6.1: Placenta Flashcards
How long can the embryo grow in vitro without a placenta?
13-14 days
What are the two main functions of the placenta?
- Implantation: so the pregnancy proceeds 2. Maintaining the pregnancy: (transport, endocrine, metabolism)
What is the role of the following extra-embryonic structures and from which cell mass are they all derived? A) yolk sac B) amnionC) allantoisD) chorion
Derived from the trophoblast cells: A) Provides nutrients until the placenta is ready to take over (degenerates after a few weeks) B) membrane surrounding the amniotic fluid C) waste disposal system, future umbilical cord D) forms the placenta
When does the development of the placenta start?
Embryo reaches blastocyst stage and a maternal-fetal connection must be formed to support the pregnancy
What are the two distinct cellular layers arising from the trophectoderm that form the placenta and what does this inner cell mass cells form?
- Syncytiotrophoblast2. Cytotrophoblast—> these two layers become the placenta Inner cell mass cells become the bilaminar disc: Epiblast (future ectoderm) and hypoblast (future endoderm)
What are the major differences between the syncytiotrophoblast cells and the cytotrophoblast cells?Include how one of the cell types nourishes the embryo prior to when links with maternal capillaries are made
Cytotrophoblast: layer of cells
Syncytiotrophoblast: derived from cytotrophoblasts, mesh of cell materials and one big nuclei: invades uterine epithelia and reaches uterine glands; the engulfed and degraded contents of uterine cells feed the embryo until links with maternal capillaries made
When should implantation be complete? How should the embryo looks at this point? (3 things)
By end of the second week:
- Two cavities: An amniotic cavity and a yolk sac
- Suspended by a connecting stalk; precursor of the umbilical cord, formed by mesenchymal cells and connects the amnion cavity and extracoelomic cavity
- Be within the supporting sac/chorionic cavity
What happens to the three embryonic spaces as development progresses?
- Yolk sac disappears2. Amniotic sac enlargers (fluid filled) 3. Chorionic sac is occupied by the expanding amniotic sac
Name three ways monozygotic twins can form
A blastocyst can separate in many ways to produce different numbers of chorion and amnions
- One zygote can split into two; resulting in two amnions, two chorion and separate implantation sites
- One morula can have two inner cell masses and bilaminar discs; so there are two amnions but one shared chorion
- One blastocyst has one bilaminar disc but TWO primitive streaks form, so there is a shared amnion and chorion between two fetuses
Why is implantation called ‘interstitial’?
Embryo goes through the uterine lining and into the ‘interstitial space’
Why is the human placenta haemomonochorial? Why is this an important aspect?
This means there’s only one layer of trophoblast cells separating maternal blood from the fetal capillary wall.
As the placenta develops the membrane becomes thinner for easier diffusion as needs of the fetus increase
What are the three major aims of implantation?
- Establishing a basic unit for exchange
- Anchoring the placenta
- Establishing maternal blood flow within the placenta
Explain what is occurring at each of these three stages:
- Primary villi
- Secondary villi
- Tertiary villi
- Early finger-like projections of the syncytiotrophoblast
- Invasion of mesenchyme into the villus core
- Fetal vessels invade the mesenchyme core (now the middle of the villus has proper blood supply)
How would the endometrium appear histologically as it prepares for implantation (3 things)? Which hormone is responsible for this?
Progesterone via corpus luteum
- Thicker
- Many glands (releasing nutrients for fetus) and elaboration of spiral arterial blood supply
- “Pre-decidual” cells
What is decidualisation?
What is the role of endometrial stromal cells and uterine NKC in this process?
Process of changing the endometrium to promote implantation
- Endometrial stromal cells; secrete GFs and signalling molecules
- Uterine NKCs; Regulate the immune response against the embryo (since it will express foreign proteins from dad)
Why is the remodelling of the spiral arteries important and how does it happen?
Normal smooth muscle cells and endothelial lining uterine blood vessels are signalled to undergo apoptosis and replaced by the trophoblast -> broadens the vessels -> forms
low resistance vascular bed that maintains the high flow/fast diffusion rates required to meet fetal demands
What is the most common implantation site in an ectopic pregnancy? Why is an ectopic pregnancy unlikely to make it to term?
Fallopian tubes.
Absence of correct spiral arteries and structures to form a placenta (if embryo implants elsewhere than the uterine lining)
Define placenta previa, why is it less dangerous than an ectopic pregnancy but still problematic?
Implantation at the bottom of the uterus; not initially problematic since there are still correct structures for implantation but as the baby grows pressure in this area can cause
- Haemorrhaging
- Blockage of the route out, baby may require a C section
Name two ‘implantation defects’
- Placental insufficiency
2. Pre-eclampsia; no meeting between invading trophoblast cells and the spiralling arteries (maternal vessels)
Describe the placenta in the first trimester. How does it change as the pregnancy progresses to i.e; 9 months?
