The chorioamnion (CA), or placental, membrane is the sac that surrounds the fetus in utero. It is comprised of two layers; the inner fibrous amnion, composed of mainly collagen, and the thick, cellular chorion. The CA membrane exhibits incredible toughness under tension, while still allowing the two layers to easily slide over one another1,2. The interface between the chorion and amnion consists of gyri (folds) and sulci (furrows)3, which enable the two layers to tightly interlock and facilitate the intact membrane’s resistance to tension. This complex structure also allows the two layers to expand during gestation without losing mechanical integrity. The gyrus and sulcus from one section are believed to uncouple, slide laterally, and then lock into their new position while the remaining membrane is unaffected by the motion3. SEM imaging of freeze-fractured CA has revealed a multitude of fine fibers connecting the two layers as illustrated in Figure 1. These are suggested to serve as “guy ropes”, which restrict the amount that the two layers could slide over one another3. Also of importance at this interface is the presence of hyaluronan, a glycosaminoglycan. The hyaluronan has been suggested to reduce the resistance between the two layers so that they may glide over one another more easily throughout gestation and with vigorous fetal movements1. The combination of the elaborate topography of the interface and the lubricating glycosaminoglycans present throughout this layer play a role in the mechanics of the interface between chorion and amnion.

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