Try out PMC Labs and tell us what you think. Learn More. Although it has been more than a century since endometriosis was initially described in the literature, understanding the etiology and natural history of the disease has been challenging. However, the broad utility of murine and rat models of experimental endometriosis has enabled the elucidation of a of potentially targetable processes which may otherwise promote this disease.
To review a variety of studies utilizing rodent models of endometriosis to illustrate their utility in examining mechanisms associated with development and progression of this disease. Use of rodent models of endometriosis has provided a much broader understanding of the risk factors for the initial development of endometriosis, the cellular pathology of the disease and the identification of potential therapeutic targets.
Although there are limitations with any animal model, the variety of experimental endometriosis models that have been developed has enabled investigation into numerous aspects of this disease. Thanks to these models, our under-standing of the early processes of disease development, the role of steroid responsiveness, inflammatory processes and the peritoneal environment has been advanced. More recent models have begun to shed light on how epigenetic alterations con-tribute to the molecular basis of this disease as well as the multiple comorbidities which plague many patients.
Continued de-velopments of animal models which aid in unraveling the mechanisms of endometriosis development provide the best oppor-tunity to identify therapeutic strategies to prevent or regress this enigmatic disease.
In his landmark paper, Dr. John Sampson [ 3 ] suggested development of endometriosis was due to ectopic implantation of refluxed menstrual tissue [ 3 ].
Alternative explanations for the occurrence of endometriosis include coelomic metaplasia [ 10 - 12 ] and peritoneal activation of embryonic cell rests [ 1314 ]. The latter theory likely explains the unusual and rare occurrence of endometriosis in men [ 1516 ]. Recently, ectopic endometrial tissue has been described in human fetuses which has been theorized to develop as a consequence of ectopic localization of primitive endometrial tissue during organogenesis [ 1718 ]. More recently, Brosens and colleagues have postulated a role for neonatal menstruation, as a consequence of early onset steroid responsiveness, in developing endometriosis as an adolescent [ 20 ].
While generally considered a benign condition, endometriosis exhibits cancer-like features and can spread throughout the peritoneal cavity and to distal sites.
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Endometriosis is frequently physically debilitating, as patients often suffer from chronic pelvic pain, dyspareunia, dysmenorrhea and subfertility. Unfortunately, most women with this disease also exhibit one or more co-morbidities including adenomyosis, adhesive disease and inflammatory conditions such as interstitial cystitis and inflammatory bowel disease [ 21 - 23 ]. However, understanding the natural history of endometriosis, as well as the myriad of equally poorly understood co-morbidities, has proven elusive, despite extensive research in each disease area. Given our lack of understanding of the etiology of endometriosis, treatment options for women with this disease remain limited and generally involve a combination of hormonal manipulation and surgery to remove diseased tissue.
Unfortunately, the side effects of hormonal therapy for endometriosis cause many women to abandon this treatment; nevertheless, surgical treatment alone is frequently non-curative and many women suffer recurrence [ 24 - 32 ]. Thus, identifying better diagnostic and treatment strategies for this disease is a major focus of many endometriosis research laboratories.
Endometriosis is rare in non-menstruating species, suggesting that shedding of endometrial tissue may be a causative factor as originally proposed by Sampson [ 3 ]. Indeed, numerous studies have demonstrated that the transfer of endometrial tissue to the peritoneal cavity can lead to the development of ectopic endometrial lesions.
Given the obvious limitations and ethical considerations of human experimentation, current endometriosis researchers rely heavily on non-human primate or rodent models in order to investigate elements of disease pathophysiology. Among various animals utilized for experimental endometriosis, rodent models have a of advantages relative to other species.
They are cost effective due to their small size and large litters while their short gestation enables transgenerational analysis. Additionally, the wide availability of genetic knock-out mice, antibodies against murine and rat proteins and knowledge of the murine genome make these animals extremely useful for the study of many diseases, including endometriosis.
Importantly, rodent models of endometriosis are often utilized for preclinical testing in an attempt to identify new therapeutic agents. The use of experimental endometriosis models for therapeutic testing has recently been reviewed [ 3334 ]; thus, these studies will not be extensively considered here. We will also explore the use of animal models that have provided insight into the development of comorbidities common among women with endometriosis. ificantly, using appropriate experimental endometriosis models for identifying therapeutic targets is a critical step prior to the examination of a treatment regimen aimed at any particular pathway or protein.
Examination of early events associated with endometriosis ie, endometrial characteristics which promote ectopic implantation and survivalrequires an appropriate model system.
Therefore, several laboratories, including ours, have utilized chimeric mouse models of endometriosis in which human endometrial tissue can be introduced into immunocompromised mice for example, [ 35 - 38 ]. Immunocompromised mice lack a fully competent immune system; therefore, these animals do not mount an immune response against human tissue xenografts; therefore, these mice are valuable for examining multiple cellular pathways associated with development of endometriosis.
The capacity to examine human endometrial cells and tissues growing in vivo make chimeric models of experimental endometriosis particularly useful for pre-clinical testing, which is critical to developing better therapeutic agents. Importantly, experimental endometriosis can be established using tissues obtained from women with and without endometriosis via endometrial biopsy, collection of menstrual effluent or from surgical specimens for example, [ 3639 - 41 ]. By utilizing tissues acquired from women with and without endometriosis, phenotypic differences between control and disease-altered eutopic endometrium can be incorporated into the experimental de.
