Summary chapter 5 embryonic development part

Placenta and Fetal Membranes

 The vessels in the umbilical cord connect the placental circulation with the fetal circulation.
 The fetal membranes include the chorion, amnion, umbilical vesicle, and allantois.
 The placenta is a feto-maternal organ that has two components
1. A fetal part that develops from the chorionic sac (the outermost fetal membrane), formed by the villous chorion
2. A maternal part that is derived from the endometrium, formed by the decidua basalis
 The amnion and chorion are fused with
each other and the decidua parietalis,
thereby obliterating the uterine cavity.
 Nutrients and oxygen pass from the
maternal blood through the placenta to
the embryo/fetal blood, and waste
materials and CO2 pass from the fetal
blood through the placenta to the
maternal blood.


 Decidua
 The decidua is the endometrium of the
uterus in a pregnant
 It is the functional layer of endometrium
 that separates from the remainder of the
uterusafter parturition (childbirth).
 3 regions of the Decidua
1. The decidua basalis: maternal part of
the placenta.
2. The decidua capsularis is the superficial
part
3. The decidua parietalis represents the
remaining parts of the decidua.
 In response to increasing progesterone
levels in maternal blood, the connective
tissue cells of the decidua
enlarge to form decidual cells.
 Note: conceptus = embryo + membranes

,  The fetal part is attached to the maternal part of the placenta by the cytotrophoblastic shell, the external layer of
trophoblastic cells on the maternal surface of the placenta


 amniotic sac enlarges faster than the chorionic sac, as a result, the amnion and smooth chorion fuse to form
the amniochorionic membrane ( that ruptures during labor)
 Membrane rupture allows amniotic fluid to escape through the vagina
 The intervillous space of the placenta, which by 8 to 10 weeks contains maternal blood, is derived from the lacunae
(small spaces) that developed in the syncytiotrophoblast during the 2nd of development


 Placental Circulation
 chorionic villi of the placenta provide a large surface area for the exchange
a) Fetal Placental Circulation
 Poorly oxygenated blood passes through the umbilical
arteries  placenta dividechorionic arteries 
branch freely in the chorionic plate villi
 well-oxygenated fetal blood in the fetal capillaries passes
 thin-walled veins  converge to the site of
attachment of the umbilical cord. They converge to form
the umbilical vein
b) Maternal Placental Circulation
 maternal blood enters the intervillous space through 80-
100 spiral endometrial arteries in the decidua basalis.
 The blood eventually returns through
the endometrial veins to the
maternal circulation.
 Reductions of uteroplacental
circulation result in fetal hypoxia &
intrauterine growth restriction (IUGR)




 Fetal and maternal circulations are
The embryo is defined as
separated by thepregnancy
the developing placental
from the time of
membrane consisting
fertilization of extrafetal
until the end
of the 8th week of
tissuesgestation, when it
becomes known as a fetus