A novel in vitro experimental model for ovarian endometriosis: the three-dimensional culture of human ovarian surface epithelial cells in collagen gels

Human Female Reproductive System Organoids: Applications in Developmental Biology, Disease Modelling, and Drug Discovery

Organoid technique has achieved significant progress in recent years, owing to the rapid development of the three-dimensional (3D) culture techniques in adult stem cells (ASCs) and pluripotent stem cells (PSCs) that are capable of self-renewal and induced differentiation. However, our understanding of human female reproductive system organoids is in its infancy. Recently, scientists have established self-organizing 3D organoids for human endometrium, fallopian tubes, oocyte, and trophoblasts by culturing stem cells with a cocktail of cytokines in a 3D scaffold. These organoids express multicellular biomarkers and show functional characteristics similar to those of their origin organs, which provide potential avenues to explore reproductive system development, disease modelling, and patient-specific therapy. Nevertheless, advanced culture methods, such as co-culture system, 3D bioprinting and organoid-on-a-chip technology, remain to be explored, and more efforts should be made for further elucidation of cell-cell crosstalk. This review describes the development and applications of human female reproductive system organoids. Graphical abstract Figure: Applications in developmental biology, disease modelling, and drug discovery of human female reproductive system organoids. ASCs: adult stem cells; PSCs: pluripotent stem cells.

In-vitro models of human endometriosis

Endometriosis is one of the most common benign gynecological diseases in women of reproductive age worldwide. In past decades, a number of in-vitro models have been used to investigate the pathology and therapeutic methods for the treatment of endometriosis. The current review summarized the majority of currently available in-vitro models, which utilize a variety of cell or tissues types, including endometriotic cell lines, primary endometrial stromal cells, endometrial stem cells, endometrial explants, peritoneal explants and immune cells. These cells or tissues are cultured individually, co-cultured in 2D or 3D systems with various matrices or cultured in chicken chorioallantotic membranes and amniotic membranes culture systems. These models are able to represent one or more aspects of the process of endometriosis. These models are helpful and can be used to investigate the development of endometriosis and the underlying mechanisms of this disorder in detail, and help investigators select appropriate models for their experiments. Recently, the new concept of endometriosis as a fibrotic condition will lead research to investigate the differentiation of myofibroblasts and the development of fibrosis in endometriotic lesions, which will increase the development of novel models that can be used to investigate endometriotic fibrosis.

Paired-box gene 2 is down-regulated in endometriosis and correlates with low epidermal growth factor receptor expression

BACKGROUND

Paired-box 2 (Pax2) is involved in the development of the female genital tract and has been associated with endometrial pathologies. The expression of Pax2 is induced by epidermal growth factor (EGF) and estrogens. In the present study, Pax2 expression and regulation were investigated in endometriosis.

METHODS AND RESULTS

Pax2 protein expression was assessed by immunohistochemistry in the eutopic (i.e. inside the uterus) and ectopic tissue (endometriosis) from 11 patients. Immunoreactivity was high in the endometrium, with strong epithelial and weaker stromal staining. Similar expression patterns of Pax2 were observed in the endometrium of women without endometriosis (n = 12). The mRNA level of Pax2 was assessed by real-time PCR in the eutopic and ectopic endometria of 14 patients and in the endometrium from women without endometriosis (n = 20). Pax2 expression was lower in endometriotic lesions than that in the eutopic endometrium of patients (P< 0.001) and controls (P= 0.007). Three possible mechanisms determining low Pax2 expression were investigated: EGF signalling, CpG DNA methylation of the Pax2 promoter and steroid response. The mRNA level of the EGF receptor (EGFR1) was assessed in the samples used for Pax2 mRNA assessment. A significant correlation between EGFR1 and Pax2 in both eutopic and ectopic tissues was observed (R = 0.58; slope regression line, 0.81; 95% CI: 0.09-1.52 and R = 0.54; slope regression line, 2.51; 95% CI: 0.02-4.99, respectively). CpG DNA methylation was analyzed by methyl-specific PCR in two regions of the Pax2 promoter but they were unmethylated in all samples. Steroid responsiveness was assessed using endometrial explant cultures and Pax2 was not regulated by either 17β-estradiol or progesterone.

CONCLUSIONS

In endometriosis patients, Pax2 is down-regulated in the lesions compared with the eutopic tissue, possibly due to low EGF signalling.

