Time series analysis of transmesothelial invasion by endometrial stromal and epithelial cells using three-dimensional confocal microscopy

OBJECTIVE

To evaluate endometrial adhesion and invasion of peritoneal mesothelium.

DESIGN

Descriptive study using confocal laser-scanning microscopy.

SETTING

University-based laboratory.

PATIENT(S)

Women undergoing surgery for benign conditions.

INTERVENTION(S)

Fluorescence-labeled peritoneal mesothelial cells (PMCs) were grown on coverslips. Fluorescence-labeled endometrial stromal cells (ESCs) and epithelial cells (EECs) and myometrial cells (Myos) were plated on the PMCs. Cultures were examined at 1, 6, 12, and 24-27 hours with differential interference contrast and confocal laser-scanning microscopy.

MAIN OUTCOME MEASURE(S)

Demonstration of adherence and invasion of endometrial cells through peritoneal mesothelium.

RESULT(S)

At 1 hour, there was adherence of the ESCs, EECs, and Myos on the perimeter of PMCs. There was no invasion by the Myos. By 6 hours, ESCs and EECs spread over the surface of the PMCs and extended cell processes through PMC junctions. Extension of pseudopodia under the PMCs followed. By 12 hours, there was vacuolization and lifting of PMCs that had been undermined by endometrial cells.

CONCLUSION(S)

This is the first time-phase study to demonstrate adherence and the process of invasion of endometrial cells through the mesothelium. The application of three-dimensional confocal laser-scanning microscopy is a novel technique that can be used to further examine mechanisms involved in the pathogenesis of the early endometriotic lesion.

Pathogenesis of endometriosis--current research

Proliferative, secretory and menstrual endometrial cells of both the stroma and epithelium adhere to intact peritoneal mesothelium and mesothelial monolayers. Endometrial attachment to the mesothelium appears to occur rapidly (within 1 h) and transmesothelial invasion occurs between 1 and 18-24 h. These results demonstrate that the mesothelium is not a 'no-stick' surface and indicates that molecules present at the surface of the mesothelium are involved in the pathogenesis of the early endometriotic lesion. The inhibition of endometrial cell adherence to peritoneal mesothelium by hyaluronidase indicates that CD44-hyaluronan binding is at least one of the mechanisms involved in the pathogenesis of endometriosis. We believe that investigation of mesothelial cell adhesion molecules is central to understanding the pathogenesis of endometriosis.

Culture of menstrual endometrium with peritoneal explants and mesothelial monolayers confirms attachment to intact mesothelial cells

BACKGROUND

To evaluate adhesion of menstrual endometrium (ME) to intact peritoneal mesothelium.

METHODS

Explants of peritoneum were cultured for 1 h with ME (n = 6). Specimens were serially sectioned for haematoxylin and eosin stain and immunohistochemistry using an anti-cytokeratin antibody to label mesothelium. Confocal laser scanning microscopy (CLSM) was performed to identify an intact layer of mesothelial cells (MC) underlying sites of ME attachment. Also, ME and MC were labelled with Cell-Tracker dyes. ME was cultured with mesothelial monolayers for 1 h (n = 10). Cultures were examined with differential interference contrast and CLSM. Optical sections were taken and a three-dimensional model was constructed.

RESULTS

In the peritoneal explants, ME adhered to intact mesothelium. There was no evidence of transmesothelial invasion. CLSM of sections of the explants demonstrated an intact monolayer of cytokeratin positive cells below the sites of ME implantation. Cytokeratin negative and positive ME cells adhered to mesothelial cells. Likewise, the ME attached to cultured mesothelium. Orthogonal sections and three-dimensional reconstruction confirmed an intact monolayer of mesothelium underlying ME attachment sites.

CONCLUSIONS

This study confirms that ME adheres rapidly to intact peritoneal mesothelium. Further studies are needed that characterize the mechanisms of ME adhesion to, and migration through, mesothelial cells.

The alpha(2)beta(1) and alpha(3)beta(1) integrins do not mediate attachment of endometrial cells to peritoneal mesothelium

OBJECTIVE

To evaluate the possible role of mesothelial alpha(2)beta(1) and alpha(3)beta(1) integrins in the attachment of endometrial stromal cells (ESCs) and endometrial epithelial cells (EECs).

DESIGN

In vitro study.

SETTING

University medical center.

PATIENT(S)

Women of reproductive age (n = 26).

MAIN OUTCOME MEASURE(S)

Mesothelial cells were grown on collagen IV. Endometrial stromal cells and EECs were plated on mesothelial cells for 1 hour. Before plating, mesothelial cells or endometrial cells were incubated with antibodies to alpha2, alpha3, and beta1 integrin subunits. The effect of these antibodies on ESC and EEC binding to collagen IV and collagen I was also examined. The expression of collagen I, collagen IV, fibronectin, and laminin by cultured ESCs and EECs was examined.

