Proliferative Activity of Lymphoid Cells in Human Endometrium throughout the Menstrual Cycle*

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Abstract
Human endometrium harbors a major population of lymphoid cells. The proliferation of these cells is assessed by the in situ labeling of endometrial sections for Ki67 (a proliferation marker) as well as in vitro incorporation of bromodeoxyuridine (BrdU; a thymidine analog) into S-phase cells in vibratome sections of endometria. Using the avidin-biotin-peroxidase complex method, sections of endometria and vibratome sections of endometria are labeled for Ki67 and BrdU, respectively. Subsequently, they are immunostained for CD45 (a lymphoid cell marker), CD3 (a T cell marker), and CD11c (a macrophage marker) molecules. Lymphoid cells scattered or aggregated in the endometrial stroma as well as intraglandular lymphoid cells exhibit Ki67 positivity throughout the menstrual cycle. The proliferative activity of the lymphoid cells, including the CD45, CD3, and CD11c positive cells scattered in the stroma, is markedly increased in the secretory phase. Similar, however less conspicuous, increased Ki67 labeling is observed in the lymphoid cells within lymphoid aggregates. The increased proliferative activity of the lymphoid cells in the secretory phase is confirmed by BrdU labeling in the vibratome sections. In the proliferative phase the non-CD45-positive cells in the endometrial stroma exhibit low proliferative activity, and in the secretory phase, Ki67 labeling in the endometrial stroma is primarily confined to the CD45-positive cells. The findings suggest that in situ proliferation is one mechanism by which the endometrial lymphoid cell pool within endometrium is restored after each menstrual shedding.