Placentation and trophoblast 1 Flashcards
Describe the placenta
An organ unique to pregnancy
Forms the interface between the mother and the fetus.
The placenta is fetal in origin at term it weighs 500-1000g
Acts as the lungs, gut and kidneys of the fetus
Acts as an endocrine organ releasing hormones into the maternal circulation such as hCG and progesterone
The placenta is a semi-allograft, the cells have genetic material from both the mother and the father
In a human pregnancy, fetal cells are in direct contact with maternal blood
This requires mechanisms to evade the maternal immune system
However the fetal and maternal circulations do not mix
Describe the process of placental development
Trophoectoderm proliferates and fuses to form a primitive syncytium (PS) beneath the implanted embryo
TC migrate or invade into the decidua
Lacunae (L) form by the action of proteases which later develop into the intervillous space
Cytotrophoblasts proliferate and migrate through the syncytium to form the anchoring villi
Further placenta structure description
Highly branched with a large surface area for exchange
Outer layer of fused cells- the syncytium
Underlying cytotrophoblast stem cells
Diffusion distance to vessels small
Growth is regulated by a number of factors including IGF I and II
Describe villous structure
2 circles, syncytium on the outside
cytotrophoblast stem cells on the inside
placental vessels like circles on the inside
Hofbauer cells are small circles around the placental vessels
the cytoplasm empty space are the ‘placental stromal cells’
Formation of syncytium
A positive feedback loop governs GCM1 activity, hCGβ expression, and placental cell differentiation. The binding of hCG to LH/CGR elevates cAMP which activates PKA.
PKA phosphorylates GCM1 on Ser269 and Ser275. This phosphorylation of GCM-1 results in hCG and syncytin 1/2 expression.
Syncytin-1 promotes placental cell fusion and differentiation. Syncytin-1 is a human endogenous retroviral envelope gene product that plays an important role in the formation and maintenance of normal syncytium throughout pregnancy.
Villous cytotrophoblast proliferation decreases with gestation by term the syncytium close to placental vessels
Syncytium is continually been shed in to the maternal circulation and is replaced by the underlying cytotrophoblasts
(GCM1 - Glial Cells Missing Homolog 1)
Describe one of the pathways- extravillous cytotrophoblast invasion
Growth of the trophoblast column
Regulated by factors such as IGF1 produced by the underlying mesenchymal cells
HIF-1α and Stox1 stimulate proliferation and inhibit differentiation
Oxygen tension and gestational age
Up until 12th week the uterine spiral arteries are plugged with trophoblasts
Placental development therefore occurs under relative hypoxia 2-3% O2
While the spiral arteries are plugged nutrition is histiotrophic nutrients being secreted by the glandular cells
Following dissolution of the trophoblast plug the placenta switches to haemotrophophic nutrition
Low oxygen early in pregnancy is important normal pregnancy progression
Prolonged low oxygen leads to placental pathologies
Methods that are used to study human placental development
1)Trophoblasts invade the decidua and maternal arterial wall and come in to direct contact with maternal blood
However there is no deep interstitial invasion of the decidua
Mice do not exhibit the same obstetric complications as humans
2)Trophoblasts invade the decidua and maternal arterial wall and come in to direct contact with maternal blood
Deep interstitial invasion of the decidua does occur
Some evidence that they do exhibit the same obstetric complications as humans
Ethically unacceptable to experiment on these animals
Describe some in vitro methods used to study human placental development
Human tissue can only be obtained either in the first trimester from TOPs or at term
Trophoblast cell lines derived from choriocarcinomas JEG3, Jar and BeWo
grow well
have lost some characteristics
Developed following transfection with oncogenes such as t- and T-antigen of SV40 or more recently hTERT
grow well
have lost some characteristics but this can depend on how they are cultured
Human embryonic stem cell-derived trophoblast cells (hESCs)
characterisation has proved problematic
Human trophoblast stem cells (hTSCs) derived from the trophectoderm and first trimester placentae
express characteristics of first trimester trophoblasts
can be induced to differentiate along either syncytial or extravillous lineages
difficult to prepare and grow
Describe culture formats
Simple mono layer cultures simple co-cultures addition of extracellular matrix effect of flow 3D environment organoid cultures
What factors influence trophoblast invasion
Trophoblast and cancer cells have mechanisms of invasion in common
However trophoblast invasion is tightly regulated.
1)Growth factors and cytokine
HGF
IGF-1
Prolactin
2)Matrix proteases
MMP-2, 9, 10, 12
3)Tissue inhibitors matrix
metalloproteinases
4)Inhibitory factors
TNF
TGFβ
IGFBP-1
Upsetting the balance between stimulatory and inhibitory factors can lead to pregnancy complications
Maternal immune cells regulating trophoblast invasion
Immune cells in decidua basalis
70% uterine natural killer cells (uNK cells)
20% are macrophages
They are recruited following implantation
They localise to maternal spiral arteries
They precede the invasion trophoblasts
They secrete factors that regulate trophoblast invasion?
Failure of the placental to develop normally can lead to common pregnancy disorders such as early pregnancy loss, pre-eclampsia and fetal growth restriction.
Failure to thrive in utero has lifelong consequences including increased risk of developing hypotension and diabetes
Overview of extravillous trophoblast differentiation
early in pregnancy cells of the placenta (trophoblasts) invade the decidua
they plug the maternal spiral arteries
they interact with and replace the cells of the vessel wall
ultimately this will result in increased blood flow to the developing baby
Describe spiral artery remodelling
Two phases
Trophoblast independent
Immune cell
Pregnancy hormones
Trophoblast dependent
We know this by studying ectopic pregnancies
How are vascular cells lost from spiral arteries?
Migration
De-differentiation
Loss of adhesion (Anoikis)
Vascular cell apoptosis
Extravillous trophoblasts in uterine spiral arteries bring about changes leading to the loss of vascular cells - crucial for the vascular remodelling
