Colocalization of prolactin and growth hormone within specific adenohypophyseal cells in male, female, and lactating female rats.

The localization of PRL and GH within adenohypophysial cells has been investigated with immunocytochemical methodology using colloidal gold of different sizes. Classically, using morphological criteria at the light and electron microscope levels, two types of individual cells have been described which, it is believed, exclusively produce either PRL or GH, i.e. mammotrophs and somatotrophs. Since it has been reported that some gonadotrophs may colocalize and secrete both FSH and LH, and that some unidentified cells release both PRL and GH, we designed a study to investigate whether PRL and GH are present only in their respective specific cells or may be colocalized in mammotrophs and somatotrophs or possibly other types of cells within the pars distalis. Using immunocytochemistry at the electron microscope level we were able to label the two hormones with different sizes of colloidal gold bound to a second antibody and visualize them within pituitary cells. Two different primary antibodies to PRL and GH as well as a more purified antibody to GH were used. Pituitaries from cycling and lactating female rats and adult males were processed appropriately for electron microscopic studies. After sectioning, individual grids were treated with either primary antibodies for PRL or GH, or the same grid was treated sequentially with these two antibodies. All primary antibodies were absorbed with the heterologous hormone before usage. The second antibodies were bound to colloidal gold particles of either 10 nm (for GH) or 20 nm (for PRL) diameter, so that selective visualization of the two hormones could be achieved within the same cells. It was observed that mammotrophs immunolabeled only for PRL, and somatotrophs labeled only for GH regardless of the source of the antibody. However, an atypical small granule cell, a possible mammosomatotroph, colabeled consistently for both PRL and GH in all types of animals used and with antibodies from all of above sources. This report gives for the first time morphological evidence for the existence of pituitary cells that colocalize both PRL and GH in the normal rat pituitary gland. The possibility that these bihormonal cells represent stem cells which may give rise to both mammotrophs and somatotrophs under appropriate stimulation or that they may release both hormones under the influence of unspecific stimuli is suggested.