A possible mechanism of rapid luteolysis in white-footed mice, Peromyscus leucopus.

Adult female white-footed mice, Peromyscus leucopus, were exposed to long (LP) or short (SP) photoperiods for 6 weeks (experiment I). Another group of animals was kept for 6 weeks in SP, then injected SC with 30 micrograms prolactin twice daily for 2, 3, 4, or 6 days (experiment II). Ovaries from the mice in both experiments were weighed and serially sectioned for light microscopic examination of regressing corpora lutea. In experiment I, it was observed that vessels supporting corpora lutea were dilated, and that their endothelium was either undergoing necrosis or it was missing. Pronounced changes of luteal capillaries led to rupture and intraluteal hemorrhage, thus opening the capillary bed. Regressing luteal cells became segregated and seemed to invade the vascular system passively. They were seen as luteal cell thrombi in medullary veins. This luteolytic course termed "rapid luteolysis" was most apparent in SP ovaries. It differed from "retarded luteolysis," which represents the well-established luteolytic model of auto- and heterophagocytosis. In experiment II, there was a statistically significant decrease in ovarian weight 4 days after prolactin treatment in comparison with saline-treated controls. At the light microscopic level, signs of both rapid and retarded luteolysis were present, but not intensified. It is concluded: (1) The concept of rapid luteolysis represents a reasonable working hypothesis. (2) Prolactin, though luteolytic at the macroscopic level, failed to produce evidence of increased rapid or retarded luteolysis at the light microscopic level.