During tests of tensile strength, the decidua and then the chorion laeve give way long before the amnion ruptures. Indeed, the membranes are elastic and can expand to twice normal size during pregnancy (Benirschke, 2012). The amnion tensile strength resides almost exclusively in the compact layer, which is composed of cross-linked interstitial collagens I and III and lesser amounts of collagens V and VI.
Collagens are the primary macromolecules of most connective tissues. Collagen I is the major interstitial collagen in tissues characterized by great tensile strength, such as bone and tendon. In other tissues, collagen III is believed to contribute to tissue integrity and provides both tissue extensibility and tensile strength. For example, the ratio of collagen III to collagen I in the walls of a number of highly extensible tissues-amnionic sac, blood vessels, urinary bladder, bile ducts, intestine, and gravid uterus-is greater than that in nonelastic tissues a efrey, 1991).
Amnion tensile strength is regulated in part by fibrillar collagen assembly. his process is influenced by the interaction fibrils with proteoglycans such as decorin and biglycan (Chap. 21, p. 409). Reduction of these proteoglycans is reported to perturb fetal membrane function (Horgan, 2014; Wu, 2014). Fetal membranes overlying the cervix have a regional shift in gene expression and lymphocyte activation that set in motion an inflammatory cascade (Marcellin, 2017). his change may contribute to tissue remodeling and loss of tensile strength in the amnion (Moore, 2009).
The amnion is metabolically active, is involved in solute and water transport for amnionic fluid homeostasis, and produces an impressive array of bioactive compounds. The amnion is responsive both acutely and chronically to mechanical stretch, which alters amnionic gene expression (Carvajal, 2013; Nemeth, 2000). This in turn may trigger both autocrine and paracrine responses that include production of MMPs, IL-8, and collagenase (Bryant-Greenwood, 1998; Mogami, 2013). Such factors may modulate changes in membrane properties during labor.
he yolk sac and the umbilical vesicle into which it develops are prominent early in pregnancy. At irst, the embryo is a lattened disc interposed between amnion and yolk sac (see Fig. 5-9). Its dorsal surface grows faster than the ventral surface, in association with the elongation of its neural tube. Thus, the embryo bulges into the amnionic sac, and the dorsal part of the yolk sac is incorporated into the embryo body to form the gut. The allantois projects into the base of the body stalk from the caudal wall of the yolk sac and later, from the anterior wall of the hindgut.
As pregnancy advances, the yolk sac becomes smaller and its pedicle relatively longer. By the middle of the third month, the expanding amnion obliterates the extraembryonic coelom, fuses with the chorion laeve, and covers the bulging placental disc and the lateral surface of the body stalk. The latter is then called the umbilical cord-or funis. A more detailed description of this cord and potential abnormalities is found in Chapter 6 (p. 117).
he cord at term normally has two arteries and one vein (Fig. 5-17). The right umbilical vein usually disappears early during fetal development, leaving only the original left vein.