Implantation, Placentation and Hormonal Changes in Pregnancy Flashcards
What is a blastocyst?
β An embryo at the 5th or 6th day of development
What does the embryoblast give rise to?
β The fetus
What are the three layers of the blastocyst?
β Trophoblast
β Blastocoel
β Embryoblast
What does the trophoblast give rise to?
β The placenta
What is the blastocoel?
β The fluid filled cavity
What are the two things needed for implantation to occur?
β The blastocyst has to be fully mature
β A receptive endometrium is needed which expresses receptivity markers to communicate with the blastocyst
Describe hatching
βThe blastocyst hatches out of the zona pellucida due to expansions and contractions
β this herniates the blastocyst out of the zona
β digestive enzymes break down the opposite pole of where the blastocyst is
Where do the enzymes dissolve the zona?
β Abembryonic pole
What are the three stages of implantation?
1) Apposition
2) Attachment
3) Invasion
What is Apposition?
β Close positioning of the blastocyst to the endometrium
What is Attachment?
β Cells of the trophoblast attaching to endometrium
What is Invasion?
β Trophoblast cells multiply and invade the endometrium
Describe how the embryo implants (days 7-11)
β The blastocyst attaches itself to the surface of the endometrial wall (decidua basalis)
β Trophoblast cells start to assemble to form a syncytiotrophoblast to facilitate invasion
β Syncytiotrophoblast further invades the decidua basalis and by day 11 it is almost completely buried
Describe the decidual reaction
β High levels of progesterone results in the enlargement and coating of the decidual cells in glycogen and lipid-rich fluid
β the fluid is taken up by the syncytiotrophoblast and helps sustain the blastocyst early on before the placenta is formed
What is a syncytiotrophoblast?
β A group of trophoblast cells which are multinucleated
How are primary villi formed?
β Cells of the syncytiotrophoblast start to protrude out to form tree-like structures known as primary villi which are formed around the blastocyst
How are lacunae formed?
β Decidual cells between the primary villi begin to clear out leaving behind empty spaces known as lacunae
How does the junctional zone form?
β Maternal arteries and veins grow into the decidua basalis
β Blood vessels merge with the lacunae
β Arteries fill the lacunae with oxygenated blood
β Veins return the deoxygenated blood to the maternal circulation
β Blood filled lacunae merge to a single large pool of blood connected to multiple arteries and veins
βthis is known as the junctional zone
When is the umbilical cord formed?
β Around week 5
Describe how chorionic frondosum form?
β Day 17 the fetal mesoderm cells start to form blood vessels within the villi
β a basic network of arteries, veins and capillaries
β capillaries connect with blood vessels in the umbilical cord
β villi grow larger in size and develop into chorionic frondosum
What separates maternal and fetal blood cells?
β The outer lining of the primary villi is in contact with the junctional zone
β The lining of the endothelial cells of the decidual from the mothers side
When do the decidual septa form?
β 4th and 5th month
What is the function of decidual septa?
β They divide the placenta into 15-20 regions
β This gives it a much larger surface area in contact with the maternal blood