Repro 8.1 Placental Function and Dysfunction Flashcards
What is the placenta?
The interface between mother and baby through which nutrients are supplied from the mother and waste is removed from the embryo. A selective barrier.
When does development of the placenta begin and from what?
Soon after fertilisation at compaction from cells of the outer cell mass
What surounds the embryo?
Amnion and chorion membranes in a protective sac. The placenta develops as a specialisation of the chorion
When does specialisation of the chorion occur?
At implantation - fingerlike projections called chorionic villi develop
What are chorionic villi?
Functional units of the placenta that represent the point of exchange between the maternal and foetal circulations. Consist of a vascularised core covered by two epithelial layers
By what means does maternofoetal exchange occur?
Simple/facilitated diffusion
Active transport
Receptor mediated endocytosis
What does a counter current achieve in the placenta?
Combined maternal and foetal circulations create a counter current supply that ensures maternofoetal exchange is efficient and simply regulated
How does the placenta protect the foetus?
Selective barrier regulates access to the foetal circulation and affords considerable protection from harmful agents. However it can be breached
How might infectious agents breach the placental barrier?
Leakage
Use of existing transport systems (HIV)
Actively penetrate (Treponema)
Opportunistically exploit gaps in the epithelium (Toxoplasma)
Other than nutrient supply and waste removal, what other functions does the placenta have?
Endocrine support of pregnancy
Provision of passive immunity that affords immune protection in the neonatal period
How does the placenta adapt as the foetus grows?
Matures to meet the increasing demand by decreasing the interhaemal distance by thinning of the trophoblast layer(s), margination of the foetal capillaries of the core of the villi and increasing the surface area for exchange through increased branching of the villus tree
What differences in placenta will be seen in women that smoke and why?
Reduced interhaemal distances. This occurs when there is a deficit of the demand for transported materials or restriction on the maternal side. Reduced placental blood flow and growth. Poor foetal nutrition -> reduced birth weight by on average 200g
If foetal demand is not met e.g. the interhaemal distance cannot compensate enough, what happens?
Major risk factor for intrauterine growth restriction and impaired foetal growth which has been linked to long-term adult health problems (Barker hypothesis - foetal origin of adult disease)
What should happen to the placenta during labour?
Sheds during parturition and the massive blood supply to the implantation site is shut down
What happens if the placenta fragments during labour?
Retained placenta which impairs shut-down of the utero-placental circulation and consequently can cause serious postpartum haemorrhage
During the first trimester, what tissue constitutes the placental barrier?
Syncytiotrophoblast
Cytotrophoblast
Connective tissue
Foetal capillary
During the third trimester, what tissue constitutes the placental barrier?
Syncytiotrophoblast and foetal capillary endothelium
What molecules pass the placenta barrier by simple diffusion?
Water
Electrolytes
Urea and uric acid
Gases
What molecules pass the placenta barrier by facilitated diffusion?
Glucose
What molecules pass the placenta barrier by active transport?
Amino acids
Iron
Vitamins
What substances pass the placenta barrier by receptor mediated transport?
Passive-immunity - Ig class specific (IgG only)
What hormones are produced by the placenta?
Human chorionic gonadotrophin Human chorionic somatomammotrophin Human chorionic thyrotrophin Human chorionic corticotrophin Progesterone Oestrogen
Describe the arrangement of the blood vessels in the umbilical cord.
2 umbilical arteries carry deoxygenated blood from foetus to placenta
1 umbilical vein carries oxygenated blood from placenta to foetus
How does alcohol cross the placenta?
Diffusion as it is lipid soluble
What implications does the ability of alcohol to diffuse across the placenta have on the baby of a mother who drinks significantly during pregnancy?
Foetal alcohol syndrome
Low birth weight
Growth retarded (potentially with mental retardation)
Head and facial abnormalities
Why might cytomegalovirus be a significant health hazard in pregnancy?
Can cross placenta and cause congenital defect (teratogenic). Normally only causes mild flu-like illness in infected adults
When would anti-D therapy be given and why?
