Co-localization of matrix metalloproteinase-1 and mast cell tryptase in the human uterus

The endometrium displays characteristic cyclical changes involving proliferation and differentiation. The differentiation that takes place requires major tissue remodelling involving the matrix metalloproteinase (MMP) family as key enzymes in this process. Mast cells, containing the tryptase and chymase enzymes that are capable of stimulating the MMP cascade, have been identified in the endometrium, but their role is still unclear. In this study, we observed that the majority of mast cells in the uterus reside in the myometrium and that they co-express mast cell tryptase and MMP-1 in the same intracellular granules. In endometrium exposed to synthetic progestogen via an intrauterine levonorgestrel system a significant increase in mast cells numbers was observed in women experiencing breakthrough bleeding compared to those in women with no reported bleeding. We conclude that mast cells contain MMP-1 and we postulate a potential role for mast cells in breakthrough bleeding.