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Much of our understanding of the molecular control of menstruation arises from laboratory models that experimentally recapitulate some, but not all, aspects of uterine bleeding observed in women. These models include: in vitro culture of endometrial explants or isolated endometrial cells, transplantation of human endometrial tissue into immunodeficient mice and the induction of endometrial breakdown in appropriately pretreated mice. Each of these models has contributed to our understanding of molecular and cellular mechanisms of menstruation, but nonhuman primates, especially macaques, are the animal model of choice for evaluating therapies for menstrual disorders. In this chapter we review some basic aspects of menstruation, with special emphasis on the macaque model and its relevance to the clinical issues of irregular and heavy menstrual bleeding (HMB).

Background: Ulipristal (UPA; CDB-2914) is a progesterone receptor modulator with contraceptive potential. To test its effects when delivered by an intrauterine system (IUS), we prepared control and UPA-filled IUS and evaluated their effects in rhesus macaques., Study Design: Short lengths of Silastic tubing either empty (n=3) or containing UPA (n=5) were inserted into the uteri of 8 ovariectomized macaques. Animals were cycled by sequential treatment with estradiol and progesterone. After 3.5 cycles, the uterus was removed., Results: During treatment, animals with an empty IUS menstruated for a mean total of 11.66+/-0.88 days, while UPA-IUS treated animals bled for only 1+/-0.45 days. Indices of endometrial proliferation were significantly reduced by UPA-IUS treatment. The UPA exposed endometria were atrophied with some glandular cysts while the blank controls displayed a proliferative morphology without cysts. Androgen receptors were more intensely stained in the glands of the UPA-IUS treated endometria than in the blank-IUS treated controls., Conclusions: In rhesus macaques, a UPA-IUS induced endometrial atrophy and amenorrhea. The work provides proof of principle that an IUS can deliver effective intrauterine concentrations of Ulipristal., (Copyright 2010 Elsevier Inc. All rights reserved.)

Despite extensive literature on vascular endothelial growth factor (VEGF) expression and regulation by steroid hormones, the lack of clear understanding of the mechanisms of angiogenesis in the endometrium is a major limitation for use of antiangiogenic therapy targeting endometrial vessels. In the current work, we used the rhesus macaque as a primate model and the decidualized mouse uterus as a murine model to examine angiogenesis during endometrial breakdown and regeneration. We found that blockade of VEGF action with VEGF Trap, a potent VEGF blocker, completely inhibited neovascularization during endometrial regeneration in both models but had no marked effect on preexisting or newly formed vessels, suggesting that VEGF is essential for neoangiogenesis but not survival of mature vessels in this vascular bed. Blockade of VEGF also blocked reepithelialization in both the postmenstrual endometrium and the mouse uterus after decidual breakdown, evidence that VEGF has pleiotropic effects in the endometrium. In vitro studies with a scratch wound assay showed that the migration of luminal epithelial cells during repair involved signaling through VEGF receptor 2-neuropilin 1 (VEGFR2-NP1) receptors on endometrial stromal cells. The leading front of tissue growth during endometrial repair was strongly hypoxic, and this hypoxia was the local stimulus for VEGF expression and angiogenesis in this tissue. In summary, we provide novel experimental data indicating that VEGF is essential for endometrial neoangiogenesis during postmenstrual/postpartum repair.

Background: Fibronectin (FN) is a component of the extracellular matrix that participates in wound healing in various tissues as an adhesive ligand for integrins (Itgs). To determine whether these molecules play similar roles during menstrual repair, we evaluated the expression and localization of FN and specific Itgs in the primate endometrium under hormonally controlled conditions., Methods: Ovariectomized rhesus macaques were treated for 2 weeks with estradiol (E(2)) followed by E(2) with progesterone for 2 weeks. On day 28, progesterone was withdrawn and uteri were collected during menstruation, postmenstrual repair, and the proliferative and secretory phases. Analysis was by focused microarray, real time PCR, in situ hybridization and immunocytochemistry., Results: Progesterone withdrawal induced significant elevations of FN, Itg alpha5 and Itg beta1 transcripts during menstruation as compared to day 28 (FN: P < 0.01; Itg alpha5: P < 0.05; Itg beta1: P < 0.05; real time PCR). These increases were concentrated in the glandular epithelium (FN) and stroma (Itg alpha5beta1) of the uppermost zones. Cyclic changes in Itg alpha3 occurred in the glandular epithelium., Conclusions: Spatially and temporally restricted peaks of expression of FN and its Itg receptors are closely correllated with menstruation and postmenstrual repair in the primate endometrium.

