Induction of nucleolar changes in rat luminal cells by single injection or low-dose infusion of estradiol

The nucleolar area of rat uterine luminal epithelial cells was measured for 41 h after beginning either of two dosage regimens of estradiol (E2): a single injection of 1.0 microgram E2 or a continuous infusion of E2 at a rate of 1.0 microgram/24 h. In response to a single injection of E2, mean nucleolar area was significantly elevated above control levels by 10 h. Nucleolar morphology showed slight changes, but the fibrillar component was always associated with 2-3 fibrillar centers by 10 h. In response to a continuous infusion of E2, mean nucleolar area was elevated by 10 h, reached a maximum by 18 h, and remained relatively constant thereafter. Nucleolar morphology showed significant changes by 10 h; the fibrillar component was commercially denser. Bivariate histograms of nuclear versus nucleolar areas showed a bimodal distribution after both injection and infusion, suggesting that two size classes of nuclei existed after estrogen stimulation. The results suggest that: 1) a continuous infusion of E2 at a rate of 1.0 microgram/24 h was more effective in inducing larger increases in nucleolar area and transcription than a single injection of 1.0 microgram E2 at time 0; 2) using analysis of nuclear and nucleolar areas two subpopulations of cells were identified; and 3) continuous infusion of E2 produced a steady state of ribosome production and cytoplasmic transport by 18 h.