384
UNIT 3
Organ Systems
Figure 19-13A.
Overview of the placenta.
H&E, left
3
13; left inset
3
55; right (
upper
)
3
704; right (
lower
)
3
748
The
placenta
consists of the
maternal portion
and the
fetal portion
. It is a temporary organ that provides a bridge for exchanging
gases, nutrients, hormones, and other materials between the maternal and fetal blood circulations. The
maternal portion
is the
decidua basalis
. The
fetal portion
consists of the
chorionic plate
(Fig. 19-11A),
chorionic villi
, and
cytotrophoblastic shell
. Fetal
blood fl
ows within the blood vessels of the chorionic villi; maternal blood is contained within the intervillous space. The placental
barrier prevents the fetal blood from mixing with the maternal blood. The decidua basalis forms when stromal ±
broblasts of the
endometrium are transformed into decidual cells at the site of implantation. The syncytiotrophoblast invades the maternal blood
vessels replacing smooth muscle in the vessel walls. Syncytiotrophoblasts also line the surface of the intervillous space. The cytotro-
phoblast forms an interface (
cytotrophoblastic shell
) between the maternal and fetal tissues.
Decidual cell
Decidual cell
Decidual cell
Cells in
Cells in
cytotrophoblastic
shell
cytotrophoblastic shell
Cells in
cytotrophoblastic
shell
Fetal blood cells
Fetal blood cells
within the chorionic villus
within the chorionic villus
Fetal blood cells
within the chorionic villus
Chorionic
plate
Amniotic
cavity
Floating
Floating
chorionic
chorionic
villus
villus
Floating
chorionic
villus
Intervilluus space
Intervilluus space
Intervillous space
Fetal
Fetal
portion
portion
Fetal
portion
Maternal portion
Maternal portion
(decidua basalis)
(decidua basalis)
Maternal portion
(decidua basalis)
Anchoring
Anchoring
villus
villus
Anchoring
villus
Decidua
Decidua
basalis
basalis
Decidua
basalis
Cytotrophoblastic
Cytotrophoblastic
shell
shell
Cytotrophoblastic
shell
A
Chorionic
Chorionic
villus
villus
Chorionic
villus
Stem
Stem
villus
villus
Stem
villus
Chorionic plate
Chorionic plate
Chorionic plate
Fetal blood vessel
Fetal blood vessel
Fetal blood vessel
Syncytiotrophoblast
Syncytiotrophoblast
Syncytiotrophoblast
Syncytiotrophoblast
Syncytiotrophoblast
Syncytiotrophoblast
Intervillous
Intervillous
space
space
Intervillous
space
Amniotic cavity
Amniotic cavity
Amniotic cavity
Cytotrophoblast
Cytotrophoblast
Cytotrophoblast
Maternal blood in the
Maternal blood in the
intervillous space
intervillous space
Maternal blood in the
intervillous space
Chorionic villus
Chorionic villus
Chorionic villus
B
Figure 19-13B.
Fetal portion of the placenta.
H&E, left
3
18; right
3
136
The chorionic plate consists of connective tissue and forms the wall of the amniotic cavity; it contains chorionic arteries and veins. The
chorionic villi can be classi± ed on the basis of their developmental stages: (1)
Primary chorionic villi
are newly formed villi at an early
stage (about the second week of implantation), and consist of only a trophoblast layer. (2)
Secondary chorionic villi
develop at the end of
the second week when mesenchymal tissue grows into the villi and forms a mesenchymal core within the trophoblastic shell. (3)
Tertiary
chorionic villi
develop at the third week, at which time the fetal blood and blood vessels are formed within the chorionic villi. By the end of
the third week, the fetal blood begins to fl ow, and gas and nutrient exchange takes place between the fetal and maternal blood by diffusion
through the placental barrier. The
placental barrier
is composed of the
syncytiotrophoblast
,
cytotrophoblast
,
connective tissue
of the
villus
,
endothelium
of the
fetal capillary
, and the
basement membranes
of the trophoblast and endothelium. The syncytiotrophoblast produces
hCG
hormone, which plays an important role in maintaining pregnancy via stimulation of the
corpus luteum
to secrete
progesterone
.
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