IMMUNOCYTOCHEMICAL LOCALIZATION OF PROLACTIN AND GROWTH-HORMONE IN THE EQUINE PITUITARY

The ultrastructural and immunoreactive staining characteristics of cel ls containing prolactin (lactotropes) and growth hormone (GH; somatotr opes) in the anterior pituitaries of gonadally intact pony mares were studied at the electron microscopic level. Lactotropes included two mo rphological subsets: Type I cells were larger and contained large, den se, polymorphic granules that were scattered throughout the cytoplasm; Type II cells were smaller and contained small, dense, polymorphic gr anules that were predominantly found in peripheral areas of the cytopl asm. Lactotropes constituted 5 to 16% of the total number of cells in the pituitary. Somatotropes were medium-sized cells containing uniform , large, dense secretory granules. The somatotropes contained the larg est secretory granules in the pituitary and represented II to 26% of t he total number of cells, Type I lactotropes and somatotropes were rea dily distinguishable without immunocytochemical staining. Double-label ing of pituitary sections allowed for characterization of cells that c ontained both hormones (mammosomatotropes). These cells were morpholog ically indistinguishable from Type I lactotropes and constituted 6.5 t o 16.5% of the total number of cells. Results from this study demonstr ated that there are two cell populations that contain only prolactin ( Type I and II lactotropes) and one cell population that contains only GH (somatotropes) in the equine pituitary, and an additional subset of cells that contains GM and prolactin in the same secretory granules.