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Development of the Placenta and Fetal Membranes
Pregnancy is divided into 3 trimesters: First Trimester is called embryogenesis
(organogenesis); the Second and Third Trimester are called fetal period  Chorion,
amnion, yolk sac, and allantois are fetal membranes that develop from zygote but do not
form parts of embryo or fetus
Placenta has fetal part (formed from chorion (synctiotrophoblast, cytotrophoblast, and
extra-embryonic mesoderm)) and maternal part (formed from endometrium (decidua)) 
At end of 2nd week, cytotrophoblast cells of chorionic sac extend into synctiotrophoblast
to form primary chorionic villi  In 3rd week, mesenchyme (extra-embryonic mesoderm)
grows into primary chorionic villi forming core of loose CT and thus becomes secondary
chorionic villi  Some of the mesenchyme differentiate into blood capillaries and blood
cells thus becomes tertiary chorionic villi; these villi extend out to the cytotrophoblast
shell and anchor chorionic sac to endometrium (these villi are stem villi (anchoring
villi)); addnl branching from stem villi called free villi constitute main sites for gas
exchange  The rupture of maternal blood vessels and endometrial glands by the lacunae
(spaces) of synctiotrophoblast marks the first stage of placental dvpt, thus maternal blood
is surrounded by fetal synctiotrophoblast  By the end of 3rd week, fetomaternal organ
of placenta is established
Implanting blastocyst and increasing progesterone levels in maternal blood cause
decidual rxn in endometrial cells, which enlarge and become decidual cells (contain large
amts of glycogen and lipids  Decidua consists of three regions based on their
relationship to implantation site: decidua basalis (maternal part of placenta; part of
decidua that lies deep to embryo); deciduas capsularis (part of decidua overlying
embryo); decidua parietalis (remaining endometrium)
Up to the 8th week, villi cover entire chorion sac  as the sac grows, the villi associated
w/ decidua capularis (over top of embryo) becomes compressed with reduced blood
supply and eventually degenerate leaving a bare area known as chorion leave (smooth
chorion)  the villi associate w/ decidua basalis increase rapidly in size and number
making chorion frondosum (bushy or villous chorion), which becomes fetal part of
placenta  the decidua basalis forms maternal component of placenta
By the 4th month, decidua basalis is almost completely replaced by chorion frondosum 
the decidua basalis forms a number of septa (decidua (placental) septa) which project into
intervillous spaces but do NOT make contact w/ chorionic plate; septa have core of
maternal tissue but surface is covered by layer of trophoblast cells (cytotrophoblast and
synctiotrophoblast) thus maternal blood and fetal blood are separated at all times;
decidual septa divide fetal part of placenta into a number of irregular compartments or
cotyledons (each cotyledon has two or more stem villi (w/ their branches)  endometrial
arteries (spiral arteries) fill intervillous spaces w/ maternal blood and these spaces are
drained by endometrial veins
2 placental circulations: fetal and maternal
Fetal circulation  deoxygenated blood leaves fetus in 2 umbilical arteries (these arteries
branch freely in chorionic plate before entering villi; w/in villi they form an extensive
capillary network, bringing fetal blood extremely close to maternal blood but there is NO
mixing of maternal and fetal blood—always a barrier btw blood supply of mother and
fetus) and passes to placenta via umbilical cord  after passing through the placenta, the
umbilical vein (only 1) collects oxygenated blood and returns it to fetus via umbilical
cord  umbilical vein is true vein and had all characteristics of vein (it is the only vein in
the fetus to carry oxygenated blood—all other veins carry deoxygenated blood)
Placental membrane always present but varies in thickness during gestation  10-20 wks
consists of synctiotrophoblast, cytotrophoblast, connective tissue (mesenchyme), and
endothelium of fetal capillary  final placental membrane towards full term consists of
synctiotrophoblast and endothelium of fetal capillary
Maternal circulation  maternal blood enters intervillous space through 80-100 spiral
arteries which penetrate endometrium (decidua basalis)  blood flow is pulsatile and
under very high pressure which spurts toward roof of chorionic plate  once here,
pressure dissipates and blood flows slowly around surfaces of branched/free villi thus
allowing gas and nutrient exchange to occur across placental membrane  gaseous and
metabolic waste products are in high concentration in fetal blood relative to maternal
blood and so diffuse down their concentration gradient into maternal blood across
placental membrane – conversely, oxygen and nutrients are in high concentration in
maternal blood relative to fetal blood and so diffuse down their concentration gradient
into fetal blood across placental membrane  when blood reaches floor of intervillous
space, it is drained by spiral veins and is returned to maternal circulation
Three Placental functions  Metabolism: especially during early pregnancy, placenta
synthesizes glycogen, cholesterol, and fatty acids, which serve as sources of nutrients and
energy for developing embryo/fetus  Transport of nutrients and wastes by simple
diffusion, facilitated diffusion, active transport, pinocytosis  gases transported: O2,
CO2, CO (interruption of O2 transport for even a few minutes will endanger fetal
survival)  nutrients transported: aas, carbohydrates (glucose), free fatty acids, vitamins,
electrolytes (there is little or no transfer of maternal cholesterol, triglycerides, or
