Immunohistochemical localization of ACTH in the adult and fetal sheep pituitary.

Light microscope peroxidase-antiperoxidase immunohistochemistry has been applied to the pituitary of adult and fetal sheep from 40 to 145 days of gestation. In the adult, immunoreactive ACTH cells were darkly stained and angular with cytoplasmic processes surrounding neighbouring unstained cells. In the fetus, cells which stained for ACTH were observed in the pars distalis at 40 days. From approximately 90 days, two morphologically distinct ACTH-positive cell types were clearly discernible. The predominant type was large and variably stained. These cells usually occurred in clusters and were often arranged in palisades. The other type resembled ACTH-positive cells in the adult. After 130 days the population of large cells declined and completely disappeared before term in most fetuses. The pars intermedia showed a different pattern of staining. In the fetus, ACTH-positive cells were observed in this region after 60 days gestation and by 90 days almost all the pars intermedia cells were strongly stained. By contrast, the cells in the adult pars intermedia were only lightly stained.