Immunological regulation of Chinese herb Guizhi Fuling Capsule on rat endometriosis model

AIM OF THE STUDY

To investigate the immunological regulation of Guizhi Fuling Capsule (GZFLC) on rat endometriosis.

MATERIALS AND METHODS

Twenty-seven rats, in which endometriotic implants were induced by transplanting autologous uterine tissue to the peritoneum, were randomly divided into three groups equally: (1) the GZFLC group of low dose (480 mg/kg/day); (2) the GZFLC group of high dose (1,920 mg/kg/day); and (3) the model group(saline solution). Another 10 rats were treated as sham operation group. After rats were treated for four weeks, we examined the alterations of implants volume, the percentage of CD4(+) T lympholeukocyte, the activity of NK cell and the expression of cytokines (MCP-1 and ICAM-1) on each group.

RESULTS

Statistical analysis showed that posttreatment volumes were significantly reduced compared with pretreatment in GZFLC groups, whereas there was no significant change in the model group. The percentage of CD4(+) T lympholeukocyte and the activity of NK cell in GZFLC groups significantly increased to the level of the sham group compared with the model. RT-PCR and immunohistochemistry showed that the endometria of the sham operation and treatment groups were similar on expression level of MCP-1 and ICAM-1.

CONCLUSIONS

GZFLC plays an important role in the regression of endometriotic implants by immunological regulation in the rat model.

Ovarian cancer plasticity and epigenomics in the acquisition of a stem-like phenotype

Aggressive epithelial ovarian cancer (EOC) is genetically and epigenetically distinct from normal ovarian surface epithelial cells (OSE) and early neoplasia. Co-expression of epithelial and mesenchymal markers in EOC suggests an involvement of epithelial-mesenchymal transition (EMT) in cancer initiation and progression. This phenomenon is often associated with acquisition of a stem cell-like phenotype and chemoresistance that correlate with the specific gene expression patterns accompanying transformation, revealing a plasticity of the ovarian cancer cell genome during disease progression.Differential gene expressions between normal and transformed cells reflect the varying mechanisms of regulation including genetic changes like rearrangements within the genome, as well as epigenetic changes such as global genomic hypomethylation with localized promoter CpG island hypermethylation. The similarity of gene expression between ovarian cancer cells and the stem-like ovarian cancer initiating cells (OCIC) are surprisingly also correlated with epigenetic mechanisms of gene regulation in normal stem cells. Both normal and cancer stem cells maintain genetic flexibility by co-placement of activating and/or repressive epigenetic modifications on histone H3. The co-occupancy of such opposing histone marks is believed to maintain gene flexibility and such bivalent histones have been described as being poised for transcriptional activation or epigenetic silencing. The involvement of both-microRNA (miRNA) mediated epigenetic regulation, as well as epigenetic-induced changes in miRNA expression further highlight an additional complexity in cancer stem cell epigenomics.Recent advances in array-based whole-genome/epigenome analyses will continue to further unravel the genomes and epigenomes of cancer and cancer stem cells. In order to illuminate phenotypic signatures that delineate ovarian cancer from their associated cancer stem cells, a priority must lie in the expansion of current technologies and further implementation of bioinformatics to handle the complexity of the cancer epigenome and the various networks that coordinate disease initiation and progression. Great potential lies in the translation of these findings into epigenetic-based therapies. Additionally, targeting chemo-resistant cancer stem cells may provide a much needed breakthrough in treatment of advanced ovarian cancer and chemoresistant disease.

Epithelial–mesenchymal interconversions in normal ovarian surface epithelium and ovarian carcinomas: An exception to the norm

Cancer that arises from the ovarian surface epithelium (OSE) accounts for approximately 90% of human ovarian cancer, and is the fourth leading cause of cancer-related deaths among women in developed countries. The pathophysiology of epithelial ovarian cancer is still unclear because of the poor understanding of the complex nature of its development and the unusual mechanism(s) of disease progression. Recent studies have reported epithelial-mesenchymal transition (EMT) in cultured OSE and ovarian cancer cell lines in response to various stimuli, but our understanding of the importance of these observations for normal ovarian physiology and cancer progression is not well established. This review highlights the current literature on EMT-associated events in normal OSE and ovarian cancer cell lines, and discusses its implication for normal ovarian function as well as acquisition of neoplastic phenotypes. The pathological changes in OSE in response to EMT during neoplastic transformation and the contribution of hormones, growth factors, and cytokines that initiate and drive EMT to sustain normal ovarian function, as well as cancer development and progression are also discussed. Finally, emphasis is placed on the clinical implications of EMT and potential therapeutic opportunities that may arise from these observations have been proposed.

