Degradation of extracellular matrix by mouse trophoblast outgrowths: a model for implantation

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Abstract
During implantation, the embryo attaches to the endometrial surface and trophoblast traverses the uterine epithelium, anchoring in the uterine connective tissue. To determine whether trophoblast can facilitate invasion of the uterus by degrading components of normal uterine extracellular matrix, mouse blastocysts were cultured on a radio-labeled extracellular matrix that contained glycoproteins, elastin and collagen. The embryos attached to the matrix and trophoblast spread over the surface. Starting on day 5 of culture, there was a release of labeled peptides into the medium. The radioactive peptides released from the matrix by the embryos had MW ranging from >25,000 to <200. By day 7, there were areas where individual trophoblast cells had separated from one another, revealing the underlying substratum that was cleared of matrix. When trophoblast cells were lysed with NH4OH on day 8, it was apparent that the area underneath the trophoblast outgrowth had been cleared of matrix. Scanning electron microscopy and time-lapse cinemicrography confirmed that the digestion of matrix was highly localized, taking place only underneath the trophoblast, with no evidence of digestion of the matrix beyond the periphery of the trophoblast outgrowth. The sharp boundaries of degradation observed may be due to localized proteinase secretion by trophoblast, to membrane proteinases on the surface of trophoblast or to endocytosis. Digestion of the matrix was not dependent on plasminogen, thus ruling out a role for plasminogen activator. Digestion was not inhibited by a variety of hormones and inhibitors, including progesterone, 17.beta.-estradiol, leupeptin, EDTA, colchicine, NH4Cl or .epsilon.-aminocaproic acid. This system of culturing embryos on extracellular matrix may be useful in determining the processes that regulate trophoblast migration and invasion into the maternal tissues during implantation.