Vervet monkeys (Chlorocebus aethiops) are Old World nonhumans that display attenuated menstruation that requires detection by vaginal swab. The physiology underlying attenuated menstruation in this species has not been previously studied. To fill this gap, we evaluated endometrial cell proliferation, steroid receptor localization and expression of menstruation-associated matrix metalloproteinase (MMP) enzymes in vervets during natural and artificial menstrual cycles. The artificial cycles were induced by sequentially treating ovariectomized animals with estradiol (E(2)) and progesterone (P). Because menstrual flow is exceptionally light in this species, menses was detected by vaginal swab. We found that both natural and artificially cycled animals menstruated 3-5 days after the decline of P at the end of the cycle. As in other primates, P withdrawal at the end of artificial cycles triggered endometrial expression of MMPs, including MMP-1, 2, 3, 7, 10, 11, 13 and 26 transcripts. In both the natural and artificial menstrual cycle, menstrual sloughing was restricted to the upper one-fourth of the endometrium, and MMP-1 and 2 were strongly expressed by the stroma of the sloughing zone. MMP-7 was localized in the endometrial glands during late menses. As in macaques, epithelial cell proliferation was localized to the functionalis zone during the estrogen-dominated proliferative phase and to the basalis zone glands during the P-dominated secretory phase. Regulation of estrogen and progestin (or estradiol and progesterone) receptors was similar to that reported for macaques. Because strong similarities exist between the endometrium of vervets, macaques and women, we conclude that vervets can provide a useful animal model for studies on hormone regulation of menstruation.

Macaques are menstruating nonhuman primates that provide important animal models for studies of hormonal regulation in the uterus. In women and macaques the decline of progesterone (P) at the end of the cycle triggers endometrial expression of a variety of matrix metalloproteinase (MMP) enzymes that participate in tissue breakdown and menstrual sloughing. To determine the minimal duration of P withdrawal required to induce menses, we assessed the effects of adding P back at various time points after P withdrawal on both frank bleeding patterns and endometrial MMP expression. Artificial menstrual cycles were induced by treating the animals sequentially with implants releasing estradiol (E2) and progesterone (P). To assess bleeding patterns, P implants were removed at the end of a cycle and then added back at 12, 24, 30, 36, 40, 48, 60, or 72 hours (h) after the initial P withdrawal. Observational analysis of frank bleeding patterns showed that P replacement at 12 and 24 h blocked menses, replacement at 36 h reduced menses but replacement after 36 h failed to block menses. These data indicate that in macaques, a critical period of P withdrawal exists and lasts approximately 36 h. In other similarly cycled animals, we withdrew P and then added P back either during (12-24 h) or after (48 h) the critical period, removed the uterus 24 h after P add back and evaluated endometrial MMP expression. Immunocytochemistry showed that replacement of P during the critical period suppressed MMP-1, -2 and -3 expression along with menses, but replacement of P at 48 h, which failed to suppress mense, suppressed MMP-1 and MMP-3 but did not block MMP-2. We concluded that upregulation of MMPs is essential to menses induction, but that after the critical period, menses will occur even if some MMPs are experimentally blocked.

According to current theory, endometriosis is initiated during retrograde menstruation when menstrual fragments flow out of the fimbriated end of the fallopian tubes and become established on the ovarian surface or other sites in the peritoneal cavity. In recent years, new data have accumulated on the properties of menstruating tissue itself, and several laboratories agree that this tissue is rich in matrix metalloproteinases (MMPs) that may facilitate endometriotic implantation. Recently, we found that vascular endothelial growth factor (VEGF) and its receptor VEGFR-2 (KDR) were dramatically upregulated in the stromal cells of the superficial endometrial zones by progesterone (P) withdrawal during the premenstrual phase. A unique role of VEGF at this stage of the cycle may be to stimulate MMP expression in stromal cells because VEGF, KDR, and MMPs were all coordinately induced in these cells in the superficial zone of the primate endometrium by P withdrawal. The rich content of MMPs and VEGF in the menstrual fragments could facilitate attachment and angiogenesis of menstrual fragments in ectopic sites. In addition, a variety of chemokines, cytokines, and cellular regulators are induced by P withdrawal in the premenstrual human endometrium. These include NFKB, prostaglandins, interleukin-8 (IL-8), cyclooxygenase-2 (COX-2), and monocyte chemotactic peptide-1 (MCP-1), among others. The perivascular expression of several of these factors may facilitate the rapid invasion of leukocytes into the endometrium, especially in the superficial zones. Consequently, menstrual fragments may be rich in IL-8 and MCP-1, both of which would add to the angiogenic potential of such fragments in ectopic sites. In sum, menstrual tissue is rich in VEGF, KDR, MMPs, leukocytes, chemokines, cytokines, and prostaglandins, all factors that may facilitate attachment and angiogenesis when menstrual fragments exit from the tubes and implant on pelvic sites. Additional research on these and other factors in premenstrual and menstrual endometrium may deepen our understanding of both the establishment and progression of this debilitating disease.

