The placenta is a temporary structure which arises from both fetal and maternal cells. Its primary function is the selective transport of nutrients and waste products between mother and fetus. This process is aided by the proximity of the maternal and fetal circulations within the placenta.
The transport of molecules
Oxygen and carbon dioxide will diffuse across the placenta in response to differences in the partial pressure of the gases.
The maternal pO2 is greater than the fetal pO2 and this causes oxygen to diffuse across the placenta. However, the same amount of oxygen will be supplied to both the fetal and maternal tissues because fetal blood has a higher haemoglobin concentration (about 50% higher than that of the mother) and a higher oxygen carrying capacity.
A lot of carbon dioxide is produced by the fetus and, because the pCO2 in fetal blood is higher than maternal blood, it diffuses from the fetal blood through the placenta, into the maternal circulation, and is got rid of by expiration from the mother's lungs.
Glucose is the main energy source for the placenta and fetus. It is transported across the placenta by facilitated diffusion.
Amino acid concentrations in the fetal blood are higher than in maternal blood and, therefore, have to be transported to the fetus by active transport.
There is a substantial transfer of immunoglobulin G from maternal to fetal circulation which confers passive immunity.
There are several fetal metabolites which must be eliminated, for example, bilirubin (derived from the breakdown of haem). In the adult, bilirubin is conjugated to make it water soluble and easier to eliminate but in the fetus conjugated bilirubin is poorly transported across the placenta in contrast unconjugated bilirubin is readily transported for elimination by the mother.
5) harmful molecules
These include carbon monoxide, viruses (eg HIV, cytomegalovirus, rubella), bacteria (eg tuberculosis) and protozoa (eg Toxoplasma), drugs (eg cocaine, alcohol, caffeine, tetracycline).Immunoglobulins can be harmful to the fetus if they are, for example, anti-Rhesus antibodies. These can arise if there is a small leak of fetal cells into the maternal circulation which will trigger an immune response.
Key stages in placental development
1) First differentiation step
About 6 days after fertilisation, the cells of the blastocyst will have differentiated into the outer cell layer – the trophectoderm – and the inner cell mass. The trophectoderm will become the placenta and the inner cell mass the fetus.
2) Attachment to the uterine tissues
Around day 6 to 7 after fertilisation the blastocyst will attach itself to the endometrial lining of the uterus.
The trophectoderm differentiates into the cytotrophoblast and syncytiotrophoblast.
3) Development of villous structure
The cytotrophoblasts and syncytiotrophoblasts go on to form a villous structure.
From around 8 weeks after fertilisation the cytotrophoblast will break through syncytiotrophoblast shell and invade into the decidua.
The cytotrophoblasts reach spiral arteries which are converted from narrow to wide vessels allowing a much greater flow of maternal blood around the villi. The villous trophoblast is the barrier between maternal and fetal circulation.
4) Remodelling of spiral arteries
The endothelium and smooth muscle of the spiral arteries are replaced by trophoblasts.
Diagram showing the relationship between the fetal and maternal circulations in the fully developed placenta and the vessels in the umbilical cord.
1. This photomicrograph shows a cross-section of a villous. What is the circular structure (containing cells) within the villous as indicated by the arrow? What are the cells within this structure?
2. This photomicrograph shows a villous. What is the name of the darkly-stained layer of cells as indicated by the arrow? What is happening at A?
3. What is this structure? What are A and B?
4. What is this structure? What are the dark blue structures shown and what is their function?
5. In the photograph is the fetal or maternal side of the placenta shown?