Extraembryonic membranes (Smyth) Flashcards
What are the 4 extra embryonic membranes of mammals?
Allantois
Chorion
Amnion
Yolk sac
What are the roles of extra embryonic membranes?
Transfer (store) of nutrients in and breakdown products out
Physically protect the embryo
- Protect the embryo from toxins and hormonal changes
- Protect the embryo from temperature changes
- Protect from maternal immune system
Allow growth (not pressing on the embryo)
Cellular differentiation - Gonadal stem cells (PGC),
- Haematopoietic stem cells
Drive hormonal changes-
- Signal to mother pregnancy is occurring (hCG)
- Maintain uterine relaxations and secretions (progesterone)
- End pregnancy and Induce parturition (estrogen- CRH)
What is the role of progesterone + 17β estradiol in uterine growth and implantation?
Proliferative phase
- 17β estradiol
- Stimulates stromal and epithelial growth
Secretory phase:
- Progesterone + 17B estradiol
- Rich in glycoproteins sugars and amino acids
- Nurtures embryo
What is the chorion, and how does it form in mammalian development?
The chorion is the first membrane to appear in embryonic development, formed from the trophectoderm
Highly motile trophoblast cells migrate from the trophectoderm into the uterine wall endometrium, forming the chorionic membrane
How does the fetal placenta form in mammals?
In mammals, the fetal placenta forms from trophoblast cells in a specific region of the chorion
The allantois fuses with the chorion, and mesoderm migrates in from the embryo to form blood vessels, establishing the chorio-allantoic placenta
How do trophoblast cells differentiate during placental development?
Trophoblast cells from the trophectoderm differentiate into two layers:
Inner Cytotrophoblasts serve as stem cells.
Outer Syncytiotrophoblasts form a continuous layer without cell boundaries.
What roles do Cytotrophoblasts play later in placental development?
Cytotrophoblasts lining the cavity can also differentiate into:
- Trophoblstic Giant Cells help remodel maternal blood vessels.
- Spongiotrophoblasts: Involved in the structural framework of the placenta.
What is a cytotrophoblast?
The cytotrophoblast is the trophoblastic stem cel
Forms four lineages
- The invasive lineage -Interstitial Cytotrophoblast
- The fusion lineage -Syncytiotrophoblast
Then later
- Through endoreduplication- TrophoblastGiant Cells
- Structural Spongiotrophoblasts
What are syncytiotrophoblasts?
Syncytiotrophoblasts control both maternal adaptive and innate immunity,
Physical separation
Express cell surface non-classical MHCI molecules (rather HLA C-E-G)
- These inhibit NK cells, suppress CTL activity and up-regulate the local CD4’ Treg cell differentiation.
They block innate immunity via the modification of neutrophil function and NET formation.
They produce hCG- signal to mother pregnancy has started
What happens to the corpus luteum if there is no embryo?
LH (from pituitary) reduces, putting the Corpus Luteum (CL) at risk after ovulation (-ve feedback Progesterone - LH)
In absence of embryo, CL will die (10-14 days after ovulation due to -ive feedback
What happens to the corpus luteum if there is a pregnancy?
Embryo implants
Syncitiotrophoblasts function and produce hCG
*hCG survival factor for CL.
*Prevents CL degeneration.
*80% identical LH.
*hCG Much longer half-life in blood
*hCG maintains progesterone formation
*Progesterone from CL switches off LH (-ve FB)
*Progesterone has no effect on hCG production (no -ve FB)
What does progesterone do during pregnancy?
- Blocks further follicle development and ovulation (blocks FSH and LH)- so single embryo (or one timed wave of ovulation).
- Produces a barrier at cervix, mucus plug stops microorganism and sperm entry. Cervical plug.
- Induces uterine endometrium to make a nutrient-rich food source for embryo (secretory phase- needs estrogen too).
- Keeps uterine myometrium ‘quiescent’.
- Progesterone initially made by CL, later in pregnancy by placental syncytiotrophoblast (4-7 weeks) luteoplacental shift.
- Induces breast tissue growth ready for lactation.
How does the placenta form?