The human endometrium displays characteristic features, both structural and functional, across the menstrual cycle. It is the sex steroid hormones, oestrogen and progesterone, that drive the endometrium through the different phases of the cycle. Oestrogen and progesterone act sequentially to regulate cellular concentrations of their respective receptors, this interaction initiates gene transcription. Thereafter a cascade of local events prepares the endometrium for implantation, but in the absence of pregnancy, progesterone withdrawal leads to menstruation and cyclic repair. Withdrawal of progesterone from an oestrogen-progesterone primed endometrium is the initiating event for the cascade of molecular and cellular interactions that result in menstruation. Progesterone withdrawal first affects cells with progesterone receptors. Early events in the menstrual process are vasoconstriction and cytokine up-regulation. The activation of lytic mechanisms is a later event and involves cells that may lack progesterone receptors, for example, uterine leucocytes and epithelial cells. Hence progesterone withdrawal results in a local increase of inflammatory mediators and the enzymes responsible for tissue breakdown. The total complex of local factors implicated in normal menstrual and aberrant menstrual bleeding are yet to be fully defined.

Background: A reliable means of menstrual suppression would greatly improve the quality of life for women. Information is lacking on the direct endometrial effects and appropriate dosages of new antiprogestins that may be useful for this purpose., Methods: The current work evaluated three different systems in macaque monkeys. First, the range of doses of two relatively new antiprogestins, ZK 137 316 and ZK 230 211, that would block progesterone action directly on the endometrium in artificially cycled, spayed rhesus macaques; second, the direct endometrial effects of ZK 230 211, a type III antiprogestin; and third, investigation of whether endometrial-suppressive doses administered chronically to intact, cycling monkeys could be used for reversible, menstrual suppression., Results: The results in naturally cycling animals showed that ZK 137 316 blocked menstruation in all animals, but doses of 0.05 mg/kg blocked ovulation in 55.5% of animals and doses of 0.1 mg/kg blocked ovulation in 66.6% of the animals. However, all doses of ZK 230 211 that blocked menstruation also blocked ovulation. All progesterone antagonist (PA)-treated animals, regardless of dose, maintained normal follicular phase concentrations of oestradiol and returned to normal menstrual cyclicity within 15--41 days post-treatment. Therefore ZK 137 316, depending on dose, can allow ovulation but block menstruation, while ZK 230 211, a much more potent PA, blocks both ovulation and menstruation at all effective doses. Both PAs block unopposed oestrogenic action on the endometrium through their antiproliferative effects., Conclusions: Reversible amenorrhoea can be achieved with these two PAs, and they can protect the endometrium from the effects of unopposed oestrogen whether or not ovulation is blocked. Chronic, low dose PA treatment may provide a new option for women who wish to suppress their menstrual periods.

Several reports indicate that vascular endothelial growth factor (VEGF) expression is increased in endometrial glands and stroma during the menstrual phase in the human endometrium. Here we report that VEGF receptor type 2 (KDR), normally expressed only in the vascular endothelium, was dramatically up-regulated in the stromal cells of the superficial endometrial zones during the premenstrual phase in both human and macaque endometrium. This increase was detectable by Northern analysis, in situ hybridization, and immunocytochemistry and was cell specific, zone specific, cycle phase specific, and VEGF receptor type specific. That is, it only occurred during the premenstrual/menstrual phase, did not occur in glandular epithelium, endothelium, or stromal cells of the deepest endometrial zones, and was not observed for VEGF receptor type 1. The upregulation of stromal KDR was induced by progesterone (P) withdrawal in both women and macaques, and adding back P 24 h after P withdrawal in macaques blocked stromal, but not vascular, endothelial KDR expression. Promatrix metalloproteinase-1 (MMP-1) was coordinately up-regulated in the same stromal cell population by P withdrawal. Because of reports that VEGF can enhance MMP expression, we hypothesize that VEGF-KDR interactions may influence MMP expression in the superficial zones of the primate endometrium during the premenstrual phase, and that these interactions play a role in the induction of menstruation.

