Flashcards in Placentation Deck (48):
Where does the fetal portion of the placenta develop from and what are its functions?
Develops from chorion, functions to protect the fetus, give nutrition, respiration, excretion, and endocrine function (hormone secretion).
What is the name for the maternal portion of the placenta? What is it derived from?
Decidua basalis- derived from the upper layers of the endometrium. it is shed during birth with the rest of the placenta
What are the three layers of the decidua?
1. Decidua basalis - forms basal plate of placenta, in direct contact with placenta directly. Directly beneath the conceptus
2. Decidua capsularis - endometrium around smooth chorion
3. Decidua parietalis - opposite endometrial wall
What is the decidua capsularis?
Intermediate layer of maternal decidua, formed by endometrium surrounding the blastocyst. It's all the way around the chorion early on, and will ultimately degenerate when pushed against the parietalis
What is the decidua parietalis?
The remainder of the endometrium not in contact with the conceptus. It is the opposite wall of the uterus. As the conceptus grows, the capsularis is pushed towards the parietalis, and it is degraded by midgestation.
What is chorion frondosum?
Villous chorion - the fetal portion of the placenta
It is in contact with the decidua basalis
What is chorion laeve?
Smooth chorion - where the chorionic villi with the mesoderm have degenerated and smoothed out. Generally contacts the decidua capsularis
What are chorionic villi and what happens to them during gestation?
Part of the chorion frondosum, they make the fetal placenta. They are the site of gas exchange, and are bathed in the maternal blood of the intervillous space. They branch more during gestation.
How does blood travel back to the fetus from the placenta?
Blood vessels from chorionic villi converge across the chorion and form the fetal vein, which carries oxygenated blood to the primordial heart. The arteries actually carry deoxygenated blood away from fetus towards chorionic villi
What is a true knot vs. false knot of umbilical cord?
False knot - just a loop in blood vessels that is harmless
True knot - knot formed by fetus moving around, can constrict blood flow if tightened, and can actually lead to death of fetal tissues.
What composes the layers of the early fetal umbilical cord?
1. Outermost layer of amnion (it ensheathes connecting stalk)
2. Mucoid mesoderm (Wharton's jelly)
3. Two umbilical arteries - deoxygenated blood
4. One umbilical vein - oxygenated blood
5. Vitelline duct - only in segment closest to fetus, contains yolk sac stalk and two blood vessels.
What is the chorionic plate?
The "roof" of the placenta, it is on fetal side. Has vascularized (extraembryonic somatopleuric) mesoderm, continuous with umbilical cord. Underside is lined with cytotrophoblast + syncytiotrophoblast. The amnion has pressed against the edges of the chorionic cavity and thus tightly opposes the mesoderm.
What are the chorionic villi?
Site of maternal-fetal exchange in the placenta, it is between chorionic plate and basal plate. Syncytiotrophoblast cells opposing the intervillous space have microvilli for increasing surface area of gas exchange. Underneath this layer is cytotrophoblasts which sit on a basal lamina. Beneath is mesoderm which is vascularized and carries blood back.
How do the chorionic villi change during gestation?
Early - fetal capillaries are located deep in the mesodermal mesenchyme.
->The "placental barrier" gets thinner over time.
Late - fetal capillaries form at periphery, and there are a reduced number of cytotrophoblasts by third trimester, making gas exchange easier
What causes circulating cell-free fetal DNA in the maternal circulation?
Syncytiotrophoblast cells contacting the intervillous space will undergo apoptosis and form syncytial knots, which are shed and enter the maternal circulation. These contain fetal DNA, and become high concentration as placenta matures
What is the floor of the placenta?
The basal plate, which is made of the cytotrophoblastic shell. This consists of anchoring villi and decidua basalis. Spiral arteries will populate this space and even penetrate into the intervillous space
How are anchoring villi made?
They are columns from proliferating cytotrophoblast cells that break through from the stem villi and anchor into the decidua basalis. Forms the cytotrophoblast shell
Where do extravillous trophoblast cells come?
From cytotrophoblasts in the cytotrophoblastic shell
What is the function of an extravillous trophoblast?
Invades the decidua basalis (endometrial wall) to remodel maternal arteries, as deep as the upper third of the myometrium. They function to break down vascular smooth muscle and endothelium that lines those arteries, to keep them permanently dilated. This increases blood volume (high capacity, lower resistance)
What are the two identifying antigenic factors expressed by extravillous trophoblasts?
1. Cytokeratin-7 - expressed by epithelial cells - ALL trophoblasts
2. Human leukocyte antigen - G - a major histocompatibility complex complex protein
Why do extravillous trophoblasts express HLA-G?
HLA-A and HLA-B are most common, but have tons of antigenic variation. HLA-G is only expressed by adult cells in immune privileged areas like the thymus because it has minimal variation. it serves to cloak these cells from maternal immune attack
What are two possible outcomes of failure for extravillous trophoblast cells to perform their function?
2. Fetal growth restriction
Since inadequate blood will be delivered unless one of these two things occurs
What are the symptoms of pre-eclampsia?
Hypertension, which leads to proteinuria from resulting kidney dysfunction.
What is the treatment for pre-eclampsia?
Delivering the baby early, usually by C-section, since hypertension and kidney dysfunction put both the mom and baby at risk. This accounts for 15% of premature births.
