Implantation, Placentation And Hormone Changes In Pregnancy Flashcards
Blastocyst
An embryo at a stage of development characterised by cells forming an outer trophoblast (trophectoderm) layer, an embryoblast (inner cell mass) and a blastocoel (fluid-filled cavity). The trophoblast layer gives rise to the placenta while the embryoblast layer gives rise to the foetus.
Endometrium
The outer mucous membrane lining the uterus, which thickens during the menstrual cycle in preparation for possible implantation of a blastocyst.
Decidua
Modified endometrium that is formed in response to progesterone, in preparation for pregnancy. Modification process is known as decidualisation.
Placenta
A large organ that is formed during pregnancy, connecting maternal and foetal blood circulation. The placenta facilitates maternal-foetal exchange which is crucial for sustaining foetal development.
Human Chorionic Gonadotrophin (hCG)
A hormone produced by the placenta after implantation and an indicator of a successful pregnancy. Urine pregnancy tests are based on hCG detection.
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.
diagram
Stages of Implantation
- Apposition
- Attachment
- Invasion
Implantation timeline Days 7 - 8
- 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.
Implantation timeline Days 9-11
Syncytiotrophoblast further invades the decidua basalis and by Day 11 its almost completely buried in the decidua.
Implantation timeline day 12
Decidual reaction occurs. 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.
Implantation timeline Around Day 14
- 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 is known as the Junctional Zone.
The Placenta
- Around day 17, foetal mesoderm cells start to form blood vessels within the villi – a basic network of arteries, veins and capillaries. Capillaries connect with blood vessels in the umbilical cord (formed around week 5).
- Villi grows larger in size, develops into the Chorionic Frondosum.
- At this point, endothelial cell wall and Syncytiotrophoblast (villi) lining separate maternal and foetal red blood cells.
in the 4TH & 5TH months of pregnancy, decidual septa form as they divide the placenta into 15-20 regions known as Cotyledons.
Numerous maternal spiral arteries supply blood to each cotyledon, facilitating the maternal-foetal exchange.
The Placenta - maternal↔foetal exchange
takes up: 02 and glucose immunoglobins hormones toxins (in some cases)
drops off:
co2
waste products
The Placenta - Functions
- 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 it’s job?
Huge maternal uterine blood supply – low pressure.
- Huge surface area in contact with maternal blood.
- Huge reserve in function.
