Placenta Formation and Function

Question 1

What are the unique properties of the placenta?

Answer 1

1. Disposable organ
2. Composed of cells from two genetically distinct organisms
3. Multi functional capable of:
   - Gas transfer
   - Nutrient transfer
   - Waste and toxin removal
   - Immune barrier
   - Endocrine function
   - Fluid balance

Question 2

What are the main components of the placenta?

Answer 2

1. Umbilical cord:
   - 2 arteries
   - 1 vein
2. Chorionic villi
   - fetal capillaries enclosed in cytotrophoblast and then syncytiotophoblast cells
3. Intervillous space:
   - lies between chorionic place and decidua basalis
   - filled with maternal blood

Question 3

Describe the formation of the placenta:

Answer 3

1. Implantation (day 8)
   - Blastocyst implants
   - Trophoblast cells differentiate into inner cytotrophoblast and outer syncytiotrophoblast
   - syncytiotrophoblast cells secrete proteolytic enzymes and invade the endometrium
2. Lacunae Formation:
   - As the syncytiotrophoblast cells extend into the endometrium gaps form calld lacunae
   - These lacunae merge to form the intervillous space
   - As maternal arteries are eroded the intervillous space will become filled with maternal blood
3. Chorionic Villi Formation:
   - Occurs in 3 stages:
4. Primary: invasion of cytotrophoblast cells into the syncytiotrophoblast
5. Secondary: invasion of extra-embryonic mesoderm into cytotrophoblast
6. Tertiary: formation of embryonic blood vessels inside the mesoderm of the villi
7. Formation of cytotrophoblastic shell:
   - The intervillous space is closed by the cytrotrophoblastic shell
8. Spiral artery remodelling:
   - extravillous trophoblast cells invade the maternal spiral arteries and cause the break down of the vessel endothelial and smooth muscle cells
   - this allow a large, continuous and low pressure supply of blood to enter the intervillous space

Question 4

What are the different morphological variations in placentation?

Answer 4

1. Diffuse:
   - chorion of the placenta covers the enter amniotic sac e.g. pigs, horses
2. Cotyledonary:
   - multiple small cotyledons cover the amniotic sac
   e. g. sheep, cows, deer
3. Zonary:
   - the placenta forms a ring/band around the centre of the amniotic sac
   - dogs, cats and bears
4. Discoid:
   - the placenta is attached to the side of the amniotic sac
   - primates, humans, mice

Question 5

What are the histological variations in placentation?

Answer 5

1. Epitheliochorial:
   - Maternal blood is in contact with fetal epithelium
   - 6 layers seperate maternal and fetal blood: maternal uterine vessel endothelial cells, mesenchyme of endometrium, uterine epithelium, fetal trophoblast cells, fetal mesenchyme and then fetal endothelial vessel cells
2. Endotheliochorial:
   - Maternal endothelium is in contact with fetal tissues:
   - 4 layers separate the maternal and fetal blood: maternal uterine vessel endothelial cells, fetal trophoblast cells, fetal mesenchyme and then fetal endothelial vessel cells
3. Hemochorial:
   - Maternal blood in contact with fetal epithelium
   - 4 layers separate the maternal and fetal blood: the fetal syncytiotrophoblast layer, the fetal cytotrophoblast layer, the fetal mesenchyme and then the fetal endothelial vessel cells

Question 6

What are the functions of the placenta?

Answer 6

1. Transfer of substances:
   - Respiratory gases
   - Nutrients
   - Wastes
2. Protection
   - Transporters and enzymes in the placenta reduce the transfer of drugs and xenobiotics across the placenta
   - Placenta is not a perfect barrier as may drugs and xenobiotics and bacteria and viruses cross
3. Endocrine function:
   - Maintain pregnancy- progesterone and estrogen
   - Placental and fetal development- hCG, estrogen, placenta lactogen, insulin-likegrowth factors
   - Promotes maternal adaptations to pregnancy: placental growth hormone and thyrotropin
   - Initiates parturition: cytokines, chemoskines

Question 7

What are some issues with placentation that may occur?

Answer 7

1. Miscarriage:
   - Due to extremely inadequate trophoblast invasion
   - Failed immune tolerance towards the trophoblast
2. Preeclampsia:
   - Due to inadequate trophoblast invasion of the spiral arteries and thus a lack of spiral artery remodelling
   - Leads to chronic hypoxia and oxidative stress in the placenta
   - The placenta releases factors that cause widespread endothelial dysfunction
3. IUGR:
   - Often characterised by underdeveloped chorionic villi
   - Inadequate transfer of material in syncytriotrophoblast
4. Placetna Accreta:
   - Placenta attaches abnormally deep (invades into myometrium)
5. Placenta Previa:
   - Placenta attaches over cervival opening
6. Placenta abruption:
   - Early detachment of placenta from uterine wall that causes significant maternal bleeding and lack of oxygen and nutrient supply to fetus