Real‐Time in Vivo Monitoring of Circulating Endometrial Cells: Uncovering Systemic Dissemination and Temporal Fluctuations in Endometriosis

ABSTRACT Endometriosis is a common gynecological disorder characterized by the ectopic growth of endometrium‐like tissue outside the uterine cavity and is associated with pelvic pain, infertility, and reduced quality of life. Although several theories have been proposed, the mechanisms by which endometrial cells disseminate and establish ectopic lesions remain incompletely understood. Here, we developed a dual‐color pseudo‐menstrual mouse model that enables real‐time in vivo tracking of circulating endometrial cells (CECs) while distinguishing their tissue origin as uterine‐derived or lesion‐derived. Using in vivo flow cytometry and two‐photon intravital microscopy, we found that both uterine‐derived and lesion‐derived CECs can enter the circulation and are detected at higher levels during menstruation‐like breakdown, potentially in association with local vascular remodeling as suggested by intravital imaging observations. These cells could localize to distant organs, including the lungs and other extrapelvic organs. In addition, both CEC populations exhibited three distinct daily peak periods, and CEC dynamics were significantly associated with fluctuations in serum prolactin levels. In conclusion, these findings provide in vivo support for hematogenous dissemination in endometriosis and suggest that hormonal rhythmicity may regulate CEC dynamics, with potential diagnostic and therapeutic implications.