Flashcards in Placental Function Deck (29):
Describe the mature placenta
It is a discoid shape, weighing ~0.5kg and 2.5cm thick at the centre. Most importantly, it lies predominantly in the upper uterine segment with 2/3 being on the posterior surface, and 1/3 on the anterior surface of the uterus.
Describe the foetal surface of the placenta
smooth and glistening surface as it is covered in amnion with the umbilical cord inserted in the centre with vessels radiating from it
Describe the maternal surface of the placenta
surface is dull and greyish, it is divided into 15-20 cotyledons where each cotyledon is formed of branches off one main villus stem (which is covered by the decidua basalis)
How does the mature placenta provide foetal circulation?
There are two umbilical arteries which travel from the foetus to the placenta, and these have smaller branches to the chorionic villi. The main site of exchange between the foetal and maternal circulation occurs at the capillary networks in the terminal branches of the chorionic villi. Then, upon leaving the placenta, there are larger venous branches which give rise to the umbilical vein which exits the placenta and delivers nutrients to the foetus.
What are the 4 stages in placental development?
1. Differentiation and maintenance of trophectoderm
2. Invasion into uterine tissues
3. Development of villous structure
4. Remodelling of spiral arteries
Have a single nucleus and divide rapidly in vivo
Derived from fused cytotrophoblasts, are multinucleated and do not divide in vivo
Describe the differentiation and maintenance of trophectoderm in placental development
The inner cell mass of the blastocyst will face the surface of the endometrium and the trophectoderm differentiates into cytotrophoblast and syncytiotrophoblast
Describe the invasion into uterine tissues in placental development
Blastocyst then attaches to the uterine wall and trophoblast cells begin to invade, which occurs partially due to the presence of the oxygen gradient between mother and foetus at the developing placenta
Describe the development of villous structure in placental development
Trophoblast cells form villous structures --> villous trophoblast is barrier between maternal and foetal circulation --> occurs at ~7 days post-fertilisation and there is very little maternal blood supply to the embryo at this stage, so it exists in a relatively hypoxic environment
Describe the remodelling of the spiral arteries in placental development
Cytotrophoblasts break through the trophoblast shell --> invade decidual tissue (extravillous trophoblast) --> spiral arteries --> narrow to wide vessels--> greater flow of maternal blood around the villi
What are the main functions of the placenta?
Site for gaseous exchange, nutrient exchange, waste exchange, synthesis of proteins and enzymes
Describe gaseous exchange at the placenta
Occurs via simple diffusion due to foetal haemoglobin having a greater oxygen affinity and carrying capacity
Describe nutrient exchange at the placenta
Simple diffusion: water, electrolytes, fatty acids, waste products
Facilitated diffusion: glucose
Active transport: amino acids
Other: IgG, hormones, antibiotics, sedatives
What hormones does the placenta produce?
hCG, hPL, oestrogen, progesterone, human placental growth hormone, insulin-like growth factor, relaxin, hypothalamic and pituitary-like hormones
What is the role of hCG produced by the placenta?
synthesis begins prior to implantation in order to maintain corpus luteum to continue progesterone and oestrogen secretion in early pregnancy. This hormone is also highly similar structurally to LH (can work on LH receptors)
What is the role of hPL produced by the placenta?
works to increase the production of free fatty acids as a result of lipolysis, it also works to inhibit gluconeogenesis, promote foetal growth and mammary duct proliferation. This hormone also works to exhibit lactogenic effects (to encourage lactation) and resembles growth factor
What is the role of oestrogen and progesterone produced by the placenta?
progesterone is produced independently from cholesterol precursors, and oestrogen is produced in concern with the foetal adrenal gland. The placenta takes over the role of the corpus luteum after the first trimester (12 weeks)
What is the role of human placental growth hormone produced by the placenta?
similar to growth hormone and works to regulate maternal blood glucose levels in order to provide adequate supply to the foetus. If there is a low maternal blood glucose level, HPGH will stimulate gluconeogenesis in the maternal liver.
What is the role of insulin-like growth factors produced by the placenta?
similar structure to insulin and stimulate proliferation and differentiation of the cytotrophoblast (cells which change spiral artery width)
What is the role of relaxin produced by the placenta?
produced by decidual cells (endometrium) and works to soften the cervix and pelvic ligaments in preparation for labour
What is the role of hypothalamic and pituitary-like hormones produced by the placenta?
these include GnRH and corticotrophin releasing factor (CRF)
Why is Rhesus factor a potential issue in pregnancy?
The mother may be Rhesus negative, whilst the baby is Rhesus positive, and therefore, due to placental transfer, the mother develops antibodies to Rh+ and if they have another child, they will transfer these antibodies to the foetus and destroy the red blood cells of the foetus.
Why is the foetus not rejected by the maternal immune system?
Don't express classic HLA but express HLA G which is not recognised by the host immune system, the foetus has infiltrating leukocytes which secrete IL-2 to regulate the immune system and there is a decidual reaction when the decidual cells become swollen and tightly compacted together around the developing foetus --> forma barrier between the mother and implanting embryo
What pathologies are associated with abnormal placental development?
Pre-eclampsia, intrauterine growth restriction and early miscarriage
What can happen if the placenta isn't delivered immediately after foetal expulsion?
Can lead to post-partum haemorrhage and infection due to retained placental tissue.
What can happen if the placenta is too thick or thin?
Too thick - association with maternal diabetes, foetal hydrops and intrauterine foetal infections
Too thin - intrauterine growth retardation of the foetus
What does pallor of the maternal surface of the placenta indicate?
Foetal anaemia and may be a sign of haemorrhage