At human E14 Cytotrophoblasts grow out into the endometrium increasing the contact surface of the primary chorionic villus
At human E16 Extra-embryonic mesoderm grows out into the primary villi increasing the contact surface further - the secondary chorionic villus
At human E20, vessels form in the secondary villi mesoderm- this forms the tertiary chorionic villus, and cytotrophoblast cells cover the syncytial cells– the cytotrophoblast shell
VIDEO ON PLACENTA FORMATION
https://www.youtube.com/watch?v=bped-RVWsLk
What are trophoblast giant cells and what do they do?
They are highlypolyploid (switched from a mitotic to endoreduplicativecell)
Like metastatic tumourcells, they breachbasement membranesand invade deeply into the maternal decidualized uterinestroma.
Highly angiogenic and vasodilatory properties, remodel arterial walls, enable them to redirectmaternal bloodflow towards theimplantationsite.
What are the four types of trophoblastic giant cells, and what are their roles in placental development?
Spiral Artery-Associated Trophoblast Giant Cells:
- These cells are involved in remodeling the maternal spiral arteries, ensuring they are wide enough to supply the necessary blood volume to the growing fetus, enhancing maternal blood flow to the placenta.
Canal-Associated Trophoblast Giant Cells:
- Located in the vascular channels, these cells facilitate the restructuring of these canals to optimize nutrient and gas exchange between the mother and fetus.
Sinusoidal Trophoblast Giant Cells:
- These cells are positioned in the sinusoidal spaces of the placenta and help regulate the blood flow within these vascular spaces, crucial for effective fetal-maternal exchange.
Parietal Trophoblast Giant Cells:
- They are responsible for strengthening the structural integrity of the placenta by adhering to the parietal tissues of the uterus, playing a crucial role in anchoring the placenta.
What are spongiotrophoblasts?
Cells forming between the villi and the cytotrophoblast cells
Between intervillous spaces which are flooded with maternal blood
Role Not Understood
- Structural
- Glycogen stores
- Intermediate stage between Cytotrophoblast and Giant cells
Where does maternal vessel entry occur?
Maternal vessel entry and placental development doesn’t occur over the whole chorion
Villus formation is biased to the upper region – decidua basalis/ placentalis
Shape and size of placenta
In humans and rodents a discoid placenta,
The placenta averages 22-24cm diameter and 2cm thick
Typically weighs 500grams
It connects to the foetus by anumbilical cordof approximately 55–60cm long
Contains twoumbilical arteriesand oneumbilical vein.
The shape alters between different mammalian species.
What are the main substances transferred to the feotus?
From themotherto thefoetus
- Water,oxygen, nutrients and IgG antibodies.
(neonatal Fc receptor (FcRn) on syncytiotrophoblast).
From thefoetusto themother
- Metabolic wastes, including urea, uric acid, andcreatinine (via developing foetal kidney and the allantois)
- Carbon dioxide and heat.
Under normal conditions, maternal cellsdo notcross the placenta during gestation.
Placental barrier. Although permeable to some substances (selective permeability), the placental barrier largely prevents the passage of cells.
However foetal cells are highly invasive – and often enter the mothers tissues
When does the amnion start to form?
Amnion precursors start to appear in man just after implantation
The epiblast forms an epithelial plate with a lumen in it
The lower surface is the epiblast and the upper are amnioblasts
As the amnion and the embryo grow, the amnion starts to rotate
What happens to the amnion as the embryo starts to grow?
As the embryo folds – head and tail, and to the sides (forming a cavity in the embryo) it pulls the amnion around
So that the amnion surrounds almost all the embryo
– except for its junction with the yolk sac/ allantois and body stalk
Amnion function
When first formed, the amniotic membrane is in contact with body of the embryo
Its begins to secrete amniotic fluid ~ 5 weeks in man and the amnion expands to ultimately adhere to the inner surface of thechorion
The amniotic fluid allows the free movements of the foetus during the later stages of pregnancy, and also protects it from trauma and maintains temperature
When is the yolk sac developed?
This develops just after implantation (final position differs in varying mammalian species)
The primitive endoderm (hypoblast) begins to migrate out from the embryonic disc
This layer (now called parietal endoderm) in time covers the inner cytotrophoblasts .
