Can luteal regression be reversed?

The corpus luteum is an endocrine gland whose limited lifespan is hormonally programmed. This debate article summarizes findings of our research group that challenge the principle that the end of function of the corpus luteum or luteal regression, once triggered, cannot be reversed. Overturning luteal regression by pharmacological manipulations may be of critical significance in designing strategies to improve fertility efficacy.     

Microvascular endothelial cells of the corpus luteum

The cyclic nature of the capillary bed in the corpus luteum offers a unique experimental model to examine the life cycle of endothelial cells, involving discrete physiologically regulated steps of angiogenesis, blood vessel maturation and blood vessel regression. The granulosa cells and theca cells of the developing antral follicle and the steroidogenic cells of the corpus luteum produce and respond to angiogenic factors and vasoactive peptides. Following ovulation the neovascularization during the early stages of corpus luteum development has been compared to the rapid angiogenesis observed during tumor formation. On the other end of the spectrum, the microvascular endothelial cells are the first cells to undergo apoptosis at the onset of corpus luteum regression. Important insights on the morphology and function of luteal endothelial cells have been gained from a combination of in vitro and in vivo studies on endothelial cells. Endothelial cells communicate with cells comprising the functional unit of the corpus luteum, i.e., other vascular cells, steroidogenic cells, and immune cells. This review is designed to provide an overview of the types of endothelial cells present in the corpus luteum and their involvement in corpus luteum development and regression. Available evidence indicates that microvascular endothelial cells of the corpus luteum are not alike, and may differ during the process of angiogenesis and angioregression. The contributions of vasoactive peptides generated by the luteal endothelin-1 and the renin-angiotensin systems are discussed in context with the function of endothelial cells during corpus luteum formation and regression. The ability of two cytokines, tumor necrosis factor alpha and interferon gamma, are evaluated as paracrine mediators of endothelial cell function during angioregression. Finally, chemokines are discussed as a vital endothelial cell secretory products that contribute to the recruitment of eosinophils and macrophages. The review highlights areas for future investigation of ovarian microvascular endothelial cells. The potential clinical applications of research directed on corpus luteum endothelial cells are intriguing considering reproductive processes in which vascular dysfunctions may play a role such as ovarian failure, polycystic ovary syndrome (PCOS), and ovarian hyperstimulation syndrome (OHSS).     

Cholesterol transport and steroidogenesis by the corpus luteum

The synthesis of progesterone by the corpus luteum is essential for the establishment and maintenance of early pregnancy. Regulation of luteal steroidogenesis can be broken down into three major events; luteinization (i.e., conversion of an ovulatory follicle), luteal regression, and pregnancy induced luteal maintenance/rescue. While the factors that control these events and dictate the final steroid end products are widely varied among different species, the composition of the corpus luteum (luteinized thecal and granulosa cells) and the enzymes and proteins involved in the steroidogenic pathway are relatively similar among all species. The key factors involved in luteal steroidogenesis and several new exciting observations regarding regulation of luteal steroidogenic function are discussed in this review.     

Angiogenesis in the corpus luteum

The corpus luteum (CL) is a site of intense angiogenesis. Within a short period, this is followed either by controlled regression of the microvascular tree in the non-fertile cycle, or maintenance and stabilisation of the new vasculature a conceptual cycle. The molecular regulation of these diverse aspects is examined. The CL provides a unique model system in which to study the cellular and molecular regulation of angiogenesis. Vascular endothelial growth factor (VEGF) has been found to have a major role in the CL. By targeting its action at specific stages of the luteal phase in vivo by antagonists, profound inhibitory effects on luteal angiogenesis and function are observed.     

