Prolactin immunoreactivity of acidophils of the small granule type

The fine structure of some oval anterior pituitary cells of the adult male rats immunostained with an antiserum to rat prolactin was investigated electron microscopically on the adjacent thin sections. Their fine structural appearance is identical with that of acidophils of the small granule type (Yoshimura et al. 1974) resembling the Kurosumi-Oota LH gonadotrophs. The secretory granules of the oval cells are spherical in shape, ranging from 130 to 200 nm in diameter. Large polymorphic granules, which are generally believed to be characteristic of prolactin cells, are absent from their cytoplasm. It is concluded that the acidophil of the small granule type with a similar fine structure to the Kurosumi-Oota LH gonadotroph is a prolactin secreting cell.