Placenta Flashcards
When is clinical gestation timed from?
Clinical gestation is timed from the 1st day of the last menstrual period.
What is the normal full term range for pregnancy?
The normal full term range is 37 to 42 weeks.
How is pregnancy split?
Pregnancy is split into 3 trimesters, each lasting 12-14 weeks.
In which trimester does the placenta form?
The placenta forms in the first trimester.
Which grows faster, the embryo or the placenta?
The placenta grows faster to ensure it can provide adequate nutrients to the embryo.
What is the embryological period?
The embryological period is the further development and differentiation of tissues up to week 8/9.
What is required for successful implantation?
A receptive endometrium is required for successful implantation.
Which hormone thickens the endometrium?
Oestrogen thickens the endometrium.
What allows the endometrium to become receptive?
During the luteal phase, high levels of oestrogen and progesterone produced by the corpus luteum drive the maturation of the endometrium.
When can implantation take place?
Implantation can take place around day 19-23 of a 28-day cycle, during the mid luteal phase.
What happens to the endometrium during the implantation window?
The endometrium reaches its thickest point and becomes glandular, secreting nutrient-rich material to support an implanting blastocyst, driven by progesterone.
What marks the receptivity of the endometrium?
The receptivity of the endometrium is marked by the formation of pinopodes on its surface.
What are the two types of epithelial cells present at this stage?
The two types of epithelial cells are ciliated epithelium and microvillus cells.
Where does implantation usually occur?
Implantation usually occurs quite high in the uterine cavity.
What happens to the oocyte between ovulation and implantation?
The oocyte is picked up by the fimbrae of the fallopian tubes, fertilized in the ampulla, undergoes cleavage to the morula stage, and enters the uterine cavity.
What needs to happen before implantation can occur?
Formation of the blastocyst, which includes an outer layer of trophectoderm, inner cell mass, and a blastocele.
Describe the three phases of implantation.
The three phases of implantation are: 1. Apposition – the blastocyst loosely associates with the uterine wall. 2. Attachment – the blastocyst firmly adheres to the uterine wall. 3. Invasion – the blastocyst secretes enzymes that degrade the endometrial stroma.
What is meant by decidualisation of the endometrium?
Decidualisation refers to the functional and morphological changes in the endometrium to form the decidual lining for implantation, driven by progesterone.
What changes does progesterone stimulate in the endometrium?
Progesterone stimulates the formation of pinopodes, generalised oedema, blood vessel growth, presence of inflammatory cells, and decidualisation of stromal cells.
What do trophoblast cells differentiate into?
Trophoblast cells differentiate into cytotrophoblasts, syncytiotrophoblasts, and extravillous trophoblasts (EVT).
Which type of trophoblast does the placenta form from?
The placenta forms from the syncytiotrophoblast.
What stimulates the placenta to form and invasion to occur?
Invasion is driven by nutrients, growth factors, and oxygen tension that act as chemotactic stimulants.
What is the chorionic plate?
The chorionic plate is a layer of vascularised fetal tissue from which the umbilical cord arises.
How does maternal blood circulate the chorionic villi?
Maternal blood pools in intervillous spaces around the chorionic villi.
What are cotyledons?
Cotyledons are the functional units of the placenta, demarcated by placental septa.
What are the 3 main types of placenta?
- Haemochoroidal: chorion in direct contact with maternal blood.
- Endotheliochoroidal: maternal blood vessel endothelium in contact with chorion (dog, cat).
- Epitheliochoroidal: maternal epithelium of uterus in contact with chorion (cows, pigs).
How is human placental blood flow classified?
Human placental blood flow is haemomonochoroidal.
How is nutrition delivered to the blastocyst up to day ~15?
Initially, histiotrophic nutrition: O2 and nutrients reach the embryo by diffusion from surrounding decidua (glandular secretions).
What are lacunae?
Lacunae grow larger, forming intervillous spaces filled with maternal blood.
In which structure do lacunae form?
Lacunae form in the syncytiotrophoblast.
What supplies the lacunae?
Spiral arteries.
Describe primary villi formation.