Structural changes and cell properties of human ovarian surface epithelium in ovarian pathophysiology

The surface epithelial cells of the ovary, which are modified peritoneal cells, form a single, focally pseudostratified layer. The Müllerian ducts differentiate after invagination of the coelomic mesothelium over the gonadal ridges during the 6th week of embryonic life. On the basis of the embryologically putative Müllerian potential of this epithelium, endometriosis can be explained by coelomic metaplasia from the peritoneum, including ovarian surface epithelium. Some pelvic endometriosis specimens have shown that epithelial cells on the ovary or pelvis are serially changed to endometriotic gland cells. Immunohistochemistry as well as scanning electron microscopy also reinforce the light-microscopical findings. A three-dimensional culture system demonstrated that human ovarian surface epithelial cells exhibited a glandular-stromal structure when they were cocultured with endometrial stromal cells in an estrogen-rich environment. Ovarian carcinomas in the epithelial-stromal category are thought to arise from the surface epithelium and its inclusions. The ovarian surface epithelium is physiologically involved in follicular rupture, oocyte release, and the subsequent repair of follicle wall during reproductive age. Simultaneously, ovulation may cause a loss of integrity of the surface epithelium, followed by accumulation of multiple mutations. The cortical invagination, surface stromal proliferation, and Müllerian differentiation of these cells are likely not to be an early step in the cancer development. However, the inclusion cysts are closely related with carcinogenesis because they are significantly more common in ovaries contralateral to those containing epithelial cancers than in control ovaries. As an in vitro study, ovarian carcinoma cell lines were established from simian virus 40 large T antigen-transformed human surface epithelial cells of the ovary. Further investigations of these cell lines may lead to insights into the preneoplastic and early stages of carcinomas. To clarify the pathogenesis of endometriosis and epithelial ovarian cancer, specifically designed studies of ovarian surface epithelium are required.

A novel in vitro model of the early endometriotic lesion demonstrates that attachment of endometrial cells to mesothelial cells is dependent on the source of endometrial cells

OBJECTIVE

To characterize the source of variability in endometrial stromal cell (ESC) binding to peritoneal mesothelial cells (PMC).

DESIGN

In vitro study.

SETTING

University medical center.

PATIENT(S)

Reproductive-age women without endometriosis undergoing surgery for benign conditions.

INTERVENTION(S)

None.

MAIN OUTCOME MEASURE(S)

Binding of ESCs (n = 9) to PMCs collected from the anterior abdominal wall (AAW) (n = 5), a commercially available mesothelial cell line (LP9) (three different passages) and normal ovarian surface epithelium (NOSE) (n = 5).

RESULT(S)

There were no differences in the binding of same-source ESCs to mesothelial cells obtained from the AAW of different women, to different passages of LP9s or to NOSE of different women. There was a trend toward increased binding of ESCs to NOSE compared to AAW PMCs. In contrast, there were significant differences in the ability of ESCs obtained from different women to bind to same-source PMCs.

CONCLUSION(S)

There is significant variability in ESC binding to PMCs. This variation is dependent primarily on the source of the ESCs. The ESC binding to LP9 PMCs was similar to AAW PMCs and NOSE.

Characterization of 17beta-hydroxysteroid dehydrogenase type 4 in human ovarian surface epithelial cells

The human ovarian surface epithelium (hOSE) is a single layer of mesothelial-type primitive epithelial cells that are potential estrogen targets. It has been reported that hOSE cells can produce estrogen. However, the mechanisms that regulate estrogen level(s) in hOSE cells are not yet known. To elucidate the enzymes involved in these reactions, we examined gene expression of 17beta-hydroxysteroid dehydrogenases (17beta-HSDs) in primary hOSE (POSE) and OSE2a cells using RT-PCR. We found that POSE cells and cells of the immortalized hOSE line, OSE2a, bidirectionally converted estrone (E1) and 17beta-estradiol (E2). Both cell types expressed mRNA for 17beta-HSD type 1 (17beta-HSD1), suggesting that the enzyme is involved in the E1 to E2 conversion. Interestingly, both cells expressed 17beta-HSD4 mRNA but not 17beta-HSD2 mRNA. We prepared an antibody against the carboxyl terminal of 17beta-HSD4 (anti-17beta-HSD4 antibody), which recognized the 80 and 48 kDa proteins in POSE and OSE2a cells based on immunoblot analysis. Furthermore, immunohistochemical study revealed the presence of 17beta-HSD4 in hOSE cells in the human ovary. These results suggest that 17beta-HSD4 is involved in estrogen inactivation and may protect against an excessive accumulation of E2 in hOSE cells.

