Serum progesterone and corpus luteum function in pregnant pigtailed monkeys (Macaca nemestrina)

Corpus luteum (CL) function and control during pregnancy and early lactation in the pigtailed macaque was investigated. Peripheral concentrations of progesterone (P) on day 10 of pregnancy were 12.98 +/- 2.21 ng/ml and decreased progressively to 7.96 +/- 1.27 ng/ml by day 21 of pregnancy. The concentration of P increased around day 27 of gestation and reached peak levels of 18.48 +/- 2.45 ng/ml on day 37, thereafter gradually decreasing to a nadir at about midgestation. Ten days before parturition P concentrations increased again (P < 0.05). Concentrations of P decreased from 6.62 +/- 1.48 ng/ml on the day of delivery to 2.16 +/- 0.43 ng/ml on day 2 of lactation and remained low thereafter. Ovariectomy on day 35 did not affect the normal course of gestation or the patterns of P secretion during pregnancy. However, in these ovariectomized animals, in spite of suckling, P was not detectable after parturition. In intact monkeys, serum concentrations of P in the utero-ovarian vein at days 80 and 159 of pregnancy were higher relative to the uterine vein. Incubation studies utilizing 3H-cholesterol as a substrate revealed that the CL were capable of synthesizing P on days 35 and 159 of gestation. Histologically, the CL contained active luteal cells at late pregnancy. Low serum concentrations of chorionic gonadotropin were detected on day 10 of gestation; concentrations of this hormone reached high levels between days 18 and 24 and the titers were nondetectable after day 40 of pregnancy. Luteinizing hormone was present in constant amounts in the circulation during pregnancy and lactation. These data suggest that the CL of pregnancy in the pigtailed monkey is functional or capable of functioning during various stages of pregnancy. However, the fetoplacental unit is the primary source of P during the latter 4.5 months of gestation. As in other primates, a functional CL is not required for maintenance of pregnancy after implantation nor for lactation. Thus, the physiological significance of CL function during pregnancy is unclear.