Embryo: Second Week Of Development Flashcards
What 2 tissues does the trophoblast form?
- Cytotrophoblast
2. Syncytiotrophoblast
What are the rules of 2?
- Embryoblast –> 2 germ layers
- Trophoblast forms 2 tissues
- Blastocyst cavity remodeled twice
- Two new cavities formed
- Extraembryonic mesoderm splits into 2 layers
What is syncytiotrophoblast?
Invasive, digestive, ingestive
What is cytotrophoblast?
Highly mitotic; contributes cells to syncytiotrophoblast
What is placenta previa?
Pregnancy in which the placenta is implanted in the lower part of the uterus (instead of the upper part)
What is an ectopic pregnancy?
Implantation of the fertilized egg outside the uterine cavity, often in the tube or ovary, or rarely in the abdominal cavity
What causes a hydatidiform mole?
- Trophoblast only; no embryonic tissue
- Paternal genes only expressed
- Produce high levels of hCG
- May produce benign or malignant tumors
What does the embryoblast become during the second week?
Bilaminar Germ Disc (at about day 8)
The embryoblast forms what 2 layers?
- Epiblast
2. Hypoblast
What is the epiblast?
The primary ectoderm that differentiates into the endoderm, mesoderm, and ectoderm during Gastrulation
- has columnar cells and an amniotic cavity with epiblast/amnioblasts
What is the hypoblast?
- Cuboidal cells
- Extraembryonic endoderm lines blastocyst (Heuser’s membrane); day 9
- This is the primary yolk sac that eventually gets pushed aside
- Definitive (secondary) yolk sac (day 13) forms smaller than primary
What is the name of the remnants of the primary yolk sac?
Exocoelomic cysts
What is the yolk sac for?
To provide nutrition for the embryo
What is the exocoelomic cavity?
Formed from additional cells of the hypoblast that migrate along the inside of the exocoelomic membrane and form a new cavity
What is the name of the vacuoles in the syncytiotrophoblast?
Lacunae primordial of intervillous spaces of placenta (day 9); concentrated at embryonic pole
- they act as barriers to maternal blood - are all fetal in origin
What happens to the endometrium during week 2?
- Blood vessels dilate to form sinusoids
2. Erosion of vessels and glands releases contents into lacunae
What happens to the cytotrophoblast during week 2?
- Forms primary villi; columns of cells extend into lacunae (day 13)
- Induced by developing extraembryonic mesoderm
What are the primary villi?
Extensions of cytotrophoblast grow out into the blood filled lacunae and carry with them a covering of syncytiotrophoblasts
Where is the extraembryonic mesoderm?
In between the embryoblasts and trophoblast cells
What happens to the extraembryonic mesoderm as it grows?
It splits in the middle and forms 2 layers
- layers form chorionic cavity
What is the chorionic cavity?
Surrounds the amniotic cavity; the yolk sac is within the chorionic cavity
- it separates where the embryo is going to grow from where the placenta will be located
What does the connecting stalk do?
Connects embryo properly to the developing placenta and becomes the umbilical cord
- Somatopleuric mesoderm connects germ disc to lining of trophoblast (chorionic plate)
- will become umbilicus
What is the chorionic villi?
These are finger-like projections that form the fetal portion of the placenta
- Primary = only trophoblast cells (day 13)
- Secondary = core of mesoderm (day 16)
- Tertiary = contain blood vessels (3rd week)
How is the blastocyst cavity remodeled twice?
Primary and secondary yolk sac
What are the 2 cavities formed?
- Amnionic cavity - forms in the epiblast
2. Chorionic cavity - forms within the extraembryonic mesoderm
Where does the extraembryonic mesoderm split into different layers?
Between the embryoblast and trophoblast
- this forms the chorionic cavity and connecting stalk
