Inflammation and endometrial bleeding

Most of the key physiological processes in the human reproductive tract involve a significant inflammatory component. These processes include follicle development, ovulation, implantation, pregnancy, labor, postpartum, remodeling and menstruation. In this context, the term 'inflammation' usually means an influx of leukocytes ('immune cells'), often of different types, into a reproductive tract tissue. These examples of inflammation are not overtly associated with any infective process. There may also be evidence that these invading leukocytes have altered their functions to take on specific and relevant local regulatory roles. Specific sequential changes in different leukocytes can be demonstrated within human endometrium during the different phases of the normal menstrual cycle. Leukocytes are fairly sparse in numbers through the proliferative phase, but increase substantially into and through the secretory phase, so much so that around 40% of all stromal cells in the premenstrual phase are leukocytes, mainly uterine natural killer cells, a large granulated lymphocyte. Other leukocytes which play key roles in menstruation appear to be macrophages, mast cells, dendritic cells, neutrophils, eosinophils and regulatory T cells. Premenstrual withdrawal of progesterone increases the endometrial expression of inflammatory mediators, including IL-8 and MCP-1, which are believed to drive endometrial leukocyte recruitment at this time. Macrophages and neutrophils are rich sources of defensins and whey acid protein motif proteins, which play important roles in ensuring microbial protection while the epithelial barrier is disrupted. Mast cells are increasingly activated as the menstrual phase approaches, and leukocyte proteases trigger a cascade of matrix metalloproteinases and degradation of extracellular matrix. Dendritic cells and other antigen-presenting cells (e.g. macrophages) almost certainly facilitate clearance of cellular debris from the uterine cavity, and reduce the amount of viable cellular material transiting the Fallopian tubes. All of these processes are influenced or controlled by regulatory T cells. Many of these leukocytes also have the potential to release regulatory molecules which stimulate endometrial repair mechanisms. Increasing recent evidence also implicates disturbances of immune cells and their cytokine mediators in contributing to symptoms of abnormal uterine bleeding and pelvic pain. These recent findings all point towards the importance of the 'inflammatory process' in both normal and abnormal endometrial bleeding.