Apoptosis induced by antigestagen RU486 in rat corpus luteum of pregnancy

Administration of RU486 to late pregnant rats results in preterm delivery 24 h after treatment and the induction of a luteolytic process after labor. We investigated whether functional changes occurring within the corpora lutea after RU486 treatment were associated with morphologic features of apoptotic cell death. Rats on d 18 of pregnancy were treated with RU486 (5 mg/kg) at 10:00 Am and killed 72 h after. We studied the number of apoptotic cells in paraffin sections of the corpora lutea by routine hematoxylin and eosin (H&E) staining, and by in situ 3' end labeling (TdT-mediated dUTP nick-end labeling [TUNEL]). The corpora lutea were also processed for electron microscopy to study ultrastructural changes after RU486 treatment. The number of cells showing apoptotic nuclei in H&E-stained sections was higher in RU486-treated animals than in controls (vehicle-treated rats). The quantification of the number of apoptotic nuclei within the corpora lutea performed by TUNEL confirmed the higher number of apoptotic nuclei in animals receiving the antigestagen compared with controls. Ultrastructurally, the luteal cells undergoing apoptosis presented a highly deteriorated cytoplasmic organization The nuclei, in an initial step of regression, displayed condensation of the chromatin, a prominent nucleolus, and a perinuclear space. In an advanced step of degeneration, the nuclei showed evidence of large irregular aggregates of condensed chromatin. Prostaglandin F-2 alpha(PGF(2 alpha)), which mediates the luteolytic action of RU486, mimicked the effect of the antigestagen on the induction of apoptosis when administered to rats on d 18 of pregnancy (100 mug at 9:00 Am and 1:00 Pm), which were killed 72 after the last injection. In conclusion, the present results indicate that functional luteolysis in rats is associated with structural luteal regression with the morphologic features of apoptotic cell death, as demonstrated by studying the luteolytic process induced by the administration of the antigestagen RU486.