Immunohistochemical Evaluation of Lumican Expression and Cellular Proliferation in Paired Eutopic and Ectopic Endometrium in Ovarian Endometriosis

Background/Objectives: This study aimed to evaluate lumican (LUM), TGF-β1, and Ki-67 expression in eutopic and ectopic endometrial tissues across the proliferative and secretory phases of the menstrual cycle in women with ovarian endometriosis. Methods: Paired eutopic and ectopic endometrial tissues were retrospectively obtained from 16 women with ovarian endometrioma and classified as proliferative (n = 8) or secretory (n = 8). LUM, TGF-β1, and Ki-67 immunoreactivities were evaluated in glandular and stromal compartments and semi-quantitatively assessed using the H-score method. Results: While no difference was found in LUM expression between eutopic and ectopic tissues in the proliferative phase, a significant increase in immunoreactivity was observed in the epithelium and stroma of the ectopic endometrium in the secretory phase (p < 0.0001). Ki-67 expression was more prominent in the eutopic stroma and ectopic epithelium in the proliferative phase. In the secretory phase, while Ki-67 expression decreased in eutopic tissue, it continued at levels close to those in the proliferative phase in ectopic tissue and was found to be higher than in eutopic tissue (p < 0.0001). TGF-β1 immunoreactivity was observed only in smooth muscle cells and perivascular areas; no evaluable TGF-β1 immunoreactivity was detected in the endometrial epithelial or stromal compartments. Conclusions: In this exploratory cohort, increased LUM expression in secretory-phase ectopic endometrium was observed alongside sustained Ki-67 immunoreactivity. However, these parallel expression patterns should not be interpreted as evidence of a direct association between LUM and cellular proliferation. TGF-β1 immunoreactivity did not differ between eutopic and ectopic tissues. Given the established role of lumican in ECM biology, its altered expression may reflect differences in the local tissue microenvironment, although ECM remodeling was not directly assessed in the present study.