For example, a therapeutic treatment directed at the endometrial tissue phenotype of an endometriosis patient can be initiated in vitro prior to introduction into mice, enabling targeting of specific cellular processes which may be involved in early lesion establishment.
Athymic nude mice have a spontaneous mutation in the forkhead box N1 Foxn1 gene resulting in an underdeveloped thymus and severely compromised of T cells [ 42 - 44 ]. Although homozygous nude mice have a normal complement of B cells, the lack of T cells prevents complete B cell maturation.
These features render nude mice suitable for receiving transplantation of human xenografts and, not surprisingly, these animals have been widely used for cancer studies. Zamah and colleagues were the first to report experimental endometriosis in nude mice [ 41 ]. For their study, human endometrial tissues eutopic and ectopic were minced and injected into the peritoneal space of nude mice. Some mice were treated with estrogen, allowing examination of the impact of this steroid on lesion growth.
In this study, the majority of animals developed a disease resembling endometriosis; however, the most extensive disease was noted in mice receiving both estrogen and ectopic tissues from women with endometriosis. Since this first report, the nude mouse model of experimental endometriosis has been used extensively by our group as well as by many other investigators to examine aspects of establishment and progression of this disease within the peritoneal cavity.
For our model of experimental endometriosis using nude mice [ 36 ], human endometrial tissue is obtained by biopsy during the proliferative phase of the menstrual cycle and initially maintained as an organ culture. Following overnight incubation in media containing estradiol, the minced endometrial tissue fragments are washed in physiological saline to remove residual blood and mucous and tissues are subsequently injected into mice either intraperitoneally or subcutaneously along the ventral midline. Recipient mice can be left intact or ovariectomized and implanted with slow-release estradiol capsules thereby mimicking the growth-promoting environment of the human proliferative phase.
As shown in Fig. Experimental human endometriosis in a nude mouse. Proliferative phase human endometrial tissue readily establishes ectopic lesions in a nude mouse. Prior to injection of endometrial fragments, mice are ovariectomized and provided a slow-release estradiol capsule to mimic the human proliferative phase. Lesions developing in mice are markedly similar to human disease both macroscopically A and microscopically following hematoxylin and eosin staining B. This genetic disorder in dysfunctional T and B lymphocytes leading to an inability of the adaptive immune system to mount or sustain an appropriate immune response.
Loss of this enzyme is associated with a failure of humoral and cellular immune systems to mature. For these reasons, SCID mice are also suitable models for establishing experimental endometriosis using human endometrial tissues. These investigators established experimental human endometriosis in both SCID and nude mice; and reported the latter to be a superior model system due to a ificantly Saint-Junien tn nude mature survival rate of human tissues in the SCID animals. However, these animals are more prone to disease and opportunistic infections compared to nude mice.
ificantly, both SCID mice and nude mice develop extrathymic immunity over time [ 6263 ]; thus, experimental endometriosis studies utilizing these animals must be completed before 3 months of age, severely limiting the utility of these animals for studies over extended periods of time.
For this reason, studies of prolonged duration require a different model. These mice were first described by Mazurier et al. In their study, human endometrium was introduced into the peritoneal cavity of mice which were then subjected to multiple artificial menstrual cycles. Their study revealed that endometriotic-like disease was present in the majority of mice even after four artificial 28 day cycles and provided strong support for the use of these mice for long-term endometriosis studies.
Although experimental endometriosis using chimeric murine models is most commonly conducted with intraperitoneal injection of human endometrial tissues, other approaches also have been utilized to an advantage. An important adaptation arising from chimeric mouse models has been the establishment of renal grafts in which human tissue, including reproductive tissues, are placed under the kidney capsule [ 6667 ].
Endometrial tissue growing under the kidney capsule allows easier identification and excision compared to the random sites of attachment following intraperitoneal injection. An additional advantage of the kidney capsule is its high vascularity, which enhances survival of ectopic tissues.
The kidney capsule lends itself not only to insertion of whole tissues explantsbut also enables isolation and recombination of endometrial cells within a collagen matrix. Renal grafting of various combinations of endometrial cells allows a more precise examination of each endometrial cell type during growth and differentiation in vivo.
Importantly, we have found that control human endometrial tissues growing at this site in mice remain responsive to ovarian steroids and exhibit classic orphological appearance and biochemical differentiation in response to progesterone treatments Fig. Human endometrium growing under the kidney capsule of nude mice. Human endometrial stromal and epithelial cells are isolated and recombined in collagen and placed under the kidney capsule. Tissues are readily visible on gross examination arrows A.
Following treatment of the animal with estradiol B or estradiol plus medroxyprogesterone acetate C lesions exhibit proliferative or secretory phenotypes, respectively. Although long Saint-Junien tn nude mature an autoimmune disease, recent research indicates that cell-mediated immunity is also disrupted in endometriosis. Thus, a major disadvantage of experimental endometriosis models established in immunocompromised mice is the limited ability to examine the role of the immune system in development and progression of disease.
Therefore, immunocompetent rodent models remain essential to examining the influence of the immune system on development and progression of experimental endometriosis.