1074
UNIT 5
Continuity
28
Formation and Roles of
the Extraembryonic Membranes
Name and describe the formation, location, and function of
the extraembryonic membranes.
Te
extraembryonic membranes
that form during the first two
to three weeks of development include the amnion, yolk sac,
allantois, and chorion (Figure 28.7c). Te
amnion
(am
9
ne-on)
develops when cells of the epiblast fashion themselves into a
transparent membranous sac. Tis sac, the amnion, becomes
filled with
amniotic fluid
. Later, as the embryonic disc curves
to form the tubular body, the amnion curves with it. Eventually,
the sac extends all the way around the embryo, broken only by
the umbilical cord (Figure 28.7d).
Sometimes called the “bag of waters,” the amnion provides
a buoyant environment that protects the developing embryo
against physical trauma, and helps maintain a constant homeo-
static temperature. Te fluid also prevents the rapidly growing
embryonic parts from adhering and fusing together and allows
the embryo considerable freedom of movement. Initially, the
fluid is derived from the maternal blood, but as the fetal kidneys
become functional later in development, fetal urine contributes
to amniotic fluid volume.
Te
yolk sac
forms from cells of the primitive gut, which
arrange themselves into a sac that hangs from the ventral sur-
face of the embryo (Figure 28.7b–d). Te amnion and yolk
sac resemble two balloons touching one another with the em-
bryonic disc at the point of contact. In many species, the yolk
sac is the main source of nutrition for the embryo, but human
differentiation of the mammary glands and ready them for lac-
tation. Te placenta also produces other hormones, such as
hu-
man placental lactogen
and
relaxin
. Shortly we will describe the
effects of these hormones on the mother.
Check Your Understanding
7.
What is the composition of the chorion?
8.
What endometrial decidua cooperates with the chorionic villi
to form the placenta?
9.
Generally speaking, when does the placenta become fully
functional?
For answers, see Appendix H.
Events of Embryonic
Development: Gastrula to Fetus
Having followed placental development into the fetal stage, we
will now backtrack and consider development of the embryo
during and a±er implantation, referring again to Figure 28.7.
Even while implantation is occurring, the blastocyst is being
converted to a
gastrula
(gas
9
troo-lah), in which the three pri-
mary germ layers form, and the extraembryonic membranes
develop. Before becoming three-layered, the inner cell mass first
subdivides into two layers—the upper
epiblast
and the lower
hy-
poblast
(Figure 28.7a, b). Te subdivided inner cell mass is now
called the
embryonic disc
.
Placenta
Decidua
basalis
Chorionic
villi
Myometrium
Maternal
veins
Maternal
arteries
Umbilical arteries
Umbilical vein
Connection to yolk sac
Umbilical cord
Chorionic villus
containing fetal
capillaries
Uterus
Lumen of
uterus
Decidua
capsularis
Fetal arteriole
Fetal venule
Amnion
Umbilical cord
Maternal blood
in lacuna
(intervillous space)
Stratum basalis
of endometrium
Maternal portion
of placenta
(decidua basalis)
Fetal portion of
placenta (chorion)
Figure 28.8
Detailed anatomy of the vascular
relationships in the mature decidua basalis.
This state of
development has been accomplished by the end of the third
month of development.
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