Physiology of the Placenta Flashcards
Characteristics of the placenta (general)
- placenta is the structure which acts as the interface between fetal and maternal circulation.
- It arises from the undifferentiated trophoblast of the implanting conceptus.
- Human placenta is a villous hemochorial placenta = maternal blood directly contacts fetal trophoblasts of placenta
- The fetal blood never directly mixes with the mothers blood.
Summary of characteristics of human placenta: vasculature, shape, interdigitaion, barrier, blood flow
- vasculature = chorioallantoic
- shape= discoid
- interdigitation = villous
- barrier= hemochorial
- blood flow=multivillous.
Stages of placental developmant
- Pre-lacunar (days 6-8)
- Lacunar/trabecular stage (days 9-12)
- Chorionic Villous Stage (days 13-18)
- Floating villi vs. anchoring villi
Characteristics of pre-lacunar stage of placental development
Prelacunar Stage (days 6-8):
a. There is no placental tissue until the conceptus implants!
b. The blastocyst attaches to endometrium and the trophectoderm begins to proliferate.
c. The outer layer of trophoblast cells fuse and form the syncytiotrophoblast, which is nonmitotic.
d. The inner layer of trophoblast cells continues to divide and are known as cytotrophoblasts.
e. These cells invade adjacent maternal endometrium.
Characteristics of lacunar/trabecular stage of placental development
a. The syncytiotrophoblasts develop large lacunae (fused vacuoles) resulting in the formation of pillars called trabeculae. (Days 9-12)
b. The lacunae make first contact with maternal endometrial capillaries.
c. This is the region that will develop into the intervillous space. (Note: implantation is complete at this point)
Characteristics of chronic villous stage of placental development
Appearance and development of chorionic villi is in this stage. (days 13-18)
b. The cytotrophoblasts proliferate and invade the trabeculae of the syncytiotrophoblasts.
i. Primary Villous – cytotrophoblast (single cell from trophectoderm) core surrounded by syncytiotrophoblast (multinucleated “bark” of chorionic tree).
ii. Secondary Villous – extraembryonic mesoderm grows into column of cytotrophoblasts, forming the villous core
iii. Tertiary Villous – Mesenchymal cells differentiate into blood vessels forming ateriocapillary network in villous. Fetal vasculature populates.
Characteristics of floating villi vs. anchoring villi
Floating villi make up the majority of the placental mass and the site of nutrient and waste exchange.
Anchoring villi provide the attachment to the uterus and are the site for invasive cytotrophoblast deployment.
Development of floating vs. anchoring villi
a. The tip of anchoring villi have cytotrophoblasts making contact with the decidua of the maternal endometrium.
b. From the cell column, these trophoblasts invade through the decidua to the inner 1/3 of the myometrium and gain access to the uterine spiral arteries.
c. This endovascular invasion results in the breakdown of smooth muscle and the replacement of endothelium.
d. These changes convert a low volume, high pressure system into a low pressure, high volume system creating a shunt from mom to baby by dumping blood into the intervillous space.
Characteristics of third-trimester placental anatomy
The maternal side is divided into cotyledons by septa. The cotyledons each contain several main stem villi and loads of branching villi. The umbilical artery runs throughout the inside of the villous structure, which is bathed in maternal blood.
The terminal villi are grapelike structures located on tertiary villi, which have many capillaries and sinusoids. This is the location where most villous growth and placental transport takes place.
Characteristics/definition of amnion and chorion
- Fetal membranes are composed of the amnion and the chorion forming the “water bag”.
- Amniotic fluid would be present on the leftside of the slide.
- Amnion is composed of a single layer of cuboidal cells.
- no blood vessels in this layer and on gross path is delicate and transparent.
- The intermediate layer (SL=spongy layer) is loosely arranged collagen and a few fibroblasts.
- Chorion is roughly analogous to the basal plate, composed of cytotrophoblast and decidua.
Production of amniotic fluid
- In early pregnancy, amniotic fluid is an ultrafiltrate of maternal plasma.
- When the fetal kidneybegins functioning around 12 weeks of life, the major source of amniotic fluid becomes fetal urine.
- Late in pregnancy, fetal lung secretions contribute a small portion to the total amniotic fluid volume.
- Critical for lung development and proper musculoskeletal function
Causes of decreased amniotic fluid volumes
- “oligohydramnios”
- most common cause is rupture of membranes (leaks out).
- Other causes include:
- pregestational diabetes, hypertension or preeclampsia leading to poor perfusion to the placenta
- twintwin transfusion
- congenital anomalies of the fetal genitourinary system
- drugs: prostaglandin synthase inhibitors and ACE inhibitors which interfere with fetal renal function.
Causes of increased amniotic fluid volumes
- “polyhydramnios” (TOO much!)
- Causes:
- congenital anomalies which interfere with normal fetal swallowing of the amniotic fluid.
- These defects include esophageal atresia, neural tube defects and hydrops (excess fluid secondary to hemolysis, heart defect or infection).
- Also seen in gestational diabetes (higher glucose > water follows glucose via osmotic pressure)
Main functions of placenta
- Transport
- Respiration
- Hepatic fxn
- Temperature regulation
- Endocrine
- Immunocompetence
Characteristics of transport across the placenta
a. Diffusion
i. Water and gases (CO2 and O2)
ii. Dependent entirely upon concentration gradient, no energy/transporters required
b. Facilitated Diffusion → Driven by a gradient, but requires specific carrier
i. Dependent on maternalfetal
gradient AND carrier proteins
ii. Glucose (GLUT1 predominates, glucose is higher in mom than baby >
hyperglycemic moms will transport MORE glucose across >
Macrobabies!)
c. Active Transport
i. Goes against the maternal fetal gradient and requires energy & ATP (against the
gradient)
ii. Amino acids (fetal side concentration is higher than on maternal side)
d. Substances that are diffusion limited cross the placenta slowly, and the ratelimiting
step is
movement between intervillous space and fetal capillaries through the
syncytiotrophoblasts.
i. Direct damage to syncytiotrophoblasts will affect O2 transport to fetus
e. Substances that are flow limited cross the placenta more rapidly and depend on both
concentration and blood flow rate
i. Pregnant women with aortic stenosis have reduced cardiac output
Characteristics of respiration fxn of placenta
Respiration
a. Fetal hemoglobin has greater affinity for O2 than adult hemoglobin, it also has lower 2,3BPG
b. This allows greater O2 delivery to fetus (left shifted O2 dissociation curve)
Image: fetal Hb binds O2 more readily compared to adult Hb. Also lower concentrations of 2,3BPG. Facilitates transfer of O2 to fetus from mom.