We sampled oviducts and endometria of 27 cynomolgus macaques during the menstrual cycle and measured the concentration of nuclear and cytoplasmic estrogen receptors in these tissues by exchange assay. We assessed the stage of the cycle by correlating serum estradiol (E2) and progesterone (P), as measured by radioimmunoassay, with the morphological condition of the ovaries, oviducts and endometrium of each animal. We have previously identified a series of oviductal stages that reflected the orderly sequence of cytological changes in the oviduct during the cycle, and we normalized receptor measurements to these stages. The amounts of nuclear and cytoplasmic estrogen receptor in both the oviduct and the endometrium were approximately twofold greater in the follicular phase than in the luteal phase. In the follicular phase, elevated receptor levels were associated with oviductal proliferation and differentiation, as well as with endometrial proliferation. During the luteal phase, lowered levels were correlated with atrophy and dedifferentiation in the oviduct, but with hypertrophy and progestational development in the endometrium. When the luteal phase of one cycle ends and the follicular phase of the next begins, it is a decline in serum P, not a rise in serum E2, that precedes the elevation in estrogen receptor level and the onset of proliferation in the oviduct and endometrium. Proliferation of the reproductive tract and elevations in nuclear estrogen receptor levels during the early follicular phase can therefore be viewed as consequences of the release of the system from antagonism by P.

We sampled the reproductive tracts of 27 cynomolgus macaques during the menstrual cycle and correlated the cytologic changes in the oviductal epithelium with changes in the serum levels of estradiol (E2) and progesterone (P) and with the histology of the ovaries and the endometria. We identified an orderly sequence of changes in the oviductal epithelium from the early follicular to the late luteal phase, and we classified this sequence into eight stages, named as follows: preciliogenic, ciliogenic, ciliogenic-ciliated, ciliated-ciliogenic, ciliated-secretory, early regression, late regression and full regression. The preciliogenic and ciliogenic phases were coincident with menses and the early follicular phase. The ciliogenic-ciliated, ciliated-ciliogenic and ciliated-secretory phases during which the oviductal epithelium became progressively more differentiated were coincident, respectively, with the midfollicular, late follicular and periovulatory phases of the cycle. The early, late and full regression stages during which the epithelium became progressively more atrophied, deciliated and nonsecretory were coincident, respectively, with the early, mid and late luteal phases of the cycle. The cyclic changes in the endometrium of cynomolgus macaques were similar to those reported for the rhesus macaque.

We examined changes in estrogen receptors (ERs) in endometrial stromal and epithelial cells in cynomolgus macaques during artificially induced menstruation and repair. We used Silastic implants filled with either estradiol (E2) or progesterone (P) to treat spayed animals for 14 days with E2 followed by 14 days of E2 plus P. We then withdrew the P (but not the E2) implants and removed uteri 0, 0.5, 1, 2, 3, 4, 5, 7, and 14 days later. Uterine tissues were assayed biochemically for ER content, fixed for histology and frozen for immunocytochemistry of ER with monoclonal antiestrophilins. On day 0, ER levels in endometrium were low [1330 +/- 201 (n = 9) fmol/mg DNA]. An increase in total receptor was evident by 3-4 days of P withdrawal 2762 +/- 190 (n = 6) fmol/mg DNA; P less than 0.001]. Total receptor concentrations increased linearly with time from 0.5-7 days of P withdrawal (r = 0.88). On day 0, staining for nuclear ER in the glandular epithelium and stroma of zones I, II, and III of the endometrium was negative. Beginning at 12-24 h and continuing through 4 days of P withdrawal, nuclear staining became detectable and increased in intensity only in endometrial stromal fibroblasts and myometrial smooth muscle cells. The glandular epithelium of the endometrium did not develop nuclear staining until 5-7 days of P withdrawal, coincident with a 10-fold increase in the mitotic index of the epithelium of the upper zones. Thus, the increase in endometrial ER levels that occurred during the first 5 days of an induced luteal-follicular transition took place almost exclusively in stromal fibroblasts.