with blood vessels
Lumen of uterus
Inner cell mass
Lumen of uterus
of the blastocyst.
Diagrammatic view of a
blastocyst that has just adhered
to the uterine endometrium
Slightly later stage of
an implanting embryo
(approximately seven days
after ovulation), depicting
the cytotrophoblast and
syncytiotrophoblast of the
micrograph of an implanted
12 days after ovulation).
(b) R. O’Rahilly and
Human Embryology and
, Wiley–Liss, 3rd Edition,
2001. This material is reproduced
with permission of Wiley–Liss,
Inc., a subsidiary of John Wiley &
Relative blood levels
Hormonal changes during pregnancy.
changes in maternal blood levels of three hormones that maintain
pregnancy are depicted, rather than actual blood concentrations.
lines the inner surface of the trophoblast
these become the
. ±e chorion develops ﬁngerlike
, which become especially elaborate where they are in
contact with maternal blood (Figure 28.7c).
Soon the mesodermal cores of the chorionic villi are invaded
by newly forming blood vessels, which extend to the embryo
as the umbilical arteries and vein. ±e continuing erosion pro-
duces large, blood-ﬁlled
, in the
stratum functionalis of the endometrium (see Figure 27.15,
p. 1039). ±e villi come to lie in these spaces totally immersed in
maternal blood, which is continouously refreshed by extravas-
cularized maternal blood (Figure 28.7d). ±e part of the en-
dometrium that lies beneath the embryo becomes the
u-ah), and that surrounding the uterine cavity
face of the implanted embryo forms the
(Figure 28.7d and e). Together, the chorionic villi and the de-
cidua basalis form the disc-shaped placenta.
±e placenta detaches and sloughs oﬀ a²er the infant is born,
so the name of the maternal portion—
(“that which falls
oﬀ”)—is appropriate. During development, the decidua capsu-
laris expands to accommodate the fetus, which eventually ﬁlls
and stretches the uterine cavity. As the developing fetus grows,
the villi in the decidua capsularis are compressed and degener-
ate, while those in the decidua basalis increase in number and
branch even more profusely.
±e placenta is usually fully functional as a nutritive, respi-
ratory, excretory, and endocrine organ by the end of the third