Invasive implantation triggers endometrial stromal responses collectively identified as decidualization. The endometrium is transformed by hyperplasia and hypertrophy of stromal cells, secretion of prolactin and ECM proteins, OPN, laminin and fibronectin, the invasion by numerous immune cells, and formation of cell cell contacts. Decidualized stroma produces many endocrine and paracrine factors not present in nondecidualized cells and controls trophoblast invasion during implantation by generating a local cytokine environment that promotes trophoblast attachment. Varying degrees of decidualization occur in all implanting species with the most extensive stomal transformation occurring with the invasive implantation of rodents and primates, moderate transformation occurring in synepitheliochorial sheep, and only minor changes occurring in the epitheliochorial pig.
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