Placental Function

Question 1

Describe the mature placenta

Answer 1

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.

Question 2

Describe the foetal surface of the placenta

Answer 2

smooth and glistening surface as it is covered in amnion with the umbilical cord inserted in the centre with vessels radiating from it

Question 3

Describe the maternal surface of the placenta

Answer 3

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)

Question 4

How does the mature placenta provide foetal circulation?

Answer 4

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.

Question 5

What are the 4 stages in placental development?

Answer 5

1. Differentiation and maintenance of trophectoderm
2. Invasion into uterine tissues
3. Development of villous structure
4. Remodelling of spiral arteries

Question 6

Describe cytotrophoblasts

Answer 6

Have a single nucleus and divide rapidly in vivo

Question 7

Describe syncytiotrophoblasts

Answer 7

Derived from fused cytotrophoblasts, are multinucleated and do not divide in vivo

Question 8

Describe the differentiation and maintenance of trophectoderm in placental development

Answer 8

The inner cell mass of the blastocyst will face the surface of the endometrium and the trophectoderm differentiates into cytotrophoblast and syncytiotrophoblast

Question 9

Describe the invasion into uterine tissues in placental development

Answer 9

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

Question 10

Describe the development of villous structure in placental development

Answer 10

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

Question 11

Describe the remodelling of the spiral arteries in placental development

Answer 11

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

Question 12

What are the main functions of the placenta?

Answer 12

Site for gaseous exchange, nutrient exchange, waste exchange, synthesis of proteins and enzymes

Question 13

Describe gaseous exchange at the placenta

Answer 13

Occurs via simple diffusion due to foetal haemoglobin having a greater oxygen affinity and carrying capacity

Question 14

Describe nutrient exchange at the placenta

Answer 14

Simple diffusion: water, electrolytes, fatty acids, waste products
Facilitated diffusion: glucose
Active transport: amino acids
Other: IgG, hormones, antibiotics, sedatives

Question 15

What hormones does the placenta produce?

Answer 15

hCG, hPL, oestrogen, progesterone, human placental growth hormone, insulin-like growth factor, relaxin, hypothalamic and pituitary-like hormones

Question 16

What is the role of hCG produced by the placenta?

Answer 16

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)

Question 17

What is the role of hPL produced by the placenta?

Answer 17

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

Question 18

What is the role of oestrogen and progesterone produced by the placenta?

Answer 18

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)

Question 19

What is the role of human placental growth hormone produced by the placenta?

Answer 19

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.

Question 20

What is the role of insulin-like growth factors produced by the placenta?

Answer 20

similar structure to insulin and stimulate proliferation and differentiation of the cytotrophoblast (cells which change spiral artery width)

Question 21

What is the role of relaxin produced by the placenta?

Answer 21

produced by decidual cells (endometrium) and works to soften the cervix and pelvic ligaments in preparation for labour

Question 22

What is the role of hypothalamic and pituitary-like hormones produced by the placenta?

Answer 22

these include GnRH and corticotrophin releasing factor (CRF)

Question 23

Why is Rhesus factor a potential issue in pregnancy?

Answer 23

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.

Question 24

Why is the foetus not rejected by the maternal immune system?

Answer 24

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

Question 25

What pathologies are associated with abnormal placental development?

Answer 25

Pre-eclampsia, intrauterine growth restriction and early miscarriage

Question 26

What can happen if the placenta isn’t delivered immediately after foetal expulsion?

Answer 26

Can lead to post-partum haemorrhage and infection due to retained placental tissue.

Question 27

What can happen if the placenta is too thick or thin?

Answer 27

Too thick - association with maternal diabetes, foetal hydrops and intrauterine foetal infections
Too thin - intrauterine growth retardation of the foetus

Question 28

What does pallor of the maternal surface of the placenta indicate?

Answer 28

Foetal anaemia and may be a sign of haemorrhage

Question 29

What is a cord knot?

Answer 29

if sufficient tension is placed on the cord before or during labour and delivery, blood flow may be cut off, and signs of foetal asphyxia may occur (oxygen starvation).