- The spaces created are the extra embryonic coelom and primary yolk sac
(in man this forms about days 9-After this forms the chorionic growth becomes predominant
What is the extra-coelomic cavity?
In man at ~ E12 extraembronic mesoderm grows out between the cytotrophoblasts and the parietal endoderm and then opens to form a wider extra-coelomic cavity
What happens after the primary yolk sac and extra-coelomic cavity is formed?
The primary yolk sac pinches into two and forms the secondary yolk sac
What differences are there in the yolk sac of humans and mice?
In the end the yolk sac in both has a stalk attaching to the abdomen of the foetus as it curves in at its head and side
In mice the rotation causes the amnion to sit around the embryo and down growth causes the yolk sac to surround them both
In man the yolk sac is pinched of and remains a bag and the embryo folds about its attachment point
What is the role of the yolk sac?
1) Early nutrient transfer important in man from E15 to ~ 3mths
- The only point of nutrient transfer in reptiles and birds
- Main storage of nutrients during embryogenesis here
2) Vascular components – preliminary circulatory cells
- First site of haematopoiesis during mammalian development.
- Yolk sac hematopoietic stem cells (YS-HSC), these form and amplify here and then migrate into the embryo proper
The yolk sac is also the first site of blood vessel endothelial development. (blood islands) in the yolk sac
3) Primordial germ cells migrate from the primary ectoderm into the yolk sac (~E20 in man) near the exit of the allantois
- The primordial germ cellsmoveback into the embryo between the fourth and sixth week along the yolk sac/ allantois wall into the wall of the gut to colonize the gonadal ridge
Where does the allantois come from?
This is an outgrowth (diverticulum) initially from the yolk sac and pushes into the stalk joining to the chorion
(part of the umbilicus), in man ~E16
In the embryo it extends from the hind gut
It fuses with the chorion and so in time forms the chorioallantoic membrane
What does the allantois do?
In contact with the bladder and lets excreted fluids, urea (kidney)- be taken to placenta where transferred to maternal circulation
In birds / reptile – no placenta so this is a large structure
In mammals it is small as chorio-allantois allows transfer over placenta to mothers circulation
What are the contents of the primitive umbilical cord?
1- Yolk sac and vitelline duct.
2- Connecting stalk with remnant of allantois
3- Umbilical and vitelline vessels.
4- Intestinal loop in its proximal part.
Do the 4 membranes have blood supply?
yolk sac, allantois and chorionall have extensive vasculature
The amnion has no blood supply
What are the differences in the membranes in the different vertebrate species?
- In amphibians – vegetal pole – cellular yolk rich (no yolk sac)
- Infish, only the yolk sac is present.
The amnion is a specialisation for terrestrial survival preventing the embryo from drying out (land animals)
Inreptilesandbirds, in addition to the yolk sac, the amnion, the chorion and the allantois are also present
- The yolk sac and allantois are far larger in these (no placenta)
- The allantois doesn’t fuse with the chorion
In placental mammals, the chorion and the allantois specialise to form theembryonic chorioallantoic placenta – filtered waste from the developing kidney is lost into the allantois and exchanged at the placenta….the endometrium forms the maternal placenta
Overview of bird membranes
Theamnionprotects the embryo in a sac filled withamniotic fluid.(shock absorber)
Theyolk saccontains yolk — the sole source of food until hatching. Yolk is a mixture of proteins andlipoproteins.- it is very large initially reducing during development
Thechorionlines the inner surface of the shell (which is permeable to gases) and participates in the exchange of O2and CO2between the embryo and the outside air.
Theallantoisstores metabolic wastes (chieflyuric acid) of the embryo and, as it grows larger through development, also participates in gas exchange.
Membranes in twins
Two foetuses share a single placenta, (monochorionic twins) in ~75% of identical twins
The placenta may occur anywhere in relation to the stalk position (the umbilical chord)– there is no set or predictable pattern
Depending on where they implant, one foetus may get a lower share of the placental exchange than it’s co-twin.
Hence although identical twins share the same genetic material, they may actually grow quite differently.
Only~ 1% of identical twins share both a single placenta and a single amniotic sac, and this poses a further risk
unequal placental sharing and umbilical knotting
