IMPLANTATION AND DECIDUALIZATION IN RODENTS

This article reviews the main events of embryo-implantation and decidu alization in rodents. In common laboratory rodents the embryo attaches to the uterine epithelial lining, usually on days 4 to 6 of pregnancy . A progressive degree of proximity between trophoblast and epithelium occurs until the epithelial cells undergo apoptosis and detach from t he basement membrane. During the attachment stage, the spindle-shaped connective tissue cells that underlie the epithelium next to the embry os transform into polyhedral and closely packed decidual cells. Follow ing the epithelial detachment and the breaching of the basement membra ne the embryo is thus in direct contact with decidual cells. These cel ls accumulate organelles associated with synthesis of macromolecules, intermediate filaments, and eventually lipid droplets and glycogen. An other remarkable feature of decidual cells is the establishment of gap and adherens intercellular junctions. Differentiation of fibroblasts into decidual cells advances antimesometrially and mesometrially, crea ting in the endometrium several regions of cells with different morpho logy. The whole phenomenon of decidualization which is normally trigge red by the embryo can be artificially induced in pseudopregnant or hor monally-prepared animals with the use of diverse stimuli. The uterine epithelium is probably responsible for the transduction of the initial stimulus. Prostaglandins have been shown to be important in the induc tion of decidualization. More recently other substances such as leukot rienes, platelet-activating factor (PAF), and transforming growth fact or (TGF) have been thought to play a role in induction. Much evidence points to prostaglandin production by the decidual cells. New proteins such as a luteotropic factor, desmin, and other molecules were shown to be produced after rat stromal cells undergo decidual transformation . The extracellular matrix of the mouse decidua contains very thick co llagen fibrils. Mouse decidual cells are also very active in phagocyto sing the thick fibrils, contributing to the remodeling and involution of the decidua that accompanies embryonic growth. Radioautographic dat a indicates that mouse decidual cells produce and secrete collagen and sulfated proteoglycans. (C) 1993 Wiley-Liss, Inc.