Morphological and functional evidence for corpus luteum activity during late pregnancy in the rhesus monkey

Structural characteristics of the corpus luteum during implantation in the rhesus monkey (Macaca mulatta)

Corpora lutea were obtained from ten pregnant rhesus monkeys during implantation, and the ultrastructure of granulosa and theca lutein cells was characterized. Specimens were individually staged with regard to the extent of implantation and the relationship to the rise in circulating progesterone and estrogen which is characteristic of early pregnancy. Structural changes characteristic of granulosa lutein cells occurring during implantation included: change in form of endoplasmic reticulum from predominantly agranular tubules to predominantly granular cisternae; reduction in size and number of lipid droplets; increase in area occupied by the Golgi and increase in length of the cisternae of the Golgi complex; development of numerous microvillus-lined intracellular spaces; increase in numbers of membrane-bound dense bodies including peroxisomelike bodies, multivesicular bodies within lobopodia, and other lysosomelike bodies; and alterations of mitochondrial cristae. These changes were suggestive of the production of a secretory protein, rapid utilization of existing steroid precursor reserves for the production of progesterone, and a reduction in capability for steroid precursor accumulation and processing by granulosa lutein cells. Structural changes characteristic of theca lutein cells occurring during implantation included an increase in size and number of lipid droplets, an increase in agranular endoplasmic reticulum, and an increase in area occupied by the Golgi complex. These changes were suggestive of an increased capability for steroid precursor accumulation and processing, perhaps for estrogen production, by the theca lutein cells.

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.

Serum estradiol and progesterone during pregnancy and the status of the corpus luteum at delivery in cynomolgus monkeys (Macaca fascicularis)

Levels of estradiol and progesterone in peripheral serum of seven cynomolgus monkeys (Macaca fascicularis) were measured from about the 30th day of pregnancy until parturition. Although the pattern of each steroid in circulation differed somewhat from the respective patterns in rhesus monkeys (Macaca mulatta), there were basic similarities. On the day of delivery, the corpus luteum was excised and in vitro incubation of dispersed luteal cells was performed. Isolated luteal cells produced progesterone under control conditions and responded to the addition of HCG with enhanced steroidogenesis. Accordingly, "rejuvenation" of the corpus luteum may occur during advanced gestation in cynomolgus monkeys. These findings, along with establishing the efficacy of the Subhuman Primate Pregnancy Test kit to diagnose pregnancy in this macaque, extend previous evidence for utility of cynomolgus monkeys as a primate model for study of steroid hormones in pregnancy.

Cholesterol side chain cleavage and aromatase activities in the corpus luteum of the pregnant rhesus monkey

Cholesterol side-chain cleavage (CSCC) and aromatase activities were measured in luteal mitochondria and tissue pieces, respectively, from rhesus monkeys on days 22, 49, 128 and 160 of gestation. CSCC activity did not vary significantly during gestation and thus probably does not respond to chorionic gonadotropin which is elevated on day 22 of pregnancy. It is not known, however, whether CSCC can be stimulated prior to day 22 when the corpus luteum is steroidogenically more active. Both 3H-pregnenolone and 3H-progesterone were synthesized from [1,2-3/]cholesterol. Aromatase activity declined from high levels on days 22 and 49 to a nadir on day 128 of pregnancy. Utilizing either [1beta-3H]androstenedione or [1beta-3H]testosterone as substrate yielded comparable results throughout gestation.

Corpus luteum function during the early postpartum interval in lactating rhesus monkeys: in vivo and in vitro response to exogenous gonadotropin

