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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

  1. Pre-lacunar (days 6-8)
  2. Lacunar/trabecular stage (days 9-12)
  3. Chorionic Villous Stage (days 13-18)
  4. 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

  1. Transport
  2. Respiration
  3. Hepatic fxn
  4. Temperature regulation
  5. Endocrine
  6. 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 >


c. Active Transport

i. Goes against the maternal fetal gradient and requires energy & ATP (against the


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


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


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.


Characteristics of hepatic fxn of placenta

a. Barrier preventing fetal drug exposure

b. Placenta has capacity for some drug metabolism

i. Oxidation, glucuronidation and sulfation

ii. When trying to increase surfactant in a baby that needs to be delivered prematurely

(ex: eclampsia), must give a corticosteroid that will NOT be deactivated by the

placenta! (ex: Betamethosone


C. Excretion of waste products


Characteristics of temp regulation fxn of placenta

Temperature regulation: normally your skin is involved in temp regulation

a. Heat exchange to maintain fetal temperature

b. Fetus generates a lot of heat due to high rate of growth and metabolism, this is transferred

to maternal blood to be lost in peripheral vessels >

moms feel warmer during

pregnancy!! Mom has to dissipate the energy heat from baby.


Characteristics of endocrine fxn of placenta

Endocrine → Placenta looks like hypothal, pituitary and adrenal gland. Alter mom’s physio so that

baby gets more glucose and nutrients.

a. Peptide hormones – βhCG, hPL, placental GH

b. Steroid hormones – Trophoblasts secrete estrogen (estriol) and progesterone


Characteristics of peptide hormones secreted by placenta

  • βhCG, hPL, placental GH
  • i. Both secreted by syncytiotrophoblasts
  • 1. hCG is one of the earliest markers of preg, peaks around 10 weeks and then declines , maintains the corpus luteum and progesterone production, regulates cytotrophoblast differentiation into syncytiotrophoblasts
    • elevated in trisomy 21 pregnancies
  • 2. hPL (human placental lactogen) is a counterregulatory hormone
    • drives materna ==> fatty acid synthesis, making more carbs available for the fetus, contributes to the insulin resistance preg women/ gestational diabetes.
  • 3. Placental Growth Hormone: similar to pituitary GH increases from 12 wks to term and replaces pituitary GH production,
    • controls maternal IGF1 levels, secretion regulated

      by glucose, lower levels observed in intrauterine growth restriction.


Characteristics of steroid hormones secreted by placenta

Progesterone and androgens are synthesized in the fetus and then processed by the placenta. Progesterone suppresses uterine contractions, necessary for pregnancy maintenance.

Estrogen production requires the maternal-placental-fetal unit 

1. Placenta lacks CYP450c17 and 16 alpha hydroxylase (to make progesterone and androgens)

2. Fetus lacks aromatase and 3βhydroxysteroid dehydrogenase (to make estratriol)

3. Mom supplies cholesterol precursors


Characteristics of immunocompetence fxn of placenta

  • Physical barrier to pathogens
  • b. Hofbauer cells are placental macrophages in the villous core
  • c. Allows maternal IgG to access the fetus (Placenta has Fc receptor that binds mom’s IgG and transports it to the fetus. This is important for Rh antigen! IgG recognizes fetal RBCs if Rh+ and baby becomes anemia > immune hydrops.)
  • i. Note: the fetal immune response is dominantly IgM, which cannot cross placenta


Steriod production by the maternal-placental-fetal

Mom – cholesterol

Fetus – progesterone, testosterone and other androgens

Placenta – estrogens (estriol), further processing of progesterone

All of these must work together as no piece of the trifecta can do everything on its own.


Classification of multiple gestation

  • zygosity
    • mono = single fertilized egg divides
    • di = two independently fertilized oocytes
      • always dichorionic & diamniotic
  • # chorions = mono vs. di
  • # amnions = mono vs. di


Chorions/amnions of monozygotic twins

  • split @ 0-5 days: dichorionic, diamniotic
  • split @ 5-8 days: monochorionic, diamniotic
  • split @ 8-13: monochorionic, monoamniotic
  • split > 13: conjoined, possibly incompatible with life