57.2 Pregnancy Flashcards
Summarise the general structure of the placenta.
The placenta is essentially the interface between the maternal blood and the fetal blood:
- Fetal vessels pass via the umbilical cord and form branches within villi
- These villi are surrounded by intervillous spaces, which are supplied by the maternal blood
- Thus, the fetal blood and maternal blood are completely separated, so the surface of the villi is required for exchange and immunological protection
What are placental villi?
Placental (a.k.a. chorionic) villi are villi that sprout from the chorion to provide maximal contact area with maternal blood.
Where on the uterus does the placenta usually form?
On the upper posterior aspect of the uterine wall.
When does the development of the placenta begin? What happens here?
DAY 9 when lacunae (fluid filled holes) form within the syncitiotrophoblasts
Describe the development of the placenta.
- Begins with implantation, including attachment, penetration and decidual reaction
- The syncytiotrophoblast cells are fused, then maternal blood fills the lacunae in the
- The cytotrophoblastic cells begin to develop and the layer buldges out, forming primary villi -> These eventually develop into tertiary villi
- Thus, the syncytiotrophoblast and crytotrophoblast form the outer lining of placental villi, which are surrounded by maternal blood
What are the stages of placenta formation?
Formed from the trophoblasts
Primary chorionic villi
Secodnary chorionic villi
Teritary chorionic villi
How do primary chorionic villi form?
Cytotrophoblast penetrates and expands in the surrounding syncitiotrophoblasts
How do secondary chorionic villi form?
Embryonic pole trophoblasts lose their cell membranes and becomes the syncitiotrophoblasts
Trophoblasts lining blastocyst keep cell membranes and become cytotrophoblasts
How do tertiary chorionic villi form?
Embryonic vessels begin to form in the poliferating embryonic mesoderm to form teritary chorionic villi
(end of third start of fourth week when placenta can now function)
Describe the differences in the composition of the different stages of the chorionic villi
Primary = trophoblast only
Secondary = trophoblast and mesoderm
Teritary = trophoblast, mesoderm and blood vessels
In summary, what tissue is the placenta derived from?
Trophoblast
Which layer other than the trophoblasts and mesoderm also forms the mature placenta?
Decidua basalis (part of endometrium that takes part in placental formation)
How is the intervillous space produced?
Lacunae filled with maternal blood fuses together to form large intervillous spaces which the synctiotrophoblasts protrude into
What is the function of the placenta?
-Synthesis of hormones
-Transfer of respiratory gases (lungs), nutrients (GI tract) and waste products (liver)
-Physical attachment of conceptus to uterus
-Invasion and digestion of endometrial tissues
What has been circles below?
Primary villi - cytotrophoblasts invading syncytiotrophoblasts
What changes in maternal metabolism does hPL cause? Why?
Reduces insulin sensitivity to maternal tissues (diabetogenic effect) to increase plasma glucose levels
Increases use of FFA used by mother and decreased glucose use to preserve the fuel
Increased foetal insulin secretion to take up the glucose
Suppression of gluconeogenesis from amino acids - useful supply for fetus
Summarise the effects of hPL.
- hPL essentially has diabetogenic effects, increasing concentrations of sugar and fatty acids in the blood for the fetus.
- This can lead to gestational diabetes
Describe the changes in concentration of the different steroids involved in pregnancy and where they are produced.
- At first, after implantation happens, hCG from the placenta (syncytiotrophoblast) maintains the corpus luteum -> Thus, the ovaries produce most of the progesterone at first
- By the second trimester, the corpus luteum has degraded and the placenta has developed -> More progesterone and estrogen are produced from the placenta, as hCG decreases
- In the third trimester, there is very little hCG, while progesterone and estrogen peak
Describe the concept of the maternal-placental-fetal unit.
- The mother requires progesterone and estrogens for maintaining pregnancy
- The mother provides the LDL cholesterol required for synthesis of these, but she lacks the enzymes to produce them, so she cannot synthesise them along
- Therefore, a maternal-placental-fetal unit is required for synthesis -> The placenta and fetal adrenal/liver provide the enzymes for progesterone and estrogen synthesis
(Details of enzymes are not core)
Why is the foetus needed in the maternal-placental-fetal unit?
Foetal liver contains the enzyme 16 alpha hydroxylase needed to make an intermediate to synthesise estriol
Foetal adrenal gland contains enzymes needed to make DHEA -sulphate (weak androgen)
Why is it important that the fetus cannot do complete biosynthesis of oestrogens and prosgesterones? How is this overcome?
It would expose dangerously high levels of hormones not needed by the fetus
Fetus cannot make anything past DHEA and 16 alpha OH -DHEA which it then delivers to the placenta which acts as a large sink of weak androgens to prevent masculinisation of female fetuses
Fetus sulphates intermediates to reduce their biological activity
What happens in amniocentesis?
50-100ml of amnionic fluid is aspirated and foetal cells are cultured and karyotyped to check for chromosomal abnormalities, single gene disorders and for alpha-fetoprotein (sign of spina bifida)
What is chorionic villus sampling?
Suction tube used to remove foetal cells from the chorion (early placenta) to look for (a) chromosomal abnormalities; (b) single gene disorders; (c) transplacental virus infection (e.g. Rubella); and (d) foetal blood grouping (e.g. Rh).
