Second Week of Development Flashcards
When does the blastocyst implant
6-10 days after fertilization
What does the “Week of twos” refer to
- Trophoblast differentiates into the syncitiotrophoblast and cytotrotrophoblast
- Embryoblast differentiates into the epiblast and hypoblast
- The bilaminar disc forms
Epiblast
derived from the