Corpus luteum size and plasma progesterone concentration in cows

It is often assumed that a larger corpus luteum will produce more progesterone and generate higher circulating plasma concentrations. The aim of the study was to determine whether the size of the corpus luteum does actually determine circulating plasma progesterone concentrations. Data were collated from a number of studies on various aspects of luteal function in non-lactating dairy cows to allow comparisons to be made between corpus luteum weight and plasma progesterone concentration across the luteal phase. In these studies oestrous cycles had been synchronised and animals slaughtered on day 5, day 8 or day 16 following oestrus. Both corpus luteum weight and plasma progesterone concentration increased between day 5 and day 8. Plasma progesterone concentration but not luteal weight also increased between day 8 and day 16. On day 5 there was a strong relationship between corpus luteum weight and plasma progesterone (R² = 0.64; P < 0.001). However, no such relationship was present on day 8 or day 16. These results indicate that while during the early stage of corpus luteum development a relationship between size and progesterone is present, by day 8 of the cycle, the size of the corpus luteum is no longer of importance in determining circulating progesterone concentrations.     

Superoxide dismutase activity, lipid peroxide production and corpus luteum steroidogenesis during natural luteolysis and regression induced by oestradiol deprivation of the ovary in pseudopregnant rabbits.

The relationship of oxygen free radicals to corpus luteum function in rabbits was explored during various stages of pseudopregnancy, including natural and induced luteal regression. Induced luteolysis was achieved during mid-pseudopregnancy by removal of an oestradiol capsule placed at the onset of pseudopregnancy, which suppressed ovarian oestradiol production. Activity of manganese superoxide dismutase (Mn SOD) was significantly and positively correlated with ovarian progesterone production (P < 0.01) throughout pseudopregnancy and during natural regression. Oestradiol deprivation for 12, 24 or 72 h resulted in declines in Mn SOD activity and progesterone secretion, although Mn SOD rose and corpus luteum steroidogenesis was restored to normal when the capsule was replaced for 48 h before assessment, having been removed for 24 h. Lipid peroxide and progesterone concentrations were not correlated, although a significant rise in lipid peroxides in the luteal tissue was detected after deprivation of oestradiol for 72 h. Changes in progesterone production and Mn SOD activity were not associated with alterations in concentration of prostaglandin F metabolite. These data suggest that Mn SOD may be involved in regulating function of the corpus luteum during pseudopregnancy in rabbits and that oxygen free radicals may play a role in regression of corpus luteum in this species.     

Changes in the distribution of tenascin and fibronectin in the mouse ovary during folliculogenesis, atresia, corpus luteum formation and luteolysis

Tenascin and fibronectin are components of the extracellular matrices that oppose and promote adhesion, respectively. Using immunohistochemical techniques, we studied the distribution of tenascin and fibronectin in the mouse ovary, in which dynamic reconstruction and degeneration occur during folliculogenesis, atresia, ovulation, corpus luteum formation and luteolysis. In growing follicles, tenascin was only detected in the theca externa layer, while fibronectin was detected in the theca externa layer, theca interna layer and basement membrane. During follicular atresia, granulosa cells, which are surrounded by the basement membrane, began to die through apoptosis. In atretic follicles, tenascin was detected in the basement membrane and theca externa layer. Distribution of fibronectin in atretic follicles was similar to that in healthy growing follicles, except that granulosa cells were slightly immunopositive for fibronectin. In young corpus luteum, luteal cells exhibit high 3 beta -hydroxysteroid dehydrogenase (3 beta -HSD) activity, an enzyme indispensable for progesterone production. Tenascin was barely detected in young luteal cells. 3 beta -HSD activity in luteal cells declines with corpus luteum age, and in older corpus luteum there is an increase in apoptotic death of luteal cells. Tenascin was intensely immunopositive in old luteal cells.In contrast, fibronectin immunostaining in luteal cells was relatively constant during corpus luteum formation and luteolysis. Our observations suggest that tenascin is critical in controlling the degenerative changes of tissues in mouse ovaries. Moreover, in all circumstances observed in this study, tenascin always co-localized with fibronectin, suggesting fibronectin is indispensable for the function of tenascin.     

