes in the expression and activity of other components mediating TH action, such as iodothyronine deiodinases, TH transporters, TH receptors and cofactors.…”

The chicken embryo as a model for developmental endocrinology: Development of the thyrotropic, corticotropic, and somatotropic axes

“…A few GH-secreting cells were found on day 14 and a significant population existed on day 16 (Porter et al, 1995 The pituitary-specific transcription factor Pit-1, an important regulator in mammalian pituitary development, is expressed very early in chicken embryogenesis, i.e. starting from day 4 or 5 (Van As et al, 2000;Nakamura et al, 2004). This may indicate that this transcription factor plays a similar role in avian pituitary development.…”

“…Whereas hepatic type 1 deiodinase activity shows a 3-fold increase up to the period of pipping and hatching, hepatic D3 activity decreases more than 10-fold from Presumably, an increased TSH secretion by the pituitary gland causes the gradual rise in plasma T 4 levels that precedes the T 3 peak, but a straightforward assay to measure plasma chicken TSH is still not available. Pituitary TSH mRNA levels increase towards day 19 of incubation (Gregory et al, 1998;Nakamura et al, 2004;De Groef et al, 2006;Ellestad et al, 2006) and so does the density of TSH-immunoreactive cells (Nakamura et al, 2004;Muchow et al, 2005). Other possible causes of an increased T 4 release, such as an increased sensitivity of the thyroid gland to TSH stimulation or an augmented sensitivity of the thyrotropes to hypothalamic releasing factors seem unlikely.…”

“…Pituitary TSH mRNA levels increase towards day 19 of incubation (Gregory et al, 1998;Nakamura et al, 2004;De Groef et al, 2006;Ellestad et al, 2006) Summarized, the marked changes in the thyrotropic, corticotropic and somatotropic axes that occur during the last week of chicken embryonic development are…”