phospholipids)  hormones transported: steroidal hormones (protein hormones do not
reach embryo/fetus)  antibodies (many of the IgG class of abs are readily transported
from maternal to fetal circulation; these maternal abs confer fetal immunity to such
diseases as diphtheria, smallpox, measles—no immunity is acquired to pertussis or
chicken pox) fetus does NOT produce its own abs until well after birth  some blood
may cross over and mix through microscopic breaks in the placental membrane  most
drugs and their metabolites cross placenta by simple diffusion (any drug taken by mother
can affect embryo/fetus directly or indirectly; fetal drug addiction may occur after
maternal use of drugs such as heroine and cocaine)  thalidomide causes major
congenital malformations specifically, limb and ear defects if taken during critical period
of limb devpmt -24-36 days after fertilization  viruses such as measles, rubella, and
poliomyelitis may pass through placental membrane and cause fetal infection and thus
congenital abnormalities and/or death (bacteria do NOT cross placental membrane) 
waste products transported: CO2, urea, uric acid, and bilirubin pass through placental
membrane from fetus to maternal circulation by simple diffusion  Endocrine secretion:
placenta synthesizes human chorionic gonadotrophin (HCG) which maintains corpus
luteum until placenta itself begins to produce its own estrogen and progesterone (HCG
hormone used for pregnancy test in early development (2nd week))  other hormones
synthesized include human placental alctogen (somatomammotropin) which promotes
breast milk production and gives fetus priority on maternal blood glucose, human
chorionic adrenocortocotropin (hACT), prolactin, relaxin, prostaglandins, progesterone,
and estrogen
Amnion forms fluid-filled sac that surrounds embryo and later fetus  attached to
margins of embryonic disc and hence its junction w/ the embryo (future umbilicus) is
located on the ventral surface after embryonic folding  as amnion increases in size, it
fuses w/ chorionic membrane (serves two functions: 1) forms amniochorionic membrane,
which ruptures upon initial stages of labor when “water breaks” 2) obliterates the
chorionic cavity and enfolds the umbilical cord which connects the embryo/fetus to
placenta  w/ subsequent growth, amniochorionic membrane will fuse w/ decidua
parietalis and thus obliterate the uterine cavity (decidua capularis thins and eventually
degenerates)
Amniotic fluid derived primarily from maternal blood and exchanged every 3 hrs
indicating enormous exchange btw amniotic cavity and maternal circulation  amniotic
fluid increases gradually w/ a maximum of ~700-1000 mL at 37 wks  fetus swallows
its own amniotic fluid  after 5th month the fetus swallows 400 mL per day  fetus also
contributes to amniotic fluid about 500 mL of urine in late pregnancy and about 400 mL
from respiratory tract
Function of Amniotic fluid:
- permits symmetrical external growth of embryo/fetus
- prevents adhesion of embryo/fetus to amnion
- cushions embryo/fetus against jolts
- helps control embryo’s/fetus’s body temperature
- enables embryo/fetus to move freely
- permits normal fetal lung development
Oligohydramnios: condition defined by low volume of amniotic fluid (~400 mL 3rd
trimester)  can occur from placental deficiency caused by diminished placental blood
flow, preterm ruptureof amniochorionic membrane which occurs in about 10% of cases
and is most common cause, or renal agenesis (absence of kidneys)  can cause
pulmonary hypoplasia (small lungs), facial defects, and limb defects, mainly caused by
the uterine compression of fetus  these defects are symptoms of potter syndrome which
also displays prominent infraorbital creases, a flattened nose, and excessive skin folds
Polyhydramnios (hydramnios): condition defined by high volume of amniotic fluid,
usually in excess of 2 liters  can occur when fetus does not swallow usual amount of
amniotic fluid  can be associated w/ severe anomalies of CNS eg meroanencephaly and
anencephaly and congenital anomalies of digestive system eg esophageal atresia  all of
these conditions prevent fetus from swallowing amniotic fluid
Yolk sac connected to midgut by narrow yolk stalk  very small and although it is
nonfunctional as far as yolk storage is concerned, it is essential for 1) transfer of nutrients
during 2nd and 3rd weeks before uteroplacental circulation established 2) blood cell
development begins in its wall 3) part of the sac is incorporated into the wall of the gut 4)
primordial germ cells arise here by 3rd week and then they migrate to developing gonads
(once there they differentiate into germ cells (spermatogonia in males and oogonia in
females)  yolk stalk usually detaches from gut by end of 6th wekk and later the yolk
stalk and sac atrophies  persistence of yolk stalk occurs in about 2% of adults as an
ileal divericulum known clinically as Meckel’s diverticulum
Allantois appears on about day 16 as a small sausage shaped diverticulum (outpouching)
from caudal wall of yolk sac that extends into connecting stalk (which later forms
umbilical cord)  allantois is large sac-like structure in embryos of reptiles, birds, and
some mammals  it has respiratory function and/or acts as a reservoir for urine during
embryonic life  in humans allantois remains very small and is not functional b/c
placenta and amniotic sac take over its functions however it does have several important
functions 1) blood cell formation occurs in its wall during 3rd to 5th week 2) its blood
vessels become the umbilical vein and arteries 3) allantois forms first a thick tube called
the urachus which after birth becomes a fibrous cord called the medial umbilical ligament
 this ligament extends from apex of urinary bladder to umbilicus