RESULT(S)

The anti-integrin antibodies had no effect on endometrial binding to mesothelium. The beta1 integrin antibody decreased binding of ESCs and EECs to the collagen matrices. In culture, ESCs and EECs expressed collagen I, collagen IV, fibronectin, and laminin to varying degrees.

CONCLUSION(S)

The initial adhesion of ESCs and EECs to mesothelium is not mediated by beta1 integrins. In contrast, ESC and EEC attachment to collagen IV and collagen I, which are present in the submesothelial extracellular matrix, is mediated by beta1 integrins.

An in vitro model to study the pathogenesis of the early endometriosis lesion

OBJECTIVE

To determine if whole fragments of endometrium can adhere to peritoneum with intact mesothelium.

DESIGN

Tissue culture and immunohistochemical study.

SETTING

University Medical Center.

PATIENTS

Reproductive-age women undergoing surgery for benign conditions.

INTERVENTIONS

Whole explants of human peritoneum from the anterior abdominal wall and the posterior surface of the uterus were cultured with whole fragments of mechanically dispersed endometrium.

MAIN OUTCOME MEASURES

Adhesion of endometrial fragments to the surface of the peritoneum was evaluated. Adherent fragments of endometrium were identified using the dissecting microscope and by performing serial sections of the peritoneum explants for light and confocal laser-scanning microscopy. Immunohistochemical staining of the mesothelium with antibodies to cytokeratin and vimentin was used to ensure an intact layer of mesothelium beneath the endometrial implants. Transmission electron microscopy was also used to evaluate the adhesion of endometrium to the mesothelium.

RESULTS

Endometrium was identified attached to the surface of the peritoneum. After 18-24 hours of culture, the majority of implants did not have identifiable mesothelium beneath them, but most had intact mesothelium running up to the point of attachment. Approximately 10% of the endometrial implants had intact mesothelium at the site of attachment. After 1 hour of culture, both endometrial stromal and epithelial cells were attached to intact mesothelium in nearly all cases. Early transmesothelial invasion involves endometrial stromal cells.

CONCLUSIONS

Endometrial stromal and epithelial cells can attach to the intact mesothelial surface of the peritoneum. Endometrial stromal cell invasion through the mesothelium occurs in less than 18-24 hours.

Mesothelial cell-associated hyaluronic acid promotes adhesion of endometrial cells to mesothelium

OBJECTIVE

To evaluate the role of hyaluronic acid in the attachment of endometrial cells to mesothelium.

DESIGN

In vitro study of adhesion of endometrial stromal and epithelial cells to mesothelial cells.

SETTING

University medical center.

PATIENT(S)

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

INTERVENTION(S)

None.

MAIN OUTCOME MEASURE(S)

The effect of hyaluronidase treatment of mesothelial cells or endometrial cells on adhesion of (51)Cr labeled endometrial stromal and epithelial cells to monolayers of mesothelium was evaluated. The expression of CD44, the hyaluronate receptor, was evaluated by western blot.

RESULT(S)

Hyaluronidase pretreatment of mesothelial cells decreased the binding of endometrial stromal and epithelial cells to mesothelium by 39% (P< .02) and 31% (P< .03), respectively. There was no effect on endometrial cell binding to mesothelial cells or to collagen IV when the endometrial cells were pretreated with hyaluronidase. CD44 expression by endometrial stromal and epithelial cells was demonstrated by western blot.

CONCLUSIONS

This study demonstrates that mesothelial cell-associated hyaluronic acid is involved in attachment of endometrial stromal and endometrial epithelial cells to the mesothelium. We hypothesize that binding of hyaluronic acid by endometrial cells is involved in the pathogenesis of the early endometriotic lesion.

Composition of the extracellular matrix of the peritoneum

OBJECTIVE

To localize the extracellular matrix proteins collagen I, collagen IV, fibronectin, and laminin in the peritoneal membrane.

STUDY DESIGN

Peritoneal biopsies (n = 13) from the anterior abdominal wall and the uterine serosa (n = 3) were incubated with antibodies to collagen IV, laminin, collagen I, and fibronectin. Specimens were examined using light and confocal laser scanning microscopy.

RESULTS

All of the extracellular matrix (ECM) proteins were present immediately under the mesothelium. Collagen (Col) IV and laminin (LM) were seen in the smooth muscle of microvascular structures, in the subendothelial basement membrane, and were present in a fascicular pattern in the peritoneal stroma. Collagen I was distributed diffusely in the peritoneal stroma. Fibronectin was also present in the subendothelial basement membrane.

CONCLUSIONS

The resolution of the confocal microscope allowed for localization of extracellular matrix proteins in relation to the mesothelium. The presence of collagen IV, laminin, collagen I, and fibronectin under the mesothelium suggests that cells invading the peritoneum must have the ability to degrade and remodel this matrix.

Short-term culture of peritoneum explants confirms attachment of endometrium to intact peritoneal mesothelium

OBJECTIVE

To evaluate the initial adhesion of endometrium to the peritoneum.