1st trimester: placenta “barrier” is still thick and has a complete cytotrophoblast layer producing more syncytiotrophoblasts
As pregnancy progresses -> SA increases -> barrier thins to single layer for fast easy diffusion + cytotrophoblast layer is lost
What is the role of the umbilical cord? Which structures are within it and what do they do? Do you need all of them?
Transport link between developing fetus and placental blood vessels. Has umbilical arteries and veins longer than the umbilical cord and twist around it to add strength and protection
Two umbilical arteries carry deoxygenated blood from the fetus -> placenta *can survive with one. -> contents carried to maternal lungs (gas exchange) + kidneys (waste), etc -> go back to umbilical cord -> umbilical vein carries oxygenated blood from the placenta -> fetus
Define cotyledons
Structures that transmit fetal blood and allow exchange of oxygen and nutrients with maternal blood
Briefly describe three metabolic things the placenta creates
- Glycogen; stores maternal glucose for transfer to fetus later if necessary
- Cholesterol; precursor of progesterone and estrogen
- Fatty acids; from breakdown of fats in maternal circulation
When is hCG produced?
First 2 months of pregnancy
Name two conditions where hCG may be present in a non-pregnant individual
Trophoblast diseases
- Molar pregnancy; a non-viable fertilized egg implants in the uterus but fails to come to term; the trophoblast develops and invades
- Choriocarcinoma; cancer of the trophoblast cells
What produces relaxin? List three of its functions
Placenta
- increases flexibility of the pubic symphysis so pelvis can increase in size
- Suppresses oxytocin release to prevent premature labour
- Causes dilation of the cervix before delivery
When is the placenta established?
11th week
Describe which two hormones influence the maternal metabolism and how
- Progesterone; increases maternal appetite (more nutrients)
- hCS/hPL (human placental lactogen):
- Increases glucose available to fetus by switching the mother from glucose -> to fatty acid metabolism (so mother doesn’t break down and use the glucose in her body)
- breast development for lactation
Describe the methods (3) in which structures can pass through the placenta
- Simple diffusion (high concentration on maternal side -> low concentration on fetal side); water, electrolytes, urea and Uric acid, gases O2 and CO2
- Facilitated diffusion: glucose (GLUT1 transporter)
- Active Transport: transporters expressed by syncytiotrophoblasts for amino acids, iron and vitamins
What limits the amount of gas exchange that can occur between mother-fetus?
The uteroplacental circulation/amount of blood flow
How does the fetus develop passive immunity?
IgG molecules passed across the placenta for first few months (passing on mother’s immunity) so IgG concentrations in fetal plasma exceed those in maternal circulation (Receptor-mediated process)
What can access the fetus via the placenta that damages fetal development? Name four examples
Teratogens; thalidomide, alcohol, drugs, smoking
What is hemolytic disease of the newborn? Is this common?
If mother is rhesus (-) but fetus is (+) from father the mother will make IgG molecules against the fetal antigens
Not common due to prophylactic treatment
Name 3 infectious agents that can be transferred from maternal to fetal circulation
- Rubella
- Zika virus; (also known to damage the placenta itself )
- Cytomegalovirus
How long and how much does the placenta weigh averagely? Describe its two surfaces and their relation to the umbilical cord
Maternal surface facing out:
-decidua/endometrial uterine lining (give the placenta its dark red appearance)
Underneath is the intervillous space: a blood filled space drained by endometrial vessels (where both sides meet!)
Underneath are 15-20 lobules/cotyledons divided by deep channels called sulci
-each lobule divides into smaller sections with one villus (emerging from the fetal side’s chorion) with fetal capillaries that bathe in the intervillous space.
Fetal surface covered by the amniotic membrane and is where the umbilical veins and arteries spreading from the umbilical cord are visualized near the organ’s centre.
What does the amniotic membrane do and why is it so important?
Secretes amniotic fluid: which
- provides a protective cushion for the fetus against uterus walls
- Maintains pressures and temperatures
- space for fetal growth
- protects against infection
What lies directly underneath the placenta’s amnion?
The placenta’s chorion, a thicker membrane which is continuous with the uterine lining
What emerges from the chorion?
The villi where a system of fetal capillaries exists to allow maximum contact with the maternal blood (also known as the intervillous space)
Where is the mesenchyme embryologically derived?
From the mesoderm (between epiblast and hypoblast)
Why is maintenance of adequate flow from placenta -> fetus especially essential?
Small fetal O2 stores
