Uneven Regional Distributions of Prolactin- and Growth Hormone-Secreting Cells and Sexually Dimorphic Proportions of Prolactin Secretors in the Adenohypophysis of Adult Chickens

Prolactin (PRL) and growth hormone (GH) proteins are structurally similar and are thought to be evolutionarily derived from a common gene. In addition, data indicate that GH and PRL cells differentiate from a common stem cell. In adult birds, females have higher levels of serum PRL than males. Levels of serum GH are reported to be higher in young male birds than in females and equal between sexes in adult birds. Furthermore, previous studies using immunocytochemistry found that PRL- and GH-containing cells were located primarily in the cephalic (Cp) and caudal (Cd) lobes. respectively, of the anterior pituitary. Two experiments were conducted to study the cellular basis for differences in PRL and GH secretion between genders or anatomical location in adult chickens. In Experiment I, anterior pituitaries from broiler breeder adult hens and roosters were subjected to reverse hemolytic plaque assays (RHPA's) for PRL and GH. The percentage of PRL-secreting cells was significantly greater in females than in males (46.1 ± 4.5% and 26.1 ± 2.4%, respectively: P < 0.001; n = 6). However, there were no significant differences between the proportions of GH-secreting cells between sexes (25.8 ± 3.1% and 30.0 ± 3.9% in females and males, respectively). Average area of plaques formed was different between sexes in GH plaque assays under basal conditions (162,465.9 ± 29,911.8 μm2 and 53,834.9 ± 31,033.5 μm2, for males and females, respectively P < 0.05). However, no differences were found in the presence of GH-releasing hormone. In Experiment II, anterior pituitaries from Leghorn hens were dissected into three regions: the extreme portions of the Cp and Cd lobes and the remaining or middle portion. The cells from each region were subjected to RHPA's for PRL and GH. We found that 52.0 ± 4.1% of all cells from the Cp lobe secreted PRL. This proportion was greater (P < 0.01; n = 4) than that found in the Cd lobe, where 2.4 ± 0.4% of all cells released PRL. In contrast, the percentage of GH-secreting cells was higher in the Cd lobe than in the Cp lobe (61.0 ± 2.3% and 0.8 ± 0.5%, respectively: P < 0.01). As expected, the middle portion contained substantial numbers of both cell types. These findings suggest that sexual dimorphism in serum PRL levels in adult chickens is due, in part, to an increased cell population of PRL secretions in females. Furthermore, PRL- and GH-secreting cells are distributed unevenly throughout the anterior pituitary of chickens, with the PRL secret