Progesterone production by the corpus luteum (CL) is essential for preparation of the endometrium for implantation and for the maintenance of gestation. Progesterone modulates its own production and opposes functional luteal regression induced by exogenous agents, such as prostaglandin F2α. In the present study, we evaluated whether progesterone is also capable of interfering with the process of structural luteal regression, which is characterized by a decrease in weight and size of the gland because of programmed cell death (i.e., apoptosis). We have found that a low number of luteal cells undergo apoptosis throughout gestation. On the day of parturition, but following the initial decline in endogenous progesterone production, a small increase in the number of luteal cells undergoing cell death was observed. This increase in apoptotic cells continued postpartum, reaching dramatic levels by Day 4 postpartum, and was accompanied by a marked decrease in average luteal weight. We have established that the exogenous administration of progesterone significantly reduces the decline in luteal weight observed during structural luteal regression postpartum. This effect was associated with a decrease in the number of cells undergoing apoptosis and with enhanced circulating levels of androstenedione. Furthermore, in vivo administration of progesterone delayed the occurrence of DNA fragmentation in postpartum CL incubated in serum-free conditions. Finally, we have shown that neither the CL of gestation nor the newly formed CL after postpartum ovulation express the classic progesterone-receptor mRNA. In summary, the present results support a protective action of progesterone on the function and survival of the CL through inhibition of apoptosis and stimulation of androstenedione production. Furthermore, this effect is carried out in the absence of classic progesterone receptors.
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