A modified medium that significantly improves the growth of human normal ovarian surface epithelial (OSE) cells in vitro

Approximately 90% of malignant ovarian tumours are epithelial and thought to arise from a single cell layer, the ovarian surface epithelium. In culture, human normal ovarian surface epithelial (OSE) cells have a very limited lifespan before they senesce, rarely progressing beyond 10 population doublings. This has restricted the use of normal OSE cells for studying the biology of ovarian surface epithelium and identifying molecular events that contribute to malignant transformation. We have investigated the conditions for culturing human, normal OSE cells in vitro using modified media. Culturing normal OSE cells in a modified medium (NOSE-CM) supplemented with epidermal growth factor, hydrocortisone, insulin and bovine pituitary extract led to significant improvements in the seeding and cloning efficiencies, overall cell growth and lifespan compared to culturing in a basic, nonsupplemented medium (BM) and previously used media (F-12 K medium and William's medium E). Cells cultured in NOSE-CM underwent, on an average, 19.0 population doublings (95% CI 16.3-21.7); cells cultured in BM underwent 0.43-3.52 population doublings over a similar time period. Growth curves established for different lines indicated that OSE cells continued to grow beyond passage 11 and up to passage 18 in NOSE-CM, but never beyond passage 7 when cultured in BM. It is likely that establishing optimal conditions for the growth of OSE cells in vitro will enable studies of the biological and genetic mechanisms of transformation in epithelial ovarian cancers.

Pathogenesis of endometriosis

Many women harbour spots of peritoneal endometriosis without having any symptoms; this is referred to as the phenomenon endometriosis. Some of these women go on to develop symptomatic endometriosis. Although we know the factors potentially involved in the aetiology and pathogenesis of endometriosis, the exact mechanism by which the phenomenon endometriosis develops into the disease endometriosis, with its associated signs and symptoms, remain obscure. The widely accepted theory is Sampson's transplantation theory. Recent findings indicate that certain properties of the endometrium, and the influence of the local environment, are crucial in the development of endometriosis. Early endometriosis lesion formation is described in detail, as this seems to be a key process in the development of peritoneal endometriosis.

Pathophysiological Dynamics of Human Ovarian Surface Epithelial Cells in Epithelial Ovarian Carcinogenesis

Epithelial ovarian cancer is responsible for almost half of all the deaths from female genital tract tumors. Major impediments to the clinical treatment of this disease are the relatively asymptomatic progression and a lack of knowledge regarding defined precursor or malignant lesions. Most epithelial ovarian cancers are thought to arise from the transformation of ovarian surface epithelial cells, a single continuous layer of flat-to-cuboidal mesothelial cells surrounding the ovary. To improve our understanding of the pathogenesis of epithelial ovarian cancer, it is necessary to study the biological characteristics of normal ovarian surface epithelial cells. However, this approach has been hampered by the inability to purify and culture such human cells. During the past decade, procedures to isolate and culture human ovarian surface epithelial cells have been developed, and, subsequently, using viral oncogenes, several immortalized cells have been established. This new experimental system is being employed to improve our understanding of the genetic changes leading to the initiation of epithelial ovarian cancer and to identify events in the cancer's development. This review mainly describes the biological dynamics of ovarian surface epithelial cells in the pathogenesis of epithelial ovarian cancer, focusing on humans and excluding small animal models.

