LM 15.2: Normal Placenta Flashcards
what structure is the placenta derived from?
trophoblasts
how many umbilical arteries and veins are there?
2 umbilical arteries and 1 umbilical vein that go to the placenta from the baby via Wharton’s jelly
arteries bring deoxygenated blood from the fetus to the placenta which returns to the fetus via the umbilical vein
where do uterine arteries originate from?
they started as the late luteal phase spiral arteries that now continue to develop due to presence of a pregnancy
do maternal and fetal blood mix?
no!
there is nutrient exchange via diffusion via the trophoblasts, arteries and veins of the baby in the chorionic plate aka the placenta!
what is the placenta?
fetal tissue attaching and intimately juxtaposed to maternal tissue
it’s an alloimmunografTT the uterus allows foreign DNA to attach without rejection while maternal blood is flowing over the fetal syncytiotrophoblasts on the outside of the villi
when does implantation occur>
day 6 after conception
the blastocyst comes into contact with endometrium, which is receptive due to absence of antiadhesive glycoprotein which is normally produced later in the luteal phase of the menstrual cycle
the blastocysts then adheres to the decidua and then the blastocyst’s trophoblasts invade the decidua, the inner 1/3 of the myometrium and uterine vasculature
trophoblasts differentiate into which 2 cells?
- syncytiotrophoblasts
2. cytotrophoblasts
what are syncytiotrophoblasts?
one of the 2 cell types that embryonic trophoblasts differentiate into
- multinucleated
- outer later
- transport functions of the placenta
- synthesizes hormones
what are cytotrophoblasts?
one of the 2 cell types that embryonic trophoblasts differentiate into
- mono nucleated cells
- inner layer
what are the 2 locations that the embryonic trophoblasts differentiate into?
- villous trophoblasts create the chorionic villi
some of these become the anchors of the placenta to the basal plate and do not go deeper than Nitabuch’s layer
- extravillous trophoblasts—these can penetrate the myometrium
endovascular trophoblast penetrate the spiral artery lumens and replace the endothelial lining and smooth muscle of the arterial wall
interstitial trophoblasts invade decidua and myometrium as well as surround maternal spiral arteries
what are chorionic villi and what is their blood supply?
chorionic villa are supplied by the branching temrinal vessels from the umbilical cord that form the base of the fetal side of the placenta
the villi are located on the maternal side of the placenta and are:
- lined by ctyotrophoblasts
- outside of the villi are covered by syncytiotrophoblast
- provide a large surface are of contact between the villi and maternal blood allowing gait and nutrient exchange
- maternal blood has direct contact to the syncytiotrophoblast layer
what is a placental cotyledon?
a collection of chorionic villi from a main stem villi and supplied by a chorionic artery and drained by a vein
what is the progression of villi development of the placenta?
week 1: primary villi develops with a cytotrophoblast core and syncytiotrophoblast sheath
week 2: secondary villus develops that now also has an extramebryonic mesoderm core within the cytotrophoblast layer
week 3: tertiary villi develop that has vessels forming in the mesenchyme aka the umbilical artery and vein branches are spreading into the villi from the chorionic plate
week 4 and beyond: stem villi develop and anchor the villi to the maternal basal plate; there is continued development of the terminal villi
what happens to the spiral arteries during pregnancy? how do they interact with the placenta?
spiral arteries of the uterus are lined by extravillous cytotrophoblasts that remodel the arterial wall to remove any decrease in flow of blood into the intervillous space aka they dilate the arterial opening into the intervillous space!
the lumen of the artery is actually increased 5-10 fold, making it a low resistance, large bore vessel.
it does not have contractile properties making it also unregulated
what are the characteristics of the placenta at term?
- fundal/posterior position in the uterus
- cephalic presentation of the fetus (head first)
- central insertion of cord into placenta
- fetal surface of placenta is smooth and covered by amnion
- maternal surface isn’t smooth
how big is a mature placenta?
diameter: 20-22 cm
mean weight: 470 gms
thickness: 25 mm
total VILLOUS surface area is 12-14 m2
that’s 11’4” X 11’4” to 12’3” X 12’3”!!
what does the maternal surface of the placenta look like?
maternal surface of the intervillous space has developed septa that correspond to the clefts of the cotyledons and derive from the cytotrophoblast of the basal plate
the septa grow into the inter villous space but not across; they probably but help with a more optimized, directed blood flow within the inter villous space
what covers the fetal surface of the placenta?
amnion
it covers the fetal surface and the umbilical cord
what is the decidua basalis?
it’s the endometrium of pregnancy; the basal plate contains the decidua basalis
it lies directly beneath the implanted blastocyst and is influenced by the invading trophoblastic tissue
it’s triggered by hormones and factors secreted by the blastocyst and it’s central to the allograft of pregnancy
because of the implanted blastocyst, the decidua basalis undergoes the most change via spiral artery growth and invasion
it is shed during the weeks after delivery
how does the decidua basalis change after implantation?
during the luteal phase of the menstrual cycle, the spiral arteries have become more developed, anticipating implantation of the blastocyst
with implantation, the spiral arteries/arterioles are invaded by trophoblasts
a shell of the original arterial wall remains having lost its endothelial cell and smooth muscle layers
the spiral arteries/arterioles become unregulated compared to normal arteries; they do not respond to vasoactive agents.
note: interestingly, the fetal chorionic blood vessels do have smooth muscle and do respond to vasoactive agents.!
what are the 3 parts of the decidua?
