The amnion at term is a tough and tenacious but pliable membrane. It is the innermost fetal membrane and is contiguous with the aminonic fluid. This particular avascular structure occupies a role of incredible importance in human pregnancy. In many obstetrical populations, preterm premature rupture of the fetal membranes is the single most common antecedent of preterm delivery. The amnion is the tissue that provides almost all of the tensile strength of the fetal membranes. Therefore, the development of the component(s) of the amnion that protects against rupture or tearing is vitally important to successful pregnancy outcome.
[...] METABOLIC FUNCTIONS The amnion is clearly more than a simple avascular membrane that functions to contain amnionic fluid. It is metabolically active, involved in solute and water transport to maintain amnionic fluid homeostasis, and produces a variety of interesting bioactive compounds, including vasoactive peptides, growth factors, and cytokines. VASOACTIVE PEPTIDES. A number of investigators have demonstrated the capacity of amnion to synthesize the vasoconstrictor endothelin-1 as well as vasorelaxant parathyroid hormone-related protein (Casey and associates 1992; Germain and co-workers, 1992). [...]
[...] This functional capacity of amnion mesenchymal cells is an important consideration in the study of amnionic fluid for evidence of labor-associated accumulation of inflammatory mediators (Garcia-Velasco and Arici, 1999). ANATOMY Reflected amnion is fused to the chorion laeve. Placental amnion covers the fetal surface of the placenta, and thereby is in contact with the adventitial surface of the chorionic vessels, which traverse the chorionic plate and branch into the cotyledons. Umbilical amnion covers the umbilical cord. In the conjoined portion of the membranes of diamnionic-dichorionic twin placentae, the fused amnions are separated by fused chorion laeve; and aside from the small area of the fetal membranes immediately over the cervical os, this is the only site at which the reflected chorion laeve is not contiguous with decidua. [...]
[...] As the amnion enlarges, it gradually engulfs the growing embryo, which prolapses into its cavity (Benirschke and Kaufman, 2000). Distension of the amnionic sac eventually brings it into contact with the interior surface of the chorion laeve. Apposition of the mesoblasts of chorion laeve and amnion near the end of the first trimester then causes an obliteration of the extraembryonic coelom. The amnion and chorion laeve, though slightly adherent, are never intimately connected, and usually can be separated easily, even at term. [...]
[...] The finding that metallothionein, a protein with high affinity for Cu2+, is expressed in high amounts in amnion epithelial cells led to the hypothesis that decreased collagen synthesis may occur in women who smoke (King and colleagues, 1997). This is the proposed mechanism leading to the increased risk for prematurely ruptured membranes. Cadmium, inhaled in cigarette smoke, enters amnionic fluid and will act on amnion epithelial cells to induce high levels of metallothionein. As a result, Cu2+ deficiency may limit the activity of lysyl oxidase and hence the capacity of mesenchymal cells to form cross- linked collagens. [...]
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