How dangerous is eclampsia?
It is the most common cause of death in mothers, 40-80%. It is seizures during pregnancy or parturition normally preceded by pre-eclampsia
How is oxygen delivered to the placenta by mom?
Intervillous space is filled with about 150 cc of maternal blood via spiral uterine arteries that empty into placental floor. Deoxygenated blood is drained by uterine veins.
What is the gas gradient across the placental membrane? How is this compensated for?
Maternal blood has pO2 of about 50 mmHg. Fetal blood is PO2 of about 30 mmHg. Oxygen diffuses via 20 mmHg gradient, and fetal hemoglobin has high carrying capacity (25% more), which reduces blood free O2 concentration.
What other substances easily cross the placental membrane other than oxygen?
nutrients, waste products including CO2, uric acid, urea, bilirubin, creatinine, antibodies - IgG only (passive immunity), DRUGS via simple diffusion
How are hormones secreted by the placenta and what are they? What are their functions?
All released by syncytiotrophoblast
1. hCG - maintains corpus luteum during first trimester
2. Human placental lactogen - breast development, fetal protein deposition, decreased maternal glucose utilization
3. Progesterone - maintains endometrium and prevents uterine contractions
4. Estrogen - enlargement of uterus, breasts, breast glands, and external genitalia (including birth canal) for delivery
When is the corpus luteum not needed and why?
By about Week 10, the syncytiotrophoblast cells of the placenta now produce enough progesterone that it is no longer needed.
Why does the chorion laeve contain avascular mesoderm?
Villi used to be present over the entire chorion through the 8th week. But they are associated with the decidua capsularis, and are ultimately stretched by growth and compressed until they lose their blood supply.
What is the leading cause of premature births?
Rupturing of amniochorionic membrane, which is the structure formed whenever the chorionic cavity is displaced by filling amnion, and the two membranes (amnion + chorion) become closely opposed.
How does the amniotic fluid enter the amniotic cavity?
Most diffuses from decidua parietalis across the amniochorionic membrane. Some comes from intervillous blood across chorionic plate. It is formed from water in maternal and fetal circulation and is approximately 1000 cc at term.
What is the function of the amniotic fluid?
1. Swallowed and excreted by fetus, exchanges with fluid in fetal lungs, intestine, and kidneys
2. Buoyant for symmetrical growth, allows for fetal movement for developing musculoskeletal system
3. Prevents adhesion of amnion to embryo
4. Acts as cushion against impacts
5. Controls temperature inside cavity
6. Protection against infection
What is oligohydramnios?
Reduced amniotic fluid (
What is polyhydramnios?
increased amniotic fluid (up to 2000 cc), caused by poor fetal swallowing of amniotic fluid.
Can be because of CNS disorders or mechanical blockage of digestive tract
What is amniocentesis?
Invasive procedure performed beginning in Week 14, transabdominal retrieval of amniotic fluid (~10 cc) via ultra-sound guided needle. Fetal cells in fluid are cultured to obtain DNA for testing
What is Chorionic Villous Sampling (CVS)?
Invasive procedure performed beginning Week 10, transabdominal or transcervical retrieval of chorionic villi. Ultrasound-guided needle or catheter insertion, tissue fragments can be cultured or directly analyzed.
When is circulating cell-free fetal DNA used?
By week 10, sufficient quantities of fetal DNA are in maternal plasma >10% of DNA can be used to sequence via next-gen sequencing. Can really only detect chromosomal # disorders, via comparison from parent
What is transcervical retrieval of fetal cells?
Some extravillous trophoblast cells migrate from the anchoring villi to the cervix. A PAP smear is done, and fetal cells can be isolated away based on immunoaffinity for HLA-G. Collects whole cells -> whole fetal DNA can be obtained, as in invasive procedures
Can be done by week 5.
What does a term placenta look like from the fetal side?
Umbilical cord attached anywhere, but usually central. Amnion and smooth chorion are present. Amnion is tight against placenta, covering chorionic plate. There are large chorionic vessels visible under surface
What does the maternal side of the placenta look like?
Cotyledons - cobblestone appearance from groups of chorionic villi, with thin, skin-like covering that is the decidua. Placenta is 2-3 cm thick, with umbilical cord is 1-2 cm thick.
What factors increase likelihood of dizygotic twinning?
1. Ovulation induction procedures that generate multiple oocytes
2. Hereditary bias
When is formation of the fetal membranes the same between monozygotic and dizygotic twins?
When monozygotic twins split prior to the blastocyst stage (totipotent). They may have separate placenta, chorion, and amnion, or the placenta and chorion may fuse.
What happens when monozygotic twins separate before blastocyst formation and how common is this?
Same things can happen as in dizygotic twinning, you just have genetically identical progeny. This happens 35% of the time.
What process represents 65% of monozygotic twinning and what characterizes it?
Separation occurs DURING blastocyst formation, thus resulting in two inner cell masses. Each ICM forms an amnion, but there is only one chorion, and therefore only one placenta.
What is the rarest form (1%) of monozygotic twinning and what characterizes it?
Separation occurs after implantation within the embryonic disc after formation of the amnion. The two fetuses will have to share a single amnion, as well as chorion and placenta.