- decidua basalis
- decidua capsularis
- decidua parietalis
what is the decidua capsular?
it lies over the implanted blastocyst creating a barrier between blastocyst and endometrial cavity
what is the decidua parietals?
it lines the rest of the endometrial cavity other than where the blastocyst is implanted
what is the decidua vera?
it’s created when the enlarging pregnancy causes the decidua capsularis and decidua parietalis to fuse
formation of the decidua vera obliterates the uterine cavity
what is the Nitabuch layer?
it’s the line that demarcates the trophoblastic invasion from the blastocyst to the decidua basalis
it’s an area of fibrinoid degeneration between trophoblasts and decidua basalis
it is defective or absent in placenta accreta spectrum disorder
what is the chorion frondosum?
when the blastocyst implants, villi persist where contact of decidua basalis occurs, to form the chorion frondosum where the villi/placenta will develop
chorionic villi that are not involved with the decidua basalis, undergo degeneration due to no longer receiving vascular support which creates the chorion leave
what is amnion?
an avascular fetal membrane lining the amniotic sac –> no blood vessels, lymphatics, smooth muscle or nerves
it’s lined by a single layer of cuboidal epithelium –> so the amnion lines the amniotic sac which contains the amniotic fluid
as early as 10 days after fertilization, the amniotic cavity is present as a space between the trophoblastic tissue and the embryonic disc and as the amniotic cavity grows, it surrounds the entire embryo and body stalk
what is the function of amniotic fluid?
- fetal movement; creates space for the fetus to move which is key for neuromusculoskeletal development
- fetal breathing for normal lung development
- fetal swallowing of AF is needed for development of GI tract
- umbilical cord flow integrity; prevents compression of the vessels within the cord
- bacteriostatic function
when does fetal urine production start?
8-11 weeks gestation
what is the composition of amniotic fluid?
it’s hypotonic compared to fetal and maternal osmolality
98% water
contains exfoliated fetal cells/DNA, proteins, etc.
how much amniotic fluid is there normally by term?
800 cc
where does the umbilical cord originate from?
the caudal region of the developing embryo
final development leaves 2 arteries and 1 vein in the umbilical cord
it’s located in the middle of the placental disc but can also arise from near the periphery or even be located completely separate from the disc
what is Wharton’s jelly?
a tissue that provides protection to the vessels and substance to the cord to prevent compression from outside forces
how is the placenta expelled?
once the baby is delivered, the uterus contracts down due to loss of uterine cavity volume
with that contraction, the area of placental attachment decreases relative to the area of the placenta leading to shearing, causing placental detachment
placenta is expelled and as it exits, peeling off the chorion from the uterine cavity, resulting in complete expulsion of placenta and its attached membranes
which hormones does the placenta make?
- steroids
ex. 17β estradiol, estriol, and progesterone - proteins
ex. hCG, hPL, adrenocorticotropin, GnRH, TRH, growth hormone variant, neuropeptide y, inhibin, activin
components for production of these steroids and proteins can cross the placenta to or from the maternal or fetal circulation or originate in the placenta
the placenta produces the most steroid and protein hormone in amount and diversity of any human organ
where does progesterone come from during pregnancy? what is its function?
progesterone is initially produced by the corpus luteum under stimulation by βhCG produced from the implanted blastocyst –> placental production begins at 7 weeks and is the primary source by 10 weeks
production is independent of health status of placenta or fetus
- required for successful preparation of endometrium and implantation
- may contribute to suppressing maternal immunologic response to fetal tissue
- serves as a substrate for the fetal adrenal gland to produce gluco- and mineralocorticoids
where does estrogen come from during pregnancy?
first 5-6 weeks: corpus luteum
after 5-6 weeks: placenta
precursors are from outside the placenta
what do estrogen levels tell you about the pregnancy?
estrogen levels can reflect the health of the fetus!
what does estrogen influence during pregnancy?
- progesterone production
- uteroplacental blood flow
- mammary gland development
- fetal adrenal gland function
- stimulates maternal and trophoblastic renin-angiotensin systems resulting in increase in maternal blood volume
- has vasodilatory effect on uteroplacental blood flow
what is hCG?
a glycoprotein related to LH, TSH, FSH—they all share a common α-unit
it is produced almost exclusively in the placenta but there are low levels produced in the fetal kidneys
αhCG production is related to placental mass –> there is very little free β hCG; measurement of β hCG is in its combined form with αhCG
when does hCG production peak?
secretion of total hCG (α and β hCG combined form) production peaks at 8-10 weeks
what is the function of hCG?
- maintains progesterone production by the corpus luteum until placenta can take over
- stimulates fetal testicular testosterone secretion
it acts as LH surrogate stimulating Leydig cell replication and synthesis of testosterone–> promotes male sexual differentiation
what is hPL?
human placental lactogen aka chorionic growth hormone aka chorionic somatomammotropin
it’s produced early in gestation prior to 6 weeks by cytotrophoblasts but then overall production is predominantly by syncytiotrophoblasts
secretion rate noted to be proportional to placental mass up to about 34-36 weeks
96% homologous to prolactin
what does hPL do?
- promotes maternal lipolysis resulting in increased free fatty acids circulating aka an energy source for maternal and fetal needs
- aids maternal adaptation to fetal energy requirements by increasing insulin resistance allowing more glucose to cross over to fetus.
- hPL increases beta cell proliferation
- prepares mammary glands for lactation
where is alpha fetoprotein produced?
yolk sac
up till about 12 weeks then later largely from the fetal liver
AGP is metabolized by the maternal liver
what do elevated AFP levels indicate?
it’s highly concentrated in the fetal CNS so any malformation of CNS resulting in less tissue between fetus and amniotic fluid results in increased levels in AFP and resultant increased levels in maternal blood
elevated maternal serum AFP is associated with:
1. CNS malformations
- multifetal gestation
- preterm birth
- pre-eclampsia risk
- fetal demise
what are the 4 main functions of the placenta?
- metabolism
- transport of oxygen, cO2, nutrients, waste
- endocrine signaling
- preventing fetal allograft rejection