A well-defined in vitro three-dimensional culture of human endometrium and its applicability to endometrial cancer invasion

A three-dimensional (3-D) endometrium culture was established, in which human endometrial stromal cells embedded in a mixture of collagen I, a major component of extracellular matrix, and matrigel, a basement membrane material, supports the epithelial cells seeded on top of the collagen/matrigel matrix. The biological growth and differentiation of the epithelial cells were studied microscopically and immunohistochemically. Transmission electron microscopy showed a polarized columnar epithelium in monolayer with basally positioned nuclei. Scanning electron microscopy revealed a confluent epithelium with an abundance of microvilli and cilia as well as pinopodes on the apical surface. An immunohistochemical staining showed that integrin alpha1, alpha4, and beta3 were co-localized with cytokeratin, confirming the epithelial origin of the cells. In contrast, immunoreactivity against cyclooxygenase-1 or -2 was positive in both epithelial and stromal cells. When epithelial cells were replaced by KLE cells, an endometrial cancer cell of epithelial origin, invasion of KLE cells into the stromal fraction was observed. The invasion was closely correlated to expression of matrix metalloproteinases and their tissue inhibitors of metalloproteinases in a manner consistent with paracrine fashion. The present 3-D culture imitates the normal endometrium physiologically as well as morphologically, thus provides an excellent in vitro tissue suitable for reproducing in vivo physiological processes, including endometrial cancer invasion.

What we have learned from isolated cells from human ovary?

In the ovary, morphodynamics of follicles with cyclic maturation, ovulation and repair occur under the control of various tropic factors. The ovarian functions have been mostly studied by using subhuman primates and non-primate animals because of the limited availability of closely staged human specimens. We have recently established the in vitro culture systems of ovarian surface epithelium (OSE) and granulosa cells of humans, and subsequently developed the immortalization of each cell. The immortalized cell lines may supply us advanced studies on ovarian disorders as well as its physiological functions. On the embryologically putative müllerian potential of coelomic epithelium, endometriosis can be explained by coelomic metaplasia from the peritoneal mesothelium, including OSE. We can microscopically observe a continuity from flat epithelial cells on the ovarian surface or within the cortical inclusion cysts to endometriotic gland cells. The primary human OSE cells exhibited a glandular-stromal structure similar to endometriosis when they were co-cultured with endometrial stromal cells in an estrogen-rich environment. Primary and immortalized OSE cells converted the estrone to estradiol, and expressed the genes for steroidogenic factor-1 (SF-1), p450arom and 17beta-HSDs. This character of OSE was, in part, similar to the granulosa cells. One of the immortalized OSE clone produces disseminated tumors mimicking undifferentiated carcinomas in nude mice. Ovarian granulosa cells play a key role in the functional maturation of the entire follicle. The molecular pathways in granulosa cells responsible for the growth, differentiation, and nursing the oocyte are still largely unknown. Our immortalized human granulosa cell line, GC1a, obtained from developing follicles, showed no steroid hormone biosynthesis, and no detectable expression of the genes for StAR or cytochrome p450 enzymes due to the lack of SF-1. Transfected SF-1 elicited estradiol secretion in GC1a cells with concomitant expression of the genes encoding the proteins for gonadal steroidogenesis. The enzymatic activity of 17beta-HSD was also achieved by SF-1 transgene. These results indicate that SF-1 controls the gene expression required for steroidogenesis in the human developing follicle. Clinically, immortalized GC1a cells from human origin, with steroidogenic capacity, may serve as a feeder layer for in vitro oocyte maturation. Further investigations of our immortalized OSE and granulosa cells of humans will allow us to clarify whether they have a single progenitor cell.

Localization of matrix metalloproteinases on endometrial cancer cell invasion in vitro

OBJECTIVES

In this study, we have established an in vitro three-dimensional (3D) coculture, where normal endometrial stromal cells and endometrial cancer cells were cocultured under defined hormonal conditions, to investigate the potential paracrine effects on synthesis and secretion of matrix metalloproteinases (MMPs) and, thus, cancer invasion.

METHODS

Endometrial stromal cells were obtained by biopsy, cultured in the presence of 100 nM estrogen for 3 days, and then mixed with extracellular matrix (ECM) composed of collagen I and matrigel in a 4:1 ratio. After 3 more days in culture, a 3D coculture was established with HEC-1A cells, an endometrial adenocarcinoma cell line, grown on top of the mixture under various ovarian steroids (no steroid, 100 nM beta-estradiol (E2), or 1.0 microM progesterone (P4)) for 10 days. The expression and localization of MMP-2, MMP-9, and integrin beta 3 subunit were visualized by immunocytochemistry and analyzed by reverse transcription polymerase chain reaction (RT-PCR). The extent of cancer invasion was quantified by Boyden's chamber assay.

