Percentages of cell types in the adenohypophysis were compared with the content of sth and prolactin at different reproductive stages (e.g. immature, oestrous cycle, pregnancy, lactation and after hysterectomy) in the pig. Acidophils represent sth and prolactin activities. Chromophobes are undifferentiated, inactive or depleted cells.
Acidophils represented half (53·5%) the cell population. Percentages of sth cells were higher in 17-day-old pigs than in mature animals. This is a period of rapid growth in the young pig. During the cycle, during pregnancy and after hysterectomy, the percentages of prolactin cells were higher than those found in immature or lactating pigs.
The chromophobes represented 13·1% of the cell population. Chromophobes were the predominant cell type during lactation, but these seemingly inactive cells may have been active acidophils that were synthesizing and secreting prolactin. By rapid turnover of hormone, the cytoplasmic granules in prolactin cells lack differentiation.
The sth activity was similar in pigs during pregnancy and lactation and after hysterectomy. There were no significant correlations in the percentages of sth cells and the concentration or content of sth during pregnancy and lactation. There was a trend of increased percentages of prolactin cells in later stages of pregnancy and after hysterectomy. Exogenous oestrogen caused a marked increase in the pituitary content of sth and prolactin in hysterectomized pigs as compared with uninjected hysterectomized animals. Prolactin levels remained relatively constant during pregnancy and after hysterectomy, but declined during lactation. Exogenous oestrogen caused a consistently higher trend in prolactin activity in hysterectomized animals.