Structural and functional differences between prolactin cells from the inner and outer zones of the male rat anterior...

The aim of this study was to compare the ultrastructure of prolactin cells in the inner and outer zones of the male rat anterior pituitary, and to relate their morphological features to their secretory activity, by means of standard and ultrastructural reverse hemolytic plaque assays (RHPA). The immuno-ultrastructural study showed that in the inner pituitary small-granulated cells represented 52% of the prolactin cells, there being only 5% with large granules, whereas the prolactin cells with large granules accounted for 52% in the outer zone, with only 7% being small-granulated. Percentages of cells with intermediate-sized granules were 43% and 41%, respectively. Analysis of RHPA data revealed that, under basal conditions, prolactin cells secreted more actively in the inner zone than in the outer zone. Stimulation with thyrotropin-releasing hormone or KCl treatment increased the percentage of secretors and the sizes of hemolytic plaques in both zones. However, in response to thyrotropin-releasing hormone, the increase in number of secretors was always higher in the outer zone, whereas the enlargement of plaque sizes was greater for the "inner" cells. These findings are in favor of the small-granulated cells, which predominate in the inner zone, being in a stage of active secretion and responsiveness.