Prophylactically to a Rh- pregnant mother or following birth if the baby is rhesus+ and the mother is rhesus-
If the foetal cells contain antigens which the mothers cells lack then the mother could raise antibodies against the antigens. Mother may therefore have antibodies to foetal rhesus antigens if blood has entered the maternal circulation. Anti-D antibody neutralises these antigens. The presence of anti-Rh antibodies in the foetal circulation causes rapid haemolysis when they bind to the foetal blood.
What are 2 complications that pregnant women may complain of due to increased progesterone levels?
GORD
Constipation
(due to smooth muscle relaxation)
What type of oestrogen level in maternal serum/urine would best indicate foetal progress?
Oestriol as it is produced as a result of foetal liver function and adrenals as well as placental function. Low levels can indicate foetal distress such that early delivery may be desirable
What hormones stimulate breast growth in pregnancy?
Progesterone
Oestrogen
Prolactin
How does inhibin prevent further pregnancies?
Inhibits FSH release, inhibiting further follicular development
What happens in the week of twos?
Differentiation:
Outer cell mass -> syncytiotrophoblast and cytotrophoblast
Inner cell mass -> bilaminar disk - epiblast and hypoblast
2 cavities form - amniotic cavity and yolk sac
Suspended by a connecting stalk within the chorionic cavity
What is the syncytium?
Multi-layerd sheet of cells that minimises resistance of diffusion (large surface area)
What happens to the embryonic spaces?
Yolk sac disappears
Amniotic sac enlarges
Chorionic sac occupied by the expanding amniotic sac
What does ‘the placenta is haemomonochorial’ mean?
One layer of trophoblast ultimately separates maternal blood form foetal capillary wall
What are the differences between primary, secondary and tertiary villi?
Primary villi: early finger-like projections of trophoblast
Secondary villi: invasion of mesenchyme into core
Tertiary villi: invasion of mesenchyme core by foetal vessels
What anchors the placenta?
Establishment of the cytotrophoblast cell
What histological changes of the endometrium occur in preparation for implantation?
Pre-decidual cells
Elaboration of spiral arterial blood supply
What does decidualisation of the endometrium do?
The decidual reaction provide the balancing force for the invasive force of the trophoblast (decidualisation is the mechanism by which the endometrium is prepared for implantation under the influence of progesterone)
Why are the spiral arteries remodelled?
To ensure transport can occur without impedance
Creation of a low resistance vascular bed
Maintains the high flow required to meet foetal demand (particularly late in gestation)
What might result if invasion is incomplete?
Placental insufficiency
Pre-eclampsia
When do the amnion and chorion become a composite membrane?
week 22
Do monozygotic twins share a placenta?
Can - various degrees to which they might share:
2 amnions and 2 chorions
2 amnions but a shared chorion
Amnion and chorion both shared (twin to twin transfusion syndrome might result)
What can the placenta metabolise?
Glycogen
Cholesterol
Fatty acids
When in pregnancy is hCG produced?
First 2 months
What is the function of hCG?
Supports secretory function of coups luteum
What produces hCG?
Syncytiotrophoblast
What hormone is used to detect pregnancy?
hCG - excreted in urine and normally specific to pregnancy as it is produced by the syncytiotrophoblast (sufficient levels within 2 weeks of fertilisation)
What types of disease can hCG be used to monitor?
Trophoblast disease e.g. molar pregnancy, choiocarcinoma
What is the role of placental steroid hormones?
Maintenance of the pregnant state
When do placental steroid hormones take over from the coups luteum?
11th week
How do placental hormones influence maternal metabolism?
Progesteron - increase appetites
hCS/hPL increase glucose availability to foetus
What limits gas exchange in the placenta?
Utero-placental flow (NOT diffusion)
Foetal oxygen stores are small therefore maintenance of adequate flow is essential
Why is it important to measure oxygen levels in labour?
Uterine contractions can limit flow
At what week of pregnancy do foetal IgG levels exceed maternal levels?
35
What infectious agents can cross the placenta?
Varicella zoster Cytomegalovirus Treponema pallidum Toxoplasma gondii Rubella
What can cause haemolytic disease of the newborn?
Rhesus blood group incompatibility of mother and foetus (lysis of red blood cells)
What common drugs are teratogenic?
Thalidomide
Alcohol
Therapeutic drugs
