Download SECTION 3 Amnion Tensile Strength Amnion Metabolic

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Placentation, Embryogenesis, and Fetal Development
particular in the case of premature membrane rupture (Mogami,
2013; Whittle, 2000).
THE UMBILICAL CORD
■ Cord Development
SECTION 3
■ Amnion Tensile Strength
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 and are the most abundant proteins in the body. 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 (Jeffrey, 1991). Although collagen III
provides some of the amnion extensibility, elastin microfibrils
have also been identified (Bryant-Greenwood, 1998).
Amnion tensile strength is regulated in part by fibrillar collagen interacting with proteoglycans such as decorin, which
promote tissue strength. Compositional changes at the time of
labor include a decline in decorin and increase in hyaluronan.
This leads to a loss of tensile strength and is further discussed in
Chapter 42 (p. 840) (Meinert, 2007). Fetal membranes overlying the cervix have a reported regional decline in expression of
matrix proteins such as fibulins. This change may contribute
to tissue remodeling and tensile strength loss (Moore, 2009).
The yolk sac and the umbilical vesicle into which it develops are
prominent early in pregnancy. At first, the embryo is a flattened
disc interposed between amnion and yolk sac. 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 exocoelom, 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 greater description of this cord and
potential abnormalities is found in Chapter 6 (p. 121).
The cord at term normally has two arteries and one vein
(Fig. 5-20). The right umbilical vein usually disappears early
during fetal development, leaving only the original left vein. In
sections of any portion of the cord near the center, the small
duct of the umbilical vesicle can usually be seen. The vesicle is
lined by a single layer of flattened or cuboidal epithelium. In
sections just beyond the umbilicus, another duct representing
the allantoic remnant is occasionally found. The intraabdominal portion of the duct of the umbilical vesicle, which extends
from umbilicus to intestine, usually atrophies and disappears,
but occasionally it remains patent, forming the Meckel diverticulum. The most common vascular anomaly is the absence
of one umbilical artery, which may be associated with fetal
anomalies (Chap. 6, p. 122).
■ Amnion Metabolic Functions
From the foregoing, it is apparent that the amnion is more than
a simple avascular membrane that contains amnionic fluid. It
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 to include production of MMPs, IL-8, and collagenase (Bryant-Greenwood, 1998; Maradny, 1996; Mogami,
2013). Such factors may modulate changes in membrane properties during labor.
■ Amnionic Fluid
Until about 34 weeks’ gestation, the normally clear fluid that
collects within the amnionic cavity increases as pregnancy progresses. After this, the volume declines. At term, the average
volume is approximately 1000 mL, although this may vary
widely in normal and especially abnormal conditions. The origin, composition, circulation, and function of amnionic fluid
are discussed further in Chapter 11 (p. 231).
FIGURE 5-20 Cross section of umbilical cord. The large umbilical
vein carries oxygenated blood to the fetus (right). To its left are
the two smaller umbilical arteries, carrying deoxygenated blood
from the fetus to the placenta. (Photograph contributed by
Dr. Mandolin S. Ziadie.)
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