Day 11-13: Cytotrophoblasts invade, forming finger-like projections in the decidua called primary villi, covering the blastocyst surface.
Describe secondary villi formation.
Extra-embryonic mesoderm invades the core of the primary villous (>day 16), forming secondary villi.
Describe tertiary villi formation.
By the end of the third week, embryonic vessels form in the extra-embryonic mesoderm of secondary chorionic villi, making them tertiary chorionic villi.
What is the structure of mature placental villi?
Mature placental villi have a branching system
What are stem villi?
Stem villi are the basal part of the villi that is attached to the chorionic plate.
What are branch/intermediate villi?
Branch/intermediate villi project from the sides of the stem villi.
What are terminal villi?
Swellings at the tips of branch villi that contain terminal vessels, forming convoluted knots where most exchange takes place.
What is the function of terminal villi?
Terminal villi continue to be produced throughout gestation and are where most exchange takes place.
What are anchoring villi?
Anchoring villi cross the intervillous space to attach to the maternal decidua.
What happens to the cytotrophoblast layer as pregnancy progresses?
The cytotrophoblast layer becomes very thin but remains mostly intact (~80% coverage in full term placenta).
How does maternal blood reach fetal circulation?
Maternal blood enters the intervillous space via the spiral arteries.
What modifications are needed for maternal blood flow in the intervillous space?
It is essential to establish a low-resistance high-flow blood supply to the intervillous space for normal pregnancy.
How are spiral arteries modified?
Spiral arteries are modified by extra villous trophoblasts that migrate into maternal decidua and coordinate the process.
What occurs during extra-villous trophoblast outgrowth in the first trimester?
Cytotrophoblast columns form at the tips of anchoring villi, differentiate into EVT, invade the decidua, and plug spiral arteries.
Why is a low oxygen environment important during pregnancy?
A low oxygen environment protects the fetus from oxidative stress.
What is the transition from histiotrophic nutrition to the haemotropic phase?
After week 14, maternal blood fills the intervillous space, transitioning from histiotrophic to haemotropic nutrition.
What is the relationship between the fetus and mother?
The fetus is dependent on maternal provision of nutrition, O2, salts, and removal of waste products across the placenta.
How do substances cross the placental barrier?
Substances cross via diffusion or active transport.
Which substances are transported via diffusion?
O2, CO2, Na+, urea, fatty acids, sugars (facilitated diffusion), non-conjugated steroids, thyroxine (T4).
Which substances are transported via active transport?
Amino acids, iron, Ca2+.
What molecules are not transported across the placenta?
Conjugated steroids and most bacteria.
What harmful substances can be transported across the placenta?
Cocaine, alcohol, caffeine, tetracycline.
Why should prescribing drugs during pregnancy be done carefully?
The placenta is not a barrier for drugs, which can enter fetal circulation and cause harm.
What are some maternal adaptations to meet the increasing oxygen/nutrient demand of the growing fetus?
Uterine blood flow increases ~20-fold, cardiac output increases by 30-40%, and maternal blood volume increases ~40%.
Describe blood flow in the placenta.
Deoxygenated blood from the fetus enters the fetal capillaries via umbilical arteries, exchanges nutrients and gases, and returns via umbilical veins.
What layers separate maternal and fetal blood after week 14?
Syncytiotrophoblast, cytotrophoblasts, connective tissue, fetal capillary endothelium.
How much blood do the intervillous spaces of a mature placenta contain?
The intervillous spaces contain ~150 ml of blood exchanged 3-4 times per minute.
How is gas exchanged in the placenta?
O2 and CO2 are transferred via passive diffusion, with fetal hemoglobin having a higher affinity for O2.
How is glucose transported across the placenta?
Via insulin insensitive hexose transporters (GLUT3, GLUT1) through facilitated diffusion.
What is the function of maternal insulin during pregnancy?
Maternal insulin regulates glucose, increasing glycogen and adipose stores.
What causes higher than normal glucose levels in mothers during pregnancy?
Maternal tissues show insulin resistance due to human placental lactogen (HPL).
