Pars tuberalis-specific cells in the ovine pituitary do express the common alpha-chain of glycoprotein hormones: an in situ hybridization and immunocytochemical study.

The ovine pituitary pars tuberalis was investigated by electron microscopy, immunocytochemistry and non-radioactive in situ hybridization in order to characterize further the pars tuberalis-specific cells, whose functional role within the endocrine system is still enigmatic. Ultrastructural analysis revealed that, besides gonadotropic cells, the vast majority of cells in the ovine pars tuberalis show the typical characteristics of pars tuberalis-specific cells with clear signs of secretory activity. Immunocytochemical staining with a polyclonal antibody directed against the alpha-subunit of ovine glycoprotein hormones and in situ hybridization with an antisense oligonucleotide complementary to the alpha-subunit mRNA showed that the common alpha-chain of glycoprotein hormones is expressed in pars tuberalis-specific cells. Antibodies against the beta-subunits failed to detect any of the known beta-chains of pituitary glycoprotein hormones in these cells. The demonstration of the glycoprotein alpha-subunit in pars tuberalis-specific cells of the adult sheep supports previously existing evidence that these cells do secrete one or more glycoprotein hormones. With respect to the thyrotropin-beta-like immunoreactivity in rats and hamsters, one might speculate that pars tuberalis-specific cells are a pluripotent cell type with a low secretory activity under basal conditions. Further studies should prove the hypothesis that a pre-existing mRNA pool in these cells can be used for sustained translation of glycoprotein hormones after physiological or pharmacological stimulation.