Placenta Flashcards
Timing of clinical gestation
First day of last period to term 40 weeks (37-42)
What is the placenta formed
Weeks 4-5, and is fully functional from week 12 (end of first trimester)
Implantation window
requires a receptive endometrium-
secretory activity peaks in mid-luteal phase
endometrial walls thick in glycogen
glands increase in number and size
surface epithelium swells to form pinopodes which cover the cilia, stopping them from wafting away the embryo helping implantation. also absorbs excess uterine fluid in the cavity.
Stages of implantation
- Apposition- blastocyst loosely associates with uterine wall
- Attachment- firm adhesion with uterine wall
- Invasion- blastocyst attachment triggers enzyme production which degrades and invades glycogen rich storm providing nutrient support
What is decidualisation
Progesterone induces changes in endometrium and occurs irregardless of implantation. Results in endometrial oedema, changes in ECM, angiogenesis and infiltration of uterine NK cells to promote vascular remodelling. The decidua completely surrounds the blastocyst by day 10.
What 3 types of trophoblast does the trophectoderm differentiate to
1.Cytotrophoblasts
2. Syncitiotrophoblasts
- invasive and hormone producing part of placenta
-formed by fusion of cytotrophoblasts
covers entire placenta forming barrier to maternal blood
3.Extravillous trophoblasts (EVT)
-differentiate from cytotrophoblasts
-remodelmaternal blood vessels
all differentiation regulated by IL-15
The human placenta is haemochoroidal, what does this mean?
The chorion is in direct contact with maternal blood, not the maternal epithelium or maternal blood vessels.
The blastocyst survives through histotrophic nutrition, what does this mean?
Oxygen and nutrients reach the embryo via diffusion through the villi. Intervillous space is intially filled with low oxygen blood from capillaries but later fills with higher oxygen blood from remodelled spinal arteries.
What are cotyledons
Lobes of the placenta, separated via septi. There are 4 villi in each cotyledon.
Primary villi
Primary villi form at day 11-13. Cytotrophoblast cells invade and extend into the syncytiotrophoblast layer, to form a finger-like projection in the decidua. Primary villi surround the entire blastocyst. Spaces form within the syncytiotrophoblast layer, to form lacunae, which become filled with blood when the primary villi extend into the maternal blood vessels.
Secondary villi
From day 16 onwards, where extra-embryonic mesoderm invades the core of the primary villus. The mesoderm surrounds the entrie chorionic sac.
Tertiary villi
Day 21 onwards, mesodermal cells differentiate to form fetal capillaries. These vessels fuse with the developing stalk and form the umbilical cord.
Structure of mature placental villi-
Stem villi, branch villi and terminal villi
Stem villi are attached to the chorionic plate, branch villi project from the stem villi and terminal villi are swellings at the tips of branch villi that contain terminal vessels.
Anchoring villi
Cross the intervillous space and attach to the maternal decidua, penetrating through the cytotrophoblastic shell
Changes in blood vessels throughout gestation
Early gestation = central vessels, thick syncytiotrophoblastic later, large diffusion distance. Late gestation = vessels move to the side, thin syncytiotrophoblast layer, short diffusion distance
Why does the spinal artery need to be remodelled
SA are resistance vessels and contract/dilate w/ normal body physiology. However, the intervillous space requires low-resistance, high-flow blood supply. The EVT cells replace the endothelium and smooth muscle cells to produce a wide, funnel-like vessel. uNK cells are also important in remodelling.
How does EVT remodel the spiral arteries
Why are the spinal arteries plugged?
EVT invades the decidua, occludes spinal arteries, replaces endothelium and smooth muscle forming an end-vascular trophoblast layer.
To protect the foetus from oxidative stress, the EVT form a plug in the spinal arteries to create a low oxygen environment. After 12 weeks, the plug dissolves and normal placental blood flow begins. Premature dissolving of EVT plug can lead to miscarridge, as fetus is under too much oxidative stress.
Umbilical arteries and veins
Are contained w/in the umbilical cord. 2 umbilical arteries leading into the foetal capillaries
• These capillaries lie within the stem, intermediate and terminal villi
• Exchange then occurs between the maternal blood in the intervillous space and foetal capillaries within the terminal villi and oxygenated blood goes to the foetus
Artery (x2) carries deoxy blood to placenta, vein (x1) brings back oxygenated blood to fetus.
Changes in mother’s physiology
Uterine blood flow increases x20, CO increases by 3-4x, maternal blood volume increases by 40%, ventilation rate increases
Parabiotic relationship w/ mother and fetus
Fetus is dependant on mother for provision of nutrients, oxygen, salts and removal of waste products. Harmful substances may be passes across placenta, e.g. cocaine, alcohol, some drugs.
What is the double Bohr effect
Gas exchange across the placenta always causes a net oxygen delivery to the foetus and CO2 delivery to the mother
Enabled by passive diffusion as foetal haemoglobin has a higher affinity for oxygen than maternal haemoglobin
There is dumping of CO2 which changes pH on maternal side – causes a double bohr shift which results in increase in pH on foetal side to promote oxygen uptake at lower pO2
Main barrier to placental transfer
Syncytiotrophoblasts, w/ many transport proteins. Oxygen, CO2, water, electrolytes and fatty acids freely diffuse across placenta.
Glucose transfer across placenta
Insulin insensitive hexose transporters via facilitated diffusion, metabolise glucose to lactate which is used as energy by fetus. There is an increase in glucose as maternal insulin insensitivity increases. Mother can be treated for gestational diabetes and fetal glucose transfer will still occur as it is insulin insensitive.
Amino acid transfer across placenta
Regulated by progestrone. Mother retains extra amino acids and actively transports into fetus.
Active transport across placenta
Calcium, B12, folic acid are taken in via active transport. Calcium is essential for ossification in third trimester.
Maternal IgG transfer
Transferred via pinocytosis, providing passive post-natal immunity
Placenta previa
Blastocyst implants lower than usual, meaning cervix is blocked. Baby may need to be delivered via C-sec. Can be marginal.
Placenta accrete
Excessive trophoblast invasion, may be through myometrium or entire uterine wall. Placenta becomes inseperable, can result in maternal haemorrhage, requiring hysterectomy. Ass. w/ scarring of myometrium from previous c-sec.
Pre-eclampsia
Poor EVT remodelling, shallow trophoblast invastion can lead to intra-uterine growth restriction and pre-eclampsia. This is because there is less blood flow, less area for exchange, etc. Symptoms present after 20 weeks.