DESIGN

Descriptive study using light and confocal laser-scanning microscopy, immunohistochemistry, and transmission electron microscopy.

SETTING

University-based laboratory.

PATIENT(S)

Women without endometriosis undergoing surgery for benign conditions.

INTERVENTION(S)

None.

MAIN OUTCOME MEASURE(S)

Explants of peritoneum (n = 20), prepared from four patients, were cultured for 1 hour with mechanically dispersed proliferative or secretory endometrium. Peritoneum was cultured with endometrium from the same patient. Specimens were fixed and serially sectioned for hematoxylin and eosin stain, immunohistochemistry using an anti-cytokeratin monoclonal antibody, and transmission electron microscopy.

RESULT(S)

In 17 of 20 explants, endometrium was adherent to intact mesothelium. There was no evidence of transmesothelial invasion at any sites of attachment. Although in most cases endometrium was adherent to mesothelium via endometrial stroma, there were many sites of endometrial epithelium-mesothelium attachment. Confocal laser scanning microscopy demonstrated an intact monolayer of cytokeratin-positive cells below the sites of endometrial implantation. Transmission electron microscopy demonstrated intact, viable, mesothelial cells below sites of attachment.

CONCLUSION(S)

This study demonstrates that endometrium rapidly adheres to intact peritoneal mesothelium. In addition, this study demonstrates that endometrial epithelial cells, as well as stroma, can attach to mesothelium. Further studies are needed that characterize the mechanism of endometrial-mesothelial cell adhesion.

Expression of the alpha2beta1 and alpha3beta1 integrins at the surface of mesothelial cells: a potential attachment site of endometrial cells

OBJECTIVE

To localize alpha2beta1 and alpha3beta1 integrins in the cell membrane of peritoneal mesothelium in vivo and in vitro.

DESIGN

Descriptive study using confocal and two-photon laser-scanning microscopy.

SETTING

University-based laboratory.

PATIENT(S)

Women without endometriosis undergoing surgery for benign conditions.

INTERVENTION(S)

None.

MAIN OUTCOME MEASURE(S)

Peritoneal biopsies (n = 9) and mesothelial monolayer cultures (n = 4) were incubated with antibodies to the alpha2 and alpha3 subunits and to the intact alpha2beta1 and alpha3beta1 integrins. Specimens were examined with laser-scanning microscopy.

RESULT(S)

The alpha2 and alpha3 subunits and the intact alpha2beta1 and alpha3beta1 integrins were identified at the base of the mesothelial cells (i.e., toward the basement membrane). There was also expression of the alpha2 and alpha3 subunits and the intact alpha2beta1 and alpha3beta1 integrins at the cell surface (i.e., toward the peritoneal cavity).

CONCLUSION(S)

The resolution of the confocal and two-photon laser-scanning microscope enabled localization of integrins in mesothelial cells. The presence of alpha2beta1 (collagen-laminin receptor) and alpha3beta1 integrins (collagen-laminin-fibronectin receptor) at the base of mesothelial cells suggests a role for these molecules in adhesion to the basement membrane. The presence of these molecules at the cell surface suggests a potential locus for cell adhesion in such processes as endometriosis and cancer metastasis.

Mesothelium expression of integrins in vivo and in vitro

OBJECTIVE

To characterize the expression of alpha subunits of integrin adhesion molecules in peritoneal tissue in vivo and in vitro.

METHOD

Peritoneum from the anterior abdominal wall (n = 22) and the serosa of the posterior uterus (n = 11) was obtained from women of reproductive age without endometriosis who were undergoing surgery for benign conditions. Immunohistochemical studies were performed on serial sections of peritoneum from the anterior abdominal wall, the uterine serosa, mesothelial monolayer cultures, and peritoneum explants from the abdominal wall using monoclonal antibodies to alpha subunits of integrin adhesion molecules. Electron microscopy was performed to localize these adhesion molecules in the mesothelium.

RESULTS

The mesothelial expression of alpha integrin subunits was identical in the anterior peritoneum and uterine serosa. In vivo the mesothelium strongly expressed alpha 2 and alpha 3 and variably expressed alpha 6. In the monolayer cultures there was moderate/strong staining for alpha 2, alpha 3, and alpha 5; there was minimal expression of alpha v. In the explants there was moderate/strong expression of alpha 2, alpha 3, alpha 5, and alpha v; alpha 6 was variably expressed. The ultrastructure of the mesothelium was unique in the anterior peritoneum, uterine serosa, and the monolayer cultures. The integrin subunits were distributed throughout the cytoplasm, were expressed in the plasma membrane, and were present on the surface (i.e., towards the peritoneal cavity) of the mesothelium.

CONCLUSION

Integrins are expressed by the mesothelium of the peritoneum. The mesothelium expression of integrins in vivo differs from that of the mesothelium integrin expression in monolayer culture and explant culture.