Human Physiology, 14th edition (2016)

742 Chapter 20

Formation of the Placenta and Amniotic Sac

As the blastocyst implants in the endometrium and the cho-

rion develops, the cells of the endometrium also undergo

changes. These changes, including cellular growth and the

accumulation of glycogen, are collectively called the decidual

reaction. This is because the maternal tissue in contact with

the chorion frondosum is called the decidua basalis. These

two structures—chorion frondosum (fetal tissue) and decidua

basalis (maternal tissue)—together form the functional unit

known as the placenta. The human placenta (Latin  5  cake) is

in the shape of a single disc.

Cells of the cytotrophoblast (see figs. 20.43 b and 20.45 a )

from the chorionic villi invade the spiral arteries of the endo-

metrium. As a result, by the end of the second trimester, the

spiral arteries have been remodeled into dilated tubes lined by

the cytotrophoblast. These produce a low vascular resistance,

so that more maternal blood flows into the placenta.

The disc-shaped human placenta is continuous at its outer

surface with the smooth part of the chorion, which bulges into

the uterine cavity. Immediately beneath the chorionic mem-

brane is the amnion, which has grown to envelop the entire

embryo ( fig.  20.46 ). The embryo, together with its umbilical

cord, is therefore located within the fluid-filled amniotic sac.

Amniotic fluid is formed initially as an isotonic secretion.

Later, the volume is increased and the concentration changed

by urine from the fetus. Amniotic fluid also contains cells that

Figure 20.46 The amniotic sac and placenta. Blood

from the embryo is carried to and from the chorion frondosum

by umbilical arteries and veins. The maternal tissue between

the chorionic villi is known as the decidua basalis; this tissue,

together with the chorionic villi, forms the functioning placenta.

The space between chorion and amnion is obliterated, and the

fetus lies within the fluid-filled amniotic sac.

Decidua

basalis

Chorion

frondosum

Placenta

Amniotic sac

containing

amniotic fluid

Chorion

Amnion

Umbilical

cord

CLINICAL APPLICATION

Amniocentesis ( fig.  20.47 ) entails aspiration of a small

amount of fluid containing fetal cells from the amniotic sac.

This procedure is usually performed when a woman is about

14 to 20 weeks pregnant and the fetus is at risk of genetic

abnormalities. Samples obtained by amniocentesis can be

analyzed by microscopic observation of the chromosomes

(a technique called karyotyping ), and by biochemical analy-

sis of DNA using a technology called chromosomal micro-

arrays. These techniques allow the detection of abnormal

numbers of chromosomes, as in Down’s syndrome ( trisomy

21 ) and Turner’s syndrome (with a single X). Additionally,

certain genetic disorders may be detected, including sickle-

cell disease, Tay-Sachs disease, cystic fibrosis, and mus-

cular dystrophy. Chorionic villus sampling ( CVS ), which

provides larger numbers of fetal cells than amniocentesis,

can be performed as early as 10 to 12 weeks of pregnancy.

A mother’s blood plasma contains a mixture of her DNA

and the DNA of the fetus, because some placental cells

undergo apoptosis and release their DNA into the blood.

Because of this, DNA analysis of the mother’s blood can

now be used to noninvasively test for the fetal sex (looking

for DNA, including the SRY, from the Y chromosome; sec-

tion 20.1). Scientists have even recently reported their abil-

ity to determine much of the genome of a fetus using DNA

in maternal plasma.

are sloughed off from the fetus, placenta, and amniotic sac.

Because all of these cells are derived from the same fertilized

ovum, all have the same genetic composition.

Amniocentesis is usually performed at about the sixteenth

week of pregnancy. By this time the amniotic sac contains

between 175 to 225 ml of fluid. Genetic diseases such as Down

syndrome (characterized by three instead of two chromosomes

number 21) can be detected by examining chromosomes;

diseases such as Tay-Sachs disease, in which degeneration of

myelin sheaths results from a defective enzyme, can be detected

by biochemical techniques.

Major structural abnormalities that may not be predict-

able from genetic analysis can often be detected by ultrasound.

Sound-wave vibrations are reflected from the interface of tissues

with different densities—such as the interface between the fetus

and amniotic fluid—and used to produce an image. This tech-

nique is so sensitive that it can be used to detect a fetal heartbeat

several weeks before it can be heard using a stethoscope.

Exchange of Molecules

Across the Placenta

The umbilical arteries deliver fetal blood to vessels within the

villi of the chorion frondosum of the placenta. This blood cir-

culates within the villi and returns to the fetus via the umbili-

cal vein. Maternal blood is delivered to and drained from the