Extrapituitary TSH in early chick embryos: Pit-1 dependence?

The expression of the thyrotropin (TSH) gene in the pituitary gland is thought to be dependent upon the pituitary-specific transcription factor, Pit-1. TSH immunoreactivity is, however, widespread in extrapituitary tissues, and the possibility that this may reflect a widespread distribution of Pit-1 was therefore investigated in embryonic chicks, prior to the ontogenic differentiation of the pituitary gland. TSH immunoreactivity in chick embryos at the end of the first trimester of the 21d incubation period was present in discrete cells in the developing brain (particularly in ependymal cells lining the diocoele and mesocoele and in cells lining the otic vesicle), spinal cord (ependymal cells), liver (hepatocytes), lungs (in the linings of the bronchi), gut (in the linings of the proventriculus) and limb bud (in skin, muscle, bone and nerve fibers). In some of these tissues (particularly in brain and spinal cord ependymal cells, cells in the otic vesicle and in liver hepatocytes), the distribution of TSH immunoreactivity was overlapped by the distribution of immunoreactive Pit-1, suggesting Pit-1 involvement in TSH expression in these sites. However, in other tissues (e.g., the trigeminal nerve in the head and the marginal mantle layer of the spinal cord), Pit-1 immunoreactivity was intense but TSH immunoreactivity was marginal. Conversely, other tissues (e.g., cells in the skin, blood vessels, limb bud, bronchus, proventriculus, and cardiopleural cavities) had intense TSH staining but little, if any, Pit-1 immunoreactivity. The expression of the TSH gene in these tissues would thus appear to be Pit-1 independent. These results demonstrate the presence of Pit-1 in pituitary and extrapituitary tissues of the domes tic fowl and suggest the involvement of Pit-1 in the extrapituitary expression of TSH in chick embryos may be tissue-specific.