Control of steroidogenesis in small and large bovine luteal cells

Evidence was cited to show that: (1) prostacyclin (PGI2) plays a luteotrophic role in the bovine corpus luteum and that products of the lipoxygenase pathway of arachidonic acid metabolism, especially 5-hydroxyeicosatetraenoic acid play luteolytic roles; (2) oxytocin of luteal cell origin plays a role in development, and possibly in regression, of the bovine corpus luteum; and (3) luteal cells arise from two sources; the characteristic small luteal cells at all stages of the oestrous cycle and pregnancy are of theca cell origin; the large cells are of granulosa cell origin early in the cycle, but a population of theca-derived large cells appears later in the cycle. Results of in vitro studies with total dispersed cells and essentially pure preparations of large and small luteal cells indicate that: (1) the recently described Ca2+-polyphosphoinositol-protein kinase C second messenger system is involved in progesterone synthesis in the bovine corpus luteum; (2) activation of protein kinase C is stimulatory to progesterone synthesis in the small luteal cells; (3) activation of protein kinase C has no effect on progesterone synthesis in the large luteal cells; and (4) protein kinase C exerts its luteotrophic effect in total cell preparations, in part at least, by stimulating the production of prostacyclin. The protein kinase C system may cause down regulation of LH receptors in the large cells.     

Prostaglandin F and progesterone secretion by porcine endometrium and corpus luteum in vitro

Slices of porcine endometrium and corpus luteum tissue obtained from mature sows throughout the luteal phase of the oestrous cycle were incubated in culture medium which was analysed at regular intervals over a period of 8 hours for prostaglandin F and progesterone. Prostaglandin F secretion was greatest by endometrium obtained during the mid III to late I luteal stage of the cycle and the increased levels secreted by this tissue were paralleled by high levels of secretion from corpus luteum tissue. The addition of indomethacin (10 μg/ml) to the culture medium completely abolished prostaglandin F secretion by both endometrium and luteal tissue indicating that the high levels of the prostaglandin were due to synthesis. Progesterone secretion by the corpus luteum was maximal from early luteal tissue and had declined to considerably lower levels by late stage tissue when prostaglandin secretion was greatest. The possible physiological significance of luteal prostaglandin F secretion is discussed.     

Tumor necrosis factor production and accumulation of inflammatory cells in the corpus luteum of pseudopregnancy and pregnancy in rabbits

The potential involvement of macrophages, T lymphocytes, and the cytokine tumor necrosis factor (TNF) in regression of the corpus luteum was investigated at different stages of pseudopregnancy and pregnancy by use of immunocytochemical methods and a TNF bioassay. Few macrophages (11 +/- 6 per high power field of 8-microns frozen sections of corpus luteum, Day 10 of pseudopregnancy) were observed until the very end of pseudopregnancy, when the number of macrophages increased greatly (176 +/- 42 per high power field, Day 19 of pseudopregnancy). Pregnancy, of 32 days duration, delayed large-scale macrophage accumulation until 3 days after parturition (154 +/- 30 per high power field). Low TNF activity (approximately 1.0 U/mg protein) was detected in incubations of luteal tissue at all stages; in response to lipopolysaccharide, TNF values in medium increased 10- to 30-fold at times of luteal regression and macrophage accumulation (1 day postpartum and Day 19 of pseudopregnancy). Class II-positive T lymphocytes were observed in luteal tissue, but unlike macrophages, the number of lymphocytes did not increase at the time of regression of the corpus luteum. These data are consistent with the hypothesis that involution of the corpus luteum is promoted through the interactions of inflammatory cells and action of TNF, although the action of TNF has not been determined in this luteal tissue. Through unknown mechanisms, pregnancy postpones the accumulation of macrophages in the corpus luteum, in association with the prolongation of luteal function until the time of parturition.     

