Human Physiology, 14th edition (2016)

Reproduction 741

Chorionic Membranes

Between days 7 and 12, as the blastocyst becomes completely

embedded in the endometrium, the chorion becomes a two-

cell-thick structure that consists of an inner cytotrophoblast

layer and an outer syncytiotrophoblast layer (see fig. 20.43 b ).

Meanwhile, the inner cell mass (which will become the fetus)

also develops two cell layers. These are the ectoderm (which

will form the nervous system and skin) and the endoderm

(which will eventually form the gut and its derivatives). A

third, middle embryonic layer—the mesoderm —is not yet

seen at this stage. The embryo at this stage is a two-layer-thick

disc separated from the cytotrophoblast of the chorion by an

amniotic cavity.

As the syncytiotrophoblast invades the endometrium,

it secretes protein-digesting enzymes that create numerous

blood-filled cavities in the maternal tissue. The cytotrophoblast

then forms projections, or villi ( fig. 20.45 ), that grow into these

pools of venous blood and produce a leafy-appearing struc-

ture called the chorion frondosum ( frond 5  leaf). This occurs

only on the side of the chorion that faces the uterine wall. As

the embryonic structures grow, the other side of the chorion

bulges into the cavity of the uterus, loses its villi, and takes on

a smooth appearance.

Since the chorionic membrane is derived from the zygote,

which inherits paternal genes that produce proteins foreign to the

mother, scientists have long wondered why the mother’s immune

system doesn’t attack the embryonic tissues. The placenta is an

immunologically privileged site (protected from immune attack,

like the testes previously discussed in section 20.3), though the

mechanisms involved are only partially understood. Part of the

answer involves the peripheral (outside the thymus) generation

of T reg cells (regulatory T cells; chapter 15, section 15.3) that are

specific for the paternal antigens of the fetus. Interestingly, the

pregnant uterus also contains a unique type of natural killer cells

(NK cells; chapter 15, section 15.5) that help remodel the spiral

arteries of the uterus. This is needed for adequate perfusion of

the placenta with blood; if perfusion is inadequate, preeclampsia

(chapter 14, section 14.7) may result.

Clinical Investigation CLUES

Linda was very happy when her pregnancy test finally

showed a positive result.

• What did the pregnancy test detect in her urine?


• What did the pregnancy kit contain that allowed it to
  detect pregnancy?

Figure 20.45 The extraembryonic membranes. After the syncytiotrophoblast has created blood-filled cavities in the

endometrium, these cavities are invaded by extensions of the cytotrophoblast ( a ). These extensions, or villi, branch extensively to

produce the chorion frondosum ( b ). The developing embryo is surrounded by a membrane called the amnion.

Chorion

(cytotrophoblast)

Ectoderm

Embryo Mesoderm

Endoderm

Yolk sac

of embryo

Amnion

Body stalk

(umbilical cord) Chorionic villi

(a)

Chorion

Umbilical blood vessels

Amnion

Amniotic sac

containing

amniotic fluid

Yolk sac

Placenta

(b)

Villi of chorion

frondosum

the first five to six weeks of pregnancy because the placenta

itself becomes an active steroid hormone-secreting gland. By the

fifth to sixth week, the mother’s corpus luteum begins to regress

(even in the presence of hCG), but by this time the placenta is

secreting more than sufficient amounts of steroids to maintain

the endometrium and prevent menstruation.

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