How is glucose metabolized in the fetus?
Glucose is also metabolized to lactate, used as an energy source by the fetus.
What is the primary barrier in the placenta to lipids?
The syncytiotrophoblast is the primary barrier.
How are lipids transported across the placenta?
Lipids are broken down by lipases into fatty acids and transported via fatty acid transporters (FATPs).
How do water and electrolytes exchange in the placenta?
Water and electrolytes transfer readily across the placenta.
What infectious agents can cross the placenta?
Viruses like cytomegalovirus, rubella, Coxsackie, varicella, polio, and HIV; bacteria like spirochetes; protozoa like Toxoplasma.
Which maternal antibodies can cross the placenta?
IgG is the only antibody class that significantly crosses the human placenta.
How does IgG cross the placenta?
IgG is transferred by pinocytosis.
What is the advantage of IgG crossing the placental barrier?
It provides passive post-natal immunity for the neonate against various diseases.
What is pre-eclampsia?
A maternal systemic syndrome caused by abnormal placentation in the first trimester, presenting clinical symptoms from 20 weeks onwards.
What are the maternal symptoms of pre-eclampsia?
Hypertension, proteinuria, headache, HELLP syndrome, and potential eclampsia.
What is the cure for pre-eclampsia?
The only cure is delivery of the placenta.
What is the treatment for pre-eclampsia?
ICU admission, antihypertensives, anticonvulsants, and possibly delivering the fetus.
What are the risk factors for pre-eclampsia?
Previous PE, family history, increased inter-pregnancy interval, multiple gestation, maternal age > 40, and insulin resistance.
What is the pathology in pre-eclampsia?
Abnormal trophoblast invasion, altered secretions, and systemic endothelial activation.
What is intrauterine growth restriction (IUGR)?
A condition where the fetus’ growth is affected due to insufficient nutrient supply.
What causes IUGR?
Failure of remodeling process of spiral arteries, compromising blood flow.
What is IUGR associated with?
Occurs in 8-14% of normal pregnancies, particularly associated with early onset pre-eclampsia.
What does reduced blood flow to the fetus result in?
Fetal hypoxia, reduced fatty acid transfer, compromised amino acid transport, acidosis, and reduced bone mineralization.
Is glucose transport affected in IUGR?
Glucose transfer is generally not affected due to high circulating glucose levels.
Describe villous development in IUGR.
Thin, poor single villi with no branching, resulting in low surface area for nutrient transport.
What happens if the migration of extra-villous trophoblasts is disturbed?
It can lead to IUGR or early onset pre-eclampsia due to insufficient penetration.
What does failure to remodel maternal arteries result in?
Low diameter of spiral arteries, affecting blood flow and nutrient supply.
What is shallow invasion of decidua?
It refers to the premature loss of EVT plugs in spiral arterioles, leading to early initiation of blood flow to the placenta, which may result in miscarriage.
What happens when there is failure to remodel maternal arteries?
It leads to disturbed placental blood flow in placental insufficiency.
What is the effect of low diameter of spiral arteries?
It results in high pressure flow, vasoconstriction, and pulsatile flow, which can cause ischemia reperfusion injury and oxidative stress.
What is intrauterine growth restriction (IUGR)?
IUGR is when fetal growth is affected due to insufficient nutrients, often caused by failure of the remodeling process of spiral arteries.
What is placenta previa?
Placenta previa occurs when the placenta partially or fully overlies the cervix, which can obstruct the exit of the fetus.
What is the incidence of placenta previa?
It occurs in 3-6 per 1000 pregnancies.
What can happen if the placenta is low lying?
If detected early, there is a good chance it will move away from the cervix as the lower section of the uterus expands.
What is placenta accreta?
Placenta accreta is when the placenta invades and is inseparable from the uterine wall, resulting from excessive trophoblast invasion.
What is the incidence of placenta accreta?
It occurs in approximately 1 in 2500 pregnancies.
What are the risks associated with placenta accreta?
It can lead to vaginal bleeding in the third trimester, maternal hemorrhage, and may necessitate pre-term C-section or hysterectomy.
