11- Implantation, the Placenta and Hormonal Changes in Pregnancy Flashcards
What do we need for implantation to occur?
A fully developed blastocyst
- fully expanded
- hatched out from the zona pellucida
A receptive endometrium
- thickened endometrial lining
- expression of embryo receptivity markers
What are the 2 definitive cell lineages that are formed at the blastocyst stage and what do they cells give rise to?
Trophoblast -
- outside cells
- gives rise to placenta
Embryoblast or inner cell mass (ICM) -
- inside cells
- gives rise to the foetus
How is the blastocyst labelled?
The position where the inner cell mass is concentrated is known as the embryonic pole and the opposite end is the abembryonic pole
What are the 3 stages of implantation?
- Apposition - the positioning of the blastocyst close to the endometrium
- Attachment - the attachment of the blastocyst to the endometrium
- Invasion - the blastocyst burrows into the endometrium and implants its self
What happens on days 7-8 on the implantation timeline?
Blastocyst attaches itself to the surface of the endometrial wall (decidua basalis).
Trophoblast cells start to assemble to form a Syncytiotrophoblast in order facilitate invasion of the decidua basalis.
What happens on days 9-11 on the implantation timeline?
Syncytiotrophoblast further invades the endometrial wall and by day 11 its almost completely buried in the decidua (endometrial wall)
What happens on day 12 on the implantation timeline?
Decidual reaction occurs - remodelling of the cells of the endometrium driven by high progesterone levels
High levels of progesterone result in the enlargement and coating of the decidual cells in glycogen and lipid-rich fluid.
This fluid is taken up by the Syncytiotrophoblast and helps to sustain the blastocyst early on before the placenta is formed.
What happens around the 14th day in the implantation timeline?
Cells of the Syncytiotrophoblast start to protrude out to form tree-like structures known as Primary Villi, which are then formed all around the blastocyst.
Decidual cells between the primary villi begin to clear out, leaving behind spaces known as Lacunae.
Maternal arteries and veins start to grow into the decidua basalis. These blood vessels merge with the lacunae – arteries filling the lacunae with oxygenated blood and the veins returning deoxygenated blood into the maternal circulation.
Blood-filled lacunae merge into a single large pool of blood connected to multiple arteries and veins. This known as the Junctional Zone.
What is the Junctional Zone known as?
The circulatory foundation for the formation of the placenta
What happens on day 17 after fertilisation to form the placenta?
The foetal mesoderm cells start to form blood vessels within the villi. Capillaries connect with blood vessels in the umbilical cord. The villi grows larger in size and develops into the Chorionic Frondosum. At this point the endothelial cell wall and Syncytiotrophoblast (villi) lining separate maternal and foetal red blood cells
When does the decidual septa form?
They form in the 4th and 5th month of pregnancy they form as they divide the placenta into 15-20 regions known as Cotyledons
How is each cotyledon supplied?
They are supplied by numerus maternal arteries facilitating the maternal foetal exchange
How does the placenta facilitate maternal and foetal exchange?
Takes up:
- oxygen and glucose
- immunoglobulins
- hormones
- toxins (in some cases)
Drops off:
- carbon dioxide
-waste products
What are the functions of the placenta?
- Provision of maternal O2, CHO, fats, amino acids, vitamins, minerals, antibodies.
- Metabolism e.g. synthesis of glycogen.
- Barrier e.g. bacteria, viruses, drugs etc.
- Removal of foetal waste products e.g. CO2, urea, NH4, minerals.
- Endocrine secretion e.g. hCG, oestrogens, progesterone, HPL, cortisol.
Why is the placenta good at its job?
- Huge maternal uterine blood supply – low pressure (allows for an efficient filtration system)
- Huge surface area in contact with maternal blood.
- Huge reserve in function.
