Fine structural criteria of prolactin cells identified immunohistochemically in the male rat.

Immunoreactive prolactin (PRL) cells in the adult male rat pituitary were observed by light microscopy to be scattered throughout the gland without special localization but sometimes to form small clusters consisting of five to ten cells. The cells had oval, polygonal, and cuplike shapes. Using the "superimposition technique," the fine structural properties of the PRL cells were examined on ultrathin sections just adjacent to the thick plastic section for immunostaining. Four cell types were distinguished: (1) oval, polygonal, and elongate cells with only small spherical granules, 130-200 nm in diameter; (2) oval or polygonal cells with both medium-sized spherical granules (250-300 nm) and about same size of polymorphic granules; (3) polygonal cells containing only large polymorphic granules (300-700 nm in maximal diameter); (4) cup-shaped PRL cells with spherical and small polymorphic granules. Furthermore, the prolactin immunoreactivity of these cell types was confirmed by the electron immunohistochemistry. Type 1 cells resemble, in fine structure, Kurosumi-Oota LH-gonadotrophs, but the former are not stained with anti-rat LH beta serum, but with anit-rat PRL serum. Although the functional relationship between these four types of cells is still unclear, it is concluded that the polymorphic shape of the granules is not necessarily an absolute criterion for identification of the PRL cell in the male.