Plasma progesterone and luteinizing hormone concentrations and the role of the corpus luteum and LH gonadotrophs in the control of delayed implantation in Schreibers' long-fingered bat (Miniopterus schreibersii)

In Schreibers' long-fingered bat from South Africa (approximately 33 degrees S) copulation, ovulation and fertilization occurred in April and May, implantation was delayed until August, and parturition occurred in December. Delayed implantation coincided with winter, during which the bats remained active, only entering prolonged periods of torpor during particularly cold spells. Plasma progesterone concentration was low during non-pregnancy (1.54 ng/ml) and during delayed implantation (1.67 ng/ml), and thereafter increased to reach a peak mean of 64.82 ng/ml in late pregnancy. Changes in size and ultrastructure of the luteal cells indicated periods of steroidogenesis just after formation of the corpus luteum, and for about 2 months after implantation; reduced steroidogenic activity during delayed implantation; and luteolysis in the last 2 months of pregnancy. Plasma luteinizing hormone (LH) concentration and pituitary LH-beta immunoreactivity were highest during follicular development and peaked just before ovulation. During early delayed implantation, plasma LH concentration was low, and both plasma LH and pituitary LH-beta immunoreactivity increased from July, reaching peaks in late pregnancy. LH may be required to activate the corpus luteum and terminate delayed implantation, or, as in some small carnivores, it may be required for luteal maintenance.     

Endocrine and local control of the primate corpus luteum

The primate corpus luteum is a transient endocrine gland that differentiates from the ovulatory follicle midway through the ovarian (menstrual) cycle. Its formation and limited lifespan is critical for fertility, as luteal-derived progesterone is the essential steroid hormone required for embryo implantation and maintenance of intra-uterine pregnancy until the placenta develops. It is well-established that LH and the LH-like hormone, CG, are the vital luteotropic hormones during the menstrual cycle and early pregnancy, respectively. Recent advances, particularly through genome analyses and cellular studies, increased our understanding of various local factors and cellular processes associated with the development, maintenance and repression of the corpus luteum. These include paracrine or autocrine factors associated with angiogenesis (e.g., VEGF), and that mediate LH/CG actions (e.g., progesterone), or counteract luteotropic effects (i.e., local luteolysis; e.g., PGF_2α). However, areas of mystery and controversy remain, particularly regarding the signals and events that initiate luteal regression in the non-fecund cycle. Novel approaches capable of gene “knockdown” or amplification”, in vivo as well as in vitro, should identify novel or underappreciated gene products that are regulated by or modulate LH/CG actions to control the functional lifespan of the primate corpus luteum. Further advances in our understanding of luteal physiology will help to improve or control fertility for purposes ranging from preservation of endangered primate species to designing novel ovary-based contraceptives and treating ovarian disorders in women.     

GnRH action on luteal steroidogenesis during pregnancy.

The results of our study presented here establishes that gonadotropin-releasing hormone (GnRH) acts directly on the corpus luteum, leading to suppressed production and release of progesterone and thus disrupting pregnancy. A GnRH-agonist (GnRH-Ag) treatment suppressed the luteal and serum progesterone levels. This suppression is neither mediated by a fall in ovarian testosterone production nor its conversion to estradiol. Although the treatment suppressed the nuclear estradiol-receptor content and binding sites for LH in the corpus luteum, it had no effect on the luteal binding sites for GnRH and prolactin within 24 h. GnRH-Ag augmented the plasma levels of luteinizing hormone, decreased the magnitude of nocturnal surges of prolactin, and had no effect on luteal cyclic adenosine 5'-monotriphosphate levels. Yet, the treatment had no effect on the luteal content of free cholesterol. We have also demonstrated, for the first time, the presence of steroidogenic acute regulatory protein and peripheral benzodiazepine receptor in the rat corpus luteum, and the suppression of these proteins by GnRH-Ag leads to reduced steroidogenesis by the corpus luteum. Concomitantly, P450 side-chain cleavage enzyme, its activity, and its mRNA content and 3beta-hydroxy-steroid dehydrogenase content in the corpus luteum decreased. The treatment suppressed the plasma levels of pregnenolone and 20alpha-dihydroprogesterone. These data suggest that the suppression of luteal steroidogenesis by GnRH-Ag may be due to its inhibitory effect on the cholesterol transport and/or on the enzymes involved in the steroidogenic pathway. Furthermore, based on other observations made in our laboratory, we propose a hypothesis that an endogenous GnRH is present in the corpus luteum/ovary during pregnancy in the rat and that this GnRH may play a physiological role in the regulation, maintenance, and/or termination of pregnancy.     

