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

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).