DISTRIBUTION OF LACTOTROPH SUBPOPULATIONS IN DIFFERENT REGIONS OF THERAT PITUITARY

In this study, we examined the distribution of lactotrophs in differen t 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 resul ting cells subjected to density sedimentation through a discontinuous Percoll gradient. The light and heavy cell subpopulations obtained wer e compared with cells not subjected to Percoll (unseparted cells). Cel l numbers were determined and prolactin positive cells were assessed b y immunocytochemistry. In OVX rats,the percentage of lactotrophs in th e unseparated cell fraction of the outer zone was significantly (P < 0 .05) lower than that in the inner zone. Estradiol increased the percen tage of PRL, cells in all cell populations, but the effect on inner zo ne 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 effe cts of estradiol. When the number of lactotrophs was calculated, the u nseparated 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 equ al distribution of lactotrophs between light and heavy cells in both i nner 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 decr eased the number of PRL, positive cells in both zones and across cell types, but this effect was most pronounced in the outer zone. Progeste rone also decreased the total pituitary cell number and this effect wa s greater than could be accounted for by the decrease in lactotroph nu mbers. We conclude that, except in untreated OVX rats, the numbers of lactotrophs in inner and outer zones are similar. The results also ind icate that estradiol and progesterone can significantly alter the prop ortion and ! or numbers of PRL positive cells within pituitary zones a nd may also affect cells that are not identified as PRL positive.