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Decreased collagen concentration in rat uterine implantation sites compared with non-implantation tissue at days 6-11 of pregnancy.

Collagen concentrations at implantation sites in the rat uterus were found to be significantly decreased compared with concentrations in adjacent non-involved uterine tissue in early pregnancy (75% by Day 11). The decrease in collagen was most marked in primary decidua and was also observed to a lesser extent (20%) in myometrium at the implantation site. There was a decrease of 20% in the concentration of total proteins at Day 7 (as measured by the ninhydrin method) and a slight increase in water content (2%) at Days 6 and 7. The differences in total protein and water content were transient, but the difference in collagen was maintained throughout early pregnancy. The localized changes in collagen content observed in this study, along with previously reported morphological changes in fibrillar and basement-membrane collagens in the uterus, give support to a theory of remodelling in early pregnancy involving simultaneous synthesis and degradation of extracellular proteins during decidualization.