'Proopiocortin fragments' in normal human adult pituitary. Distribution and ultrastructural characterization of immunoreactive cells.

The distribution and ultrastructure of cells immunoreactive towards antisera against synthetic fragments of proopiocortin were studied in human pituitaries by immunohistochemical methods. Anterior lobe cells exhibiting the ultrastructural characteristics of corticolipotropes, as well as all epithelial cells 'invading' the posterior lobe, display beta-endorphin/beta-lipotropin-ACTH immunoreactivities. At low dilution a beta-lipotropin antiserum, raised with highly purified antigen, stains also somatotropes. alpha-MSH antibodies bind to certain corticolipotropic cells in the pars distalis and to only a few of the cells 'invading' the pars nervosa. Met-enkephalin antisera react only with isolated cells in the anterior lobe. These results indicate a striking difference in the processing of the proopiocortin precursor molecule in different corticolipotropic cells of the human hypophysis. Furthermore, the hypothesis of a homology between cells 'invading' the human neurohypophysis and those of the intermediate lobe of lower vertebrates, is questioned.