L13: the placenta and support of pregnancy

Question 1

Describe the process of implantation

Answer 1

Begins at day 6
Trophoblast cells interact with the endometrial lining of the uterus once it has ‘hatched’ and lost the zona pellucida
Blastocyst then becomes embedded within the endometrium -> interact with the increased vasculature and secretory glands of the endometrium that have developed during the menstrual cycle

Question 2

What is placenta accreta?

Answer 2

Invasion that is too deep

Question 3

Describe the development of the placenta

Answer 3

Specialisation of the ‘fetal membranes’ surrounding the fetus – develops from the trophoblast cells to support the pregnancy
Amniotic sac enlarges, chorionic sac is displaced -> amniotic membrane fuses with chorionic membrane to produce a single amniotic cavity
Projections around the outer surface of the membrane are initially balanced over the entire surface, however as this growth occurs -> projections concentrated in a single disc-like space = placenta

Question 4

Describe the chorionic villi

Answer 4

Villi come from the trophoblast cells
Branch out as tree-like structures, with an outer layer of syncytiotrophoblast & core comprised of connective tissue – fetal blood vessels can develop
Maternal blood vessels then surround these villi, allowing exchange to occur – two circulations never mix
1st trimester: barrier is relatively thick, but as pregnancy progresses the barrier becomes thinner

Question 5

Describe the decidua

Answer 5

The cells of the endometrium that become specialised to modulate the degree of invasion of the conceptus once it has implanted – occurs through a decidual reaction
Important -> if conceptus implants in a location where there is no decidua, there is no inhibition of decidualisation and therefore no control of extent of invasion

Question 6

Describe the maternal and fetal blood vessels

Answer 6

Maternal blood vessels – endometrial arteries and veins, which bathe the outside of the villi for exchange to occur
Fetal blood vessels – bring waste products to the villi through the umbilical arteries (paired) and takes oxygen and nutrients to the fetus via the umbilical vein

Question 7

Describe the endocrine function of the placenta

Answer 7

Produces protein hormones, such as hCG -> analysed in urinary and serum pregnancy testing
Sustains the corpus luteum in the first trimester
Also produces steroid hormones, so that it can take over the role of the corpus luteum -> produces enough oestrogen and progesterone to keep HPG axis in a ‘pregnant state’

Question 8

Describe metabolic changes that occur due to hormones

Answer 8

Progesterone – increase in appetite to allow an increased fat deposition to help support the fetus & breastfeeding later in the pregnancy
Human placental lactogen – creates a diabetogenic state to cause insulin resistance in the mother, increasing glucose availability to the fetus

Question 9

Describe gas exchange in the fetus

Answer 9

Rate of exchange is flow limited, not diffusion limited

Adequate uteroplacental circulation is required & if compromised can lead to fetal distress

Question 10

Describe the transfer of immunity from mother to fetus

Answer 10

Antibodies can be transported across the placenta into the fetal circulation so that fetus has some sort of defence against infection
Only certain types of immunoglobulins can cross the placenta, specifically IgG
Teratogens have their greatest effect in early pregnancy, particularly in embryonic stage (3-8 weeks) – key time for development of the body systems
Pregnancy = immunocompromised state for mother

Question 11

How does the placenta sustain the corpus luteum in the 1st trimester?

Answer 11

Production of protein hormones eg. hCG

Produced by syncytiotrophoblast

Question 12

Outline placental transport mechanisms and substances they transport

Answer 12

Simple diffusion - water, electrolytes & gases
Facilitated diffusion - glucose
Active transport - amino acids