Distribution of lactotroph subpopulations in different regions of the rat pituitary

In this study, we examined the distribution of lactotrophs in different regions of pituitaries of ovariectomized (OVX) rats either untreated or treated with estradiol, progesterone or a combination of estradiol and progesterone. Anterior pituitaries were cut into inner and outer zones. Each of these regions was enzymatically dispersed and the resulting cells subjected to density sedimentation through a discontinuous Percoll gradient. The light and heavy cell subpopulations obtained were compared with cells not subjected to Percoll (unseparated cells). Cell numbers were determined and prolactin positive cells were assessed by immunocytochemistry. In OVX rats, the percentage of lactotrophs in the unseparated cell fraction of the outer zone was significantly (P < 0.05) lower than that in the inner zone. Estradiol increased the percentage of PRL cells in all cell populations, but the effect on inner zone light cells was significantly less than on any other type of cells. Progesterone given alone did not affect the percentage of lactotrophs in pituitary compared to untreated OVX rats nor did it alter the effects of estradiol. When the number of lactotrophs was calculated, the unseparated cells of inner and outer zone were equivalent except in the untreated OVX group where there was a significantly greater number of lactotrophs in the outer zone. In untreated OVX rats there was an equal distribution of lactotrophs between light and heavy cells in both inner and outer zones. Estradiol significantly increased the number of PRL cells in both zones and the effect was greater in the heavy cell fractions especially in the outer zone. In contrast, progesterone decreased the number of PRL positive cells in both zones and across cell types, but this effect was most pronounced in the outer zone. Progesterone also decreased the total pituitary cell number and this effect was greater than could be accounted for by the decrease in lactotroph numbers. We conclude that, except in untreated OVX rats, the numbers of lactotrophs in inner and outer zones are similar. The results also indicate that estradiol and progesterone can significantly alter the proportion and/or numbers of PRL positive cells within pituitary zones and may also affect cells that are not identified as PRL positive.