The Placenta Flashcards
What are the two layers that the trophoblasts differentiates into?
Syncytiotophoblast
Cytotrophoblast
By which day is the blastocyst fully embedded within the endometrium?
Day 10
What does it mean that the placenta is haemomonochorial?
One layer of trophoblast ultimately separates maternal blood in the intervillous space from the fetal capillary wall
Aims of implantation?
Establish a basic unit of exchange
Anchor the placenta
Establish maternal blood flow within the placenta
Describe the units of exchange
Primary villi - early finger-like projections of the trophoblast
Secondary villi - invasion of mesenchyme into core
Tertiary villi - invasion of mesenchyme core by fetal vessels
What anchors the placenta?
Establishment of outermost cytotrophoblast cell
What is decidualisation?
Pre-decidual cells which are cells of the endometrium that fall away in the absence of pregnancy
How is the endometrium prepared for implantation?
Predecidual cells prepare it
Elaboration of spiral arterial blood supply
What is the decidual reaction?
Interaction between predecidual cells and trophoblast cells
Provides the balancing force for the invasive force of the trophoblast
What is the decidual reaction stimulated by?
Progesterone
What can happen without the decidual reaction?
Complications such as haemorrhage
What is remodelling of spiral arteries? Why does it happen?
Creation of low-resistance vascular bed
Maintains the high flow required to meet the demand of the fetus, particularly later on in gestation.
Their lining is replaced with fetal tissue
What happens if remodelling of the spiral arteries is inadequate?
Get pre-eclampsia
What are some implantation defects?
Ectopic pregnancy
Placenta praevia - implantation in lower uterine segment
Incomplete invasion - placental insufficiency and pre-eclampsia
What is the fetal portion of the placenta made up of?
The chorionic plate which has the villi projecting out of it containing the umbilical arteries and vein
What is found between the chorionic and decidual plates?
Intervillous spaces, filled with maternal blood
What happens to the intervillous space during the fourth and fifth months?
Decidua basalis forms a number of decidual septa which project into the intervillous spaces but don’t reach the chorionic plate
The septa divide into compartments called cotyledons
What percentage of the internal surface of the uterus does the placenta cover?
15-30%
Give features of the first trimester placenta
Placental barrier is relatively thick
Has complete cytotrophoblast layer beneath the syncytiotrophoblast
Give features of the term placenta
Surface area for exchange has dramatically increased
Placental barrier is now thin
Cytotrophoblast maher beneath syncytiotrophoblast is lost
What do the umbilical arteries and veins project into?
Tertiary villi which are bathed in oxygenated maternal blood in the intervillous space
What do the two umbilical arteries transport?
Deoxygenated blood from the foetus to the placenta
What does the umbilical vein transport?
Oxygenated blood from the placenta to the fetus
What do cotyledons receive blood via?
80-100 spiral arteries that pierce the decidual plate.
Pressure in these arteries forces oxygenated blood deep into the intervillous spaces and bathes numerous small villi of the villus tree in oxygenated blood
As pressure decreases, blood flows back from the chorionic plate towards the decidua, where it enters the endometrial veins. (Jacuzzi)
What factors influence passive diffusion of substances across the placenta?
Concentration gradient
Barrier to diffusion - placental membrane gradually thins throughout pregnancy as demand increases
Diffusion distance - haemomonochorial
Which substances pass through the placenta by simple diffusion?
Water
Electrolytes
Urea and uric acid
Gases, limited by flow
Why is maintenance flow of adequate oxygen to the fetus essential?
The fetus has low stores of oxygen
What passes through the placenta by facilitated diffusion?
Glucose
What is the maternal part of the placenta?
The decidua basalis/decidual plate which contains the spiral arteries and veins
What is actively transported across the placenta?
Amino acids
Iron
Vitamins
What are some teratogens which can cross the placenta?
Thalidomide
Alcohol
Drugs
Smoking
What are some pathogens that can cross the placenta?
Varicella zoster Cytomegalovirus Rubella Treponema pallidum Toxoplasma gondii
What hormones does the placenta produce?
Protein hormones: hCG - human chorionic gonadotropin hCS/hPL - human placental lactogen Human chorionic thyrotrophin Human chorionic corticotrophin
Steroid hormones:
Progesterone
Oestrogen
What does hCG do?
Produced during first 2 months
Supports secretory function of corpus luteum
Produced by the syncytiotrophoblast so is pregnancy-specific
What does hCS do?
Influences maternal metabolism, increasing the availability of glucose to the fetus
What does progesterone produced by the placenta do?
Takes over progesterone production from the corpus luteum
Influences maternal metabolism by increasing appetite
Relaxes smooth muscle, decreasing total peripheral resistance
What substances can the placenta synthesise?
Fatty acids
Glycogen
Cholesterol
Which hormone is used in a pregnancy test and why?
Human chorionic gonadotrophin
Produced during first two months of pregnancy by the syncytiotrophoblast
Excreted in maternal urine
Describe the process of implantation
Involves interaction between trophoblast cells and epithelium of the uterus
Further embedding of the blastocyst into the endometrium is dependent upon the invasive property of the trophoblast which by now has separated into its two layers
Uterine epithelium is breached and the conceptus implants within the stroma
When does immunological competence develop?
Begins late in the first trimester, by which time the fetus makes all of the components of complement
Which immunoglobulin is transported to the fetus?
When does this begin?
The maternal IgG
At approximately 14 weeks
How is IgG transported to the fetus
Receptor-mediated pinocytosis
When are adult levels of IgG attained?
Age 3
How does haemolytic disease of the newborn happen?
When the rhesus group of the mother and fetus are incompatible
Mother is previously sensitised to rhesus antigen in a previous pregnancy
IgG against the fetal rhesus crosses the placenta and attacks red blood cells
How is haemolytic rhesus disease of the newborn prevented?
Prophylactic treatment
Rhesus negative mothers pregnant with rhesus positive fetus are given rhesus specific IgG throughout pregnancy to prevent sensitisation in the event of exposure to the antigen
The given IgG will bind to the antigen before mother’s immune system can mount a response