RESULTS

Integrin beta 3 subunit was localized along the cell surface of HEC-1A cell under all three hormonal conditions. MMP-2 was located in the cytoplasm of stromal cells and on the surface of HEC-1A cells. Synthesis and secretion of stromal MMP-2 were increased in the presence of ovarian steroids. In contrast, expression of stromal MMP-9 was suppressed in the presence of ovarian steroids. No MMPs were synthesized in HEC-1A cells. Invasion assay revealed that HEC-1A cells achieved high tumoral invasiveness in the presence of beta-estradiol.

CONCLUSIONS

These findings suggest that stromal cell-derived MMP-2 is translocated to the surface of HEC-1A cells. Integrin beta 3 subunit might contribute, in part, to providing a binding site for MMP-2. Thus, HEC-1A cells invade by recruiting MMP-2 secreted by stromal cells, which is greatly enhanced in the presence of beta-estradiol. The 3D coculture provides an excellent experimental system in which single parameters can be isolated from a complex in vivo system in the process of endometrial cancer invasion.

Role of cytokines in endometriosis

OBJECTIVE

To review the literature on the role of cytokines in the pathogenesis of endometriosis and endometriosis-associated infertility.

DESIGN

Pertinent studies were identified by a computer search of MEDLINE. References of selected articles were hand-searched for additional citations.

RESULT(S)

Recent studies suggest that the peritoneal fluid of women with endometriosis contains an increased number of activated macrophages that secrete various local products, such as growth factors and cytokines. Levels of several cytokines were reported to be elevated in the peritoneal fluid of women with endometriosis. Because the peritoneal environment may be controlled by locally regulated factors, cytokines are believed to play a role in the development and progression of endometriosis and endometriosis-associated infertility. A possible pathogenic mechanism links cytokines with endometriosis.

CONCLUSION(S)

Cytokines, which are produced by many cell types including endometriotic tissues, play diverse roles in the pathogenesis of endometriosis and endometriosis-associated infertility. More studies about the specific role of these cells and soluble factors are needed to improve understanding of endometriosis and to develop novel therapies.

Ovarian surface epithelium: biology, endocrinology, and pathology

The epithelial ovarian carcinomas, which make up more than 85% of human ovarian cancer, arise in the ovarian surface epithelium (OSE). The etiology and early events in the progression of these carcinomas are among the least understood of all major human malignancies because there are no appropriate animal models, and because methods to culture OSE have become available only recently. The objective of this article is to review the cellular and molecular mechanisms that underlie the control of normal and neoplastic OSE cell growth, differentiation, and expression of indicators of neoplastic progression. We begin with a brief discussion of the development of OSE, from embryonic to the adult. The pathological and genetic changes of OSE during neoplastic progression are next summarized. The histological characteristics of OSE cells in culture are also described. Finally, the potential involvement of hormones, growth factors, and cytokines is discussed in terms of their contribution to our understanding of the physiology of normal OSE and ovarian cancer development.

Development of endometriosis-like lesions after transplantation of human endometrial fragments onto the chick embryo chorioallantoic membrane

The chick embryo chorioallantoic membrane (CAM) bioassay was used to investigate the early pathogenesis of endometriosis. Endometrial fragments were explanted onto the CAM. The grafts including the surrounding CAM were excised at 24, 48 or 72 h after explantation, fixed and embedded in paraffin. Immunohistochemical analysis was used to distinguish endometrial cells. To identify cells of human origin, in-situ hybridization was performed using a probe specific for human chromosome 1. After 24 h, direct contact between endometrial stromal as well as epithelial cells and the mesenchymal layer of the CAM was observed. Invasion of both stromal cells and intact endometrial glands into the mesenchymal layer was observed after 48 h. At 72 h, endometriosis-like lesions were observed in the mesenchymal layer. Positive staining with antibodies to vimentin and pan-cytokeratin was observed in the invading cells as well as in the lesions. In the lesions these positively stained cells showed in-situ hybridization signals for human chromosome 1, confirming their human origin. In conclusion, after 3 days of incubation, endometriosis-like lesions consisting of human endometrial glands and stromal cells were found in the mesenchymal layer of the CAM. These lesions apparently resulted from the invasion of intact human epithelial structures and stromal cells.