Apoptosis induced by antigestagen RU486 in rat corpus luteum of pregnancy

Administration of RU486 to late pregnant rats results in preterm delivery 2 4 h after treatment and the induction of a luteolytic process after labor. We investigated whether functional changes occurring within the corpora lut ea after RU486 treatment were associated with morphologic features of apopt otic 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 cel ls in paraffin sections of the corpora lutea by routine hematoxylin and eos in (H&E) staining, and by in situ 3' end labeling (TdT-mediated dUTP nick-e nd labeling [TUNEL]). The corpora lutea were also processed for electron mi croscopy to study ultrastructural changes after RU486 treatment. The number of cells showing apoptotic nuclei in H&E-stained sections was higher in RU 486-treated animals than in controls (vehicle-treated rats). The quantifica tion of the number of apoptotic nuclei within the corpora lutea performed b y TUNEL confirmed the higher number of apoptotic nuclei in animals receivin g the antigestagen compared with controls. Ultrastructurally, the luteal ce lls undergoing apoptosis presented a highly deteriorated cytoplasmic organi zation The nuclei, in an initial step of regression, displayed condensation of the chromatin, a prominent nucleolus, and a perinuclear space. In an ad vanced step of degeneration, the nuclei showed evidence of large irregular aggregates of condensed chromatin. Prostaglandin F-2 alpha(PGF(2 alpha)), w hich mediates the luteolytic action of RU486, mimicked the effect of the an tigestagen 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 t he last injection. In conclusion, the present results indicate that functio nal luteolysis in rats is associated with structural luteal regression with the morphologic features of apoptotic cell death, as demonstrated by study ing the luteolytic process induced by the administration of the antigestage n RU486.