The response of the postpartum corpus luteum to exogenous gonadotropin was studied in 12 lactating rhesus monkeys given daily injections of either human chorionic gonadotropin (HCG, n = 6) or saline (control, n = 6) for 4 days immediately following parturition. Peripheral blood samples were collected daily. On the 5th day postpartum, luteectomy was performed progesterone production by dispersed luteal cells was examined. Whereas progesterone in the peripheral circulation of control monkeys progressively declined between days 1 and 5 postpartum, progesterone levels increased significantly (p less than 0.025) with the onset of HCG treatment and remained significantly (p less than 0.025) elevated above the controls throughout the period of HCG treatment. However, despite the daily administration of HCG, circulating progesterone levels declined (p less than 0.05) between days 3 and 5 postpartum. The weight of the corpus luteum excised from HCG-treated macaques was significantly (p less than 0.005) greater than that of the controls. Dispersed cells from corpora lutea of saline-treated monkeys produced progesterone in vitro under control conditions (nutrient medium alone) and responded to the addition of high (100 ng/ml), but not low (1 ng/ml), levels of HCG with increased steroidogenesis. Although luteal cells from HCG-treated macaques tended to produce more progesterone in vitro than cells from control monkeys, they also exhibited a 50-fold reduction in sensitivity to HCG in vitro. These data suggest that the corpus luteum of lactating postpartum rhesus monkeys exhibited steroidogenic function which was stimulated by exogenous gonadotropin. However, prolonged exposure of the corpus luteum to high levels of exogenous gonadotropin appeared to produce a state of refractoriness to additional gonadotropic stimuli.

In vitro evaluation of corpus luteum function of cycling and pregnant rhesus monkeys: Progesterone Production by dispersed luteal cells

Corpus luteum function in the cycling and the pregnant rhesus monkey (Macaca mulatta) was evaluated through short term in vitro studies of progesterone production by suspensions of collagenase-dispersed luteal cells in the presence and absence of exogenous gonadotropin (human chorionic gonadotropin, HCG). Cells from mid-luteal phase of the menstrual cycle secreted progesterone, as measured by accumulation of this hormone in the incubation medium, and responded to the addition of 100 ng HCG/ml with a marked increase in progesterone secretion significantly above basal level (63.7 +/- 13.1 versus 24.7 +/- 5.5 ng progesterone/ml/5 x 10(4) cells/3 hr, X +/- S.E., n =6 ; p less than 0.05). However, luteal cells from early pregnancy (23-26 days after fertilization) secreted siginificantly less progesterone than cells of the non-fertile menstrual cycle (3.6 +/- 2.4 versus 24.7 +/- 5.5 ng/ml/5 x 10(4) cells/3 hr, n =3 ; p less than 0.05) and did not respond to HCG with enhanced secretion. By mid-pregnancy (108-118 days gestation ) luteal cells exhibited partially renewed function, and near the time of parturition (163-166 days gestation) basal and HCG-stimulated progesterone secretion (30.2 +/- 5.6 and 63.0 +/- 13.0 ng/ml/5 x 10(4) cells/3 hr, respectively; n = 3) was equivalent to that of cells from the luteal phase of the non-fertile menstrual cycle. The data suggest that following a period around the fourth week of gestation, when steroidogenic activity is markedly diminished, the corpus luteum of pregnancy progressively reacquires its functional capacity and at term exhibits gonadotropin-sensitive steroidogenesis similar to that the corpus luteum of the menstrual cycle.

Structural modifications of lutein cell gap junctions during pregnancy in the rat and the mouse

By use of lanthanum tracer and freeze-fracture procedures it was found that granulosa-lutein cells of the pregnant mouse and rat ovaries are connected by gap junctions and septate-like zones of contact. Lutein cell gap junctions enlarge and become partially internalized by the end of the first week of gestation. Expansion of the gap junction domain appears to be due initially to intercalation of particles along borders of small gap junctions devoid of smaller non-junctional particles. The number of gap junction lined processes appearing at the cell border increases concomitantly with hypertrophy of the lutein cell during the second week of pregnancy. Strands of particulate or grooved membrane emanate from the margin of larger gap junctions undergoing interiorization. Most large gap junctions are intimately associated with elements of the smooth endoplasmic reticulum. Spherical gap junctional profiles assume a deeper location in the lutein cell and may form concentric arrays by term while true surface gap junctions appear to fragment in the post-partum corpus luteum. The modifications observed are interpreted with respect to biogenesis of the gap junction and the hormonal control of lutein cell function.