The thyrotropic cells of the guinea pig pituitary. Electron microscopic study after characterization by immunocytochemical means.

Three different immunocytoenzymatic techniques were used to identify and characterize the thyrotropic cells in the pituitary of normal guinea pigs at the ultrastructural level (superimposition technique, immunocytochemical technique using P.A.P. and indirect immunohistoenzymatic method before embedding). These cells are characterized by a dark cytoplasm with granules ranging from 1500 to 2000 A in diameter. The appearance of these granules is very variable: some display a marked electron density and are homogeneous but some have a less marked electron density with a more electron dense peripherally situated region. The TSH molecules are essentially confined to the granules but when the immunocytochemical reactions are carried out before embedding, positive staining is also seen in the cytoplasm and the outer surface of most of the rough endoplasmic reticulum membranes. These results are discussed.