Development and function of the placenta

Question 1

What happens during implantation

Answer 1

• implantation begins around 6 days post-implantation
• the zona pellucida degenerates and the mature blastocyst forms (trophoblast, inner cell mass, blastocyst cavity)
• implantation is completed by day 10

Question 2

What happens to the blastocyst around day 7

Answer 2

• trophoblast differentiates into two layers
• cytotrophoblast (inner)
• synctiotrophoblast (outer)
• the inner cell mass differentiates
• epiblast
• hypoblast

proliferating synctiontrophoblasts
- produce HCG
- undergo significant division and contain degradation enzymes that degrade maternal layer allowing blastocyst to borrow further

hCG signals to corpus luteum that fertilisation has occurred

Question 3

What changes occur in the developing embryo on day 12

Answer 3

• the blastocyst is now completely inside the endometrium
• the amniotic cavity forms from the epiblast
• the yolk sac forms from the hypoblast
• the proliferating syncytiotrophoblast can break down the walls of maternal blood vessels

Question 4

What occurs in the decidual reaction

Answer 4

• reaction triggered by implantation and high concentrations of progesterone and estrogen
• uterine cells become laden with glycogen and lipids (they are swollen)
• the rupturing of the decidual (uterine) cells provides nutritional support
• decidual cells also protect the embryo from immune rejection

Question 5

What changes are present in the decidua (maternal tissue)

Answer 5

• decidua all shed during labour
• the functional endometrium is now the ‘decidua’
• it forms three layers which act to protect and develop the growing embryo
• decidua BASALIS: key for nutritional support
  (becomes part of maternal placenta)
• decidua CAPSULARIS
• decidua PARIETALIS

Question 6

What does the developing embryo look at around day 16

Answer 6

• the extraembryonic mesoderm: forms a coat around inner cell mass and blastocyst cavity

lucuna: maternal blood floods into intervillous space from the breakdown of blood vessels

chorionic cavity forms and beginning of umbilical cord forms

Question 7

How does chorion formation occur

Answer 7

around day 12
- cytotrophoblast proliferates forming primary chorionic villi

• the extraembryonic mesoderm protrudes as the mesoderm fills the core forming secondary chorionic villi

chorionic villi form foetal part of placenta
- some mesoderm cells differentiate into epithelial cells leading to blood vessels appearing forming tertiary chorionic villi

Question 8

What is the placental blood flow

Answer 8

• spiral arteries empty maternal blood into intervillous spaces
• they are convoluted and diluted to reduce pressure and allow maximum exchange of nutrients between blood
• fetal blood contained within vessels, oxygenated blood enters within umbilical vein and deoxygenated blood leaves via umbilical arteries
• arteries are more constricted and show poorer relaxation in pregnancies with fetal growth restriction but this can be improved

Question 9

What is the structure of the mature placenta

Answer 9

fetomaternal organ
- Decidua basalis (maternal)
- Chorionic villi (fetal)

fetal side of placenta is smooth as lined by amnion, maternal side is divided into 35 lobes which increase surface area

disk shaped (3cm thick)
500g in weight
surface area of 10m2
- maximises oxygen and nutrient delivery

Question 10

What are the different functions of the placenta

Answer 10

• protection
• nutrition
• respiration
• excretion
• hormone production

Question 11

How does the placenta have a role in protection

Answer 11

• placenta transfers some of the maternal antibodies
• selective barrier to some microbes
• prevents the fetus being destroyed by maternal immune system

Question 12

How does the placenta have a role in nutrition

Answer 12

• placenta cells can synthesise glycogen, cholesterol and fatty acids
• glucose easily crosses placenta by facilitated diffusion (GLUT 1)
• amino acids and vitamins can also cross

Question 13

How does the placenta have a role in respiration

Answer 13

• oxygen diffuses from maternal to fetal blood
• carbon dioxide diffuses from fetal to maternal blood

Question 14

How does the placenta have a role in excretion

Answer 14

• excretion of urea and uric acid
• conjugated bilirubin

Question 15

How does the placenta have a role in hormone production

Answer 15

• protein hormones: hCG and hPL

hCG: prevents breakdown of corpus luteum
hPL: prepares breast tissue and formation of milk

• steroid hormones: estrogen and progesterone
• maintains endometrial lining