Human uterine endometrium exhibits unique properties of cyclical regeneration and remodeling throughout reproductive life and also is subject to endometriosis through ectopic implantation of retrogradely shed endometrial fragments during menstruation. Here we show that functional endometrium can be regenerated from singly dispersed human endometrial cells transplanted beneath the kidney capsule of NOD/SCID/␥ c null immunodeficient mice. In addition to the endometrium-like structure, hormonedependent changes, including proliferation, differentiation, and tissue breakdown and shedding (menstruation), can be reproduced in the reconstructed endometrium, the blood to which is supplied predominantly by human vessels invading into the mouse kidney parenchyma. Furthermore, the hormone-dependent behavior of the endometrium regenerated from lentivirally engineered endometrial cells expressing a variant luciferase can be assessed noninvasively and quantitatively by in vivo bioluminescence imaging. These results indicate that singly dispersed endometrial cells have potential applications for tissue reconstitution, angiogenesis, and human-mouse chimeric vessel formation, providing implications for mechanisms underlying the physiological endometrial regeneration during the menstrual cycle and the establishment of endometriotic lesions. This animal system can be applied as the unique model of endometriosis or for other various types of neoplastic diseases with the capacity of noninvasive and real-time evaluation of the effect of therapeutic agents and gene targeting when the relevant cells are transplanted beneath the kidney capsule.animal model ͉ bioluminescence imaging ͉ endometriosis ͉ menstruation ͉ angiogenesis H uman endometrium lines the uterus and comprises luminal and glandular epithelial cells, stromal fibroblasts, vascular smooth muscle cells, endothelial cells, and immune competent cells. These cell components coordinately participate in the cyclical changes of human endometrium, including proliferation, differentiation, and tissue breakdown and shedding under the influence of estrogen and progesterone during the menstrual cycle. This unique system of cyclic tissue regeneration also depends on the cyclical growth and regression of the blood vessels that supply the endometrium (1). In addition, angiogenesis is deeply involved in the pathogenesis of endometriumderived disorders such as endometriosis (2). Endometriosis, one of the most common gynecological diseases, is characterized by the presence of functional endometrial-like tissue outside the uterine cavity. It is an estrogen-dependent disorder associated with substantial morbidity; however, the etiology and pathophysiology are not well elucidated (3). To study the physiology of normal endometrium and the pathogenesis of endometriosis, a variety of in vivo models using small animals has been developed by using the transplantation of autologous or heterologous endometrial cells/tissues or endometriotic tissues (4).In the present study, taking advantage of newly de...