Dynamics of endometrial collagen skeleton at implantation and during pregnancy

Changes in endometrial collagen skeleton occur dw ring various reproductive stages as reaction of the stroma to specific hormonal and mechanical stimuli. Using 2N-NaOH maceration technique collagen fibrils are successfully exposed without alterations in their morphology and arrangement. Seen tinder the scanning electron microscope they appear organized in a continuous network which reveals some particular features in the superficial and deep stroma of the endometrium. In estrus rabbits the superficial areas are composed by wavy bundles of fibrils with a rosette-like appearance around hollow spaces containing endometrial glands and their openings. The deep zones, situated in the central part of the mucosal folds and near the myometrium, consist of parallel lamellae of fibrils housing small spaces for blood vessels. 7.5 days after stimulation with hCG (which corresponds to the implantational stage of the endometrium) significant changes are found mainly in the superficial endometrial stroma which consist in dispersed and very irregular distributed collagen fibrils. As a consequence, their characteristic radial arrangement around glandular openings is Lost. In the deep stroma the total content of collagen fibrils, bundles and lamellae is reduced. These changes are caused by an increased epithelial proliferation and glandular secretion. Both phenomena are rather enhanced during pregnancy and result in more evident alterations of the endometrial collagen skeleton. In pregnancy at term, an extremely deformed fibrillar network is detected. Numerous thin projections and channel-like invaginations containing endometrial glands make the superficial areas highly convoluted and irregular. In the deep stroma the lamellar arrangement of collagen network is less affected but the amount of their fibrils is significantly reduced. These three-dimensional results demonstrate that the collagen fibrillar skeleton is an important connective tissue component of the endometrium very likely responsible for its tissue integrity. Therefore, it modifies its arrangement during implantation and pregnancy in response to morphodynamic changes occurring primarily in other endometrial structures. Certainly, the collagen re-organisation during these stages has a specific functional role facilitating embryo-maternal interaction.