Low serum levels of FSH, LH and prolactin in luteal phase inadequacy

In order to elucidate the relationship between prolactin (PRL) levels and corpus luteum function in humans, assessment of temporal relationship between levels of PRL, LH, FSH, estradiol and progesterone was made in eleven normal cycling women and six short luteal women. All hormones were determined by specific radioimmunoassay. The mean PRL level in the luteal phase was higher than that in the follicular phase in normal women. On the other hand, no difference mean was seen between the PRL levels of follicular and luteal phases in short luteal women. In addition, follicular and luteal phase secretion of PRL in the short luteal phase (SLP) was lower than that in the normal control. LH and FSH in the follicular and luteal phases, estradiol secretion in the late follicular and early to mid-luteal phases in SLP were also lower than those in the control. These observations were consistent with the hypothesis that SLP is a sequel to aberrant folliculogenesis. In addition, it is inferred that low PRL levels in the SLP might be due to inadequate augmentation by estrogen, rather than giving PRL any positive controlling role in the maintenance of corpus luteum function.     

Hammond memorial lecture. New concepts of the control of corpus luteum function

Five new concepts concerning the control of corpus luteum function in the cow have been developed in recent years. Prostacyclin (PGI-2) plays a luteotrophic role. Conversely, products of the lipoxygenase pathway of arachidonic acid metabolism, particularly 5 hydroxyeicosatetraenoic acid (5-HETE), play luteolytic roles. Luteal cells arise from two sources. The small luteal cells are all of theca cell origin; the large cells found early in the cycle (Days 2-6) are mainly of granulosa cell origin. However, a population of large cells found after Day 10 of the cycle are of theca cell origin. Oxytocin of luteal cell origin plays a role in development of the corpus luteum and possibly in its regression. The recently described Ca2+-polyphosphoinositol-protein kinase C second messenger system, as well as the LH-cAMP system, is involved in control of progesterone synthesis in the bovine corpus luteum. Progesterone synthesis in the small theca-derived luteal cells is primarily controlled by the cAMP system. However, elevated intracellular calcium diminishes cAMP-mediated progesterone synthesis in these cells. These findings modify our current concepts of the mechanisms of control of progesterone secretion by the corpus luteum and suggest several new lines of research.     

Single nucleotide polymorphisms in genes that are common targets of luteotropin and luteolysin in primate corpus luteum: Computational exploration

Luteal insufficiency affects fertility and hence study of mechanisms that regulate corpus luteum (CL) function is of prime importance to overcome infertility problems. Exploration of human genome sequence has helped to study the frequency of single nucleotide polymorphisms (SNPs). Clinical benefits of screening SNPs in infertility are being recognized well in recent times. Examining SNPs in genes associated with maintenance and regression of CL may help to understand unexplained luteal insufficiency and related infertility. Publicly available microarray gene expression databases reveal the global gene expression patterns in primate CL during the different functional state. We intend to explore computationally the deleterious SNPs of human genes reported to be common targets of luteolysin and luteotropin in primate CL. Different computational algorithms were used to dissect out the functional significance of SNPs in the luteinizing hormone sensitive genes. The results raise the possibility that screening for SNPs might be integrated to evaluate luteal insufficiency associated with human female infertility for future studies.     

Rescue of the corpus luteum and an increase in luteal superoxide dismutase expression induced by placental luteotropins in the rat: action of testosterone without conversion to estrogen

The superoxide radical and its scavenger, superoxide dismutase (SOD), play important roles in the regulation of corpus luteum function. The present study was undertaken to investigate whether SOD is related to pregnancy-induced maintenance of corpus luteum function. Placentae obtained from rats on Day 12 of pregnancy were incubated for 24 h, and the supernatant was used as placental luteotropins. Pseudopregnant rats were given the placental incubation medium from Day 9 to Day 12 of pseudopregnancy. The treatment significantly increased serum progesterone concentrations on Day 12 of pseudopregnancy. Both activities and mRNA levels of copper-zinc SOD (Cu,Zn-SOD) and manganese SOD (Mn-SOD) in the corpus luteum were also increased on Day 12 of pseudopregnancy. Treating the placental incubation medium with charcoal significantly eliminated the stimulatory effects of placental incubation medium on serum progesterone concentrations and luteal Mn-SOD expression, but not on Cu,Zn-SOD expression. The inhibitory effect of the charcoal treatment on luteal Mn-SOD expression was reversed by supplementation with testosterone or dihydrotestosterone (DHT), but serum progesterone concentrations were recovered only by DHT. Testosterone or DHT alone had no effect on serum progesterone concentrations and luteal SOD expression. In conclusion, placental luteotropins increased SOD expression in the corpus luteum and stimulated progesterone production, suggesting that SOD is involved in the maintenance of the corpus luteum function by placental luteotropins. In addition, androgen, with other placental luteotropins, acted to stimulate progesterone production and Mn-SOD expression in pseudopregnant rats.     

Morphological changes in the corpus luteum of tuatara (Sphenodon punctatus) during gravidity

Tuatara (Sphenodon spp.) are rare reptiles, members of the reptilian order Sphenodontida, inhabiting small offshore islands of New Zealand. Females usually require about three years to yolk a clutch of eggs followed by an 8-month period of in utero egg shelling. As in other vertebrates, the post-ovulatory follicle forms a transitory endocrine structure, the corpus luteum. The tuatara Sphenodon punctatus exhibits a corpus luteum having several unusual morphological features as compared to turtles and squamate reptiles. Like the crocodilians, the tuatara has a corpus luteum in which the luteal cell mass never fills the central cavity and in which the thecal fibroblasts do not close the ovulation aperture. As in all oviparous reptiles examined, however, the corpus luteum appears to persist throughout gravidity based on its histological appearance. During gravidity, plasma progesterone concentrations are detectable, even though gravidity lasts an exceptionally long time (8 months) for an oviparous species. Luteolysis is initiated within two months following oviposition. The initial stages of luteolysis appear rapid, but luteal scar tissue is apparent in the ovaries of all adult females we examined and probably persists for many years post-oviposition. J Morphol 232:79–91, 1997. © 1997 Wiley-Liss, Inc.     

Gelatinases, endonuclease and Vascular Endothelial Growth Factor during development and regression of swine luteal tissue

Background  

The development and regression of corpus luteum (CL) is characterized by an intense angiogenesis and angioregression accompanied by luteal tissue and extracellular matrix (ECM) remodelling. Vascular Endothelial Growth Factor (VEGF) is the main regulator of angiogenesis, promoting endothelial cell mitosis and differentiation. After the formation of neovascular tubes, the remodelling of ECM is essential for the correct development of CL, particularly by the action of specific class of proteolytic enzymes known as matrix metalloproteinases (MMPs). During luteal regression, characterized by an apoptotic process and successively by an intense ECM and luteal degradation, the activation of Ca⁺⁺/Mg⁺⁺-dependent endonucleases and MMPs activity are required. The levels of expression and activity of VEGF, MMP-2 and -9, and Ca⁺⁺/Mg⁺⁺-dependent endonucleases throughout the oestrous cycle and at pregnancy were analyzed.     

Ovarian oxytocin and the maternal recognition of pregnancy

The secretion of oxytocin by the corpus luteum is thought to stimulate the episodic release of PGF-2 alpha from the uterus, thereby contributing to luteolysis. In pregnancy corpus luteum function is maintained, and secretion of oxytocin, or its actions on the uterus, appear to be inconsistent with the successful establishment of gestation. Protection against the effects of oxytocin is ensured by a number of mechanisms, including the cessation of luteal oxytocin secretion, which is evident by Day 20 after mating in sheep, and the maintenance of low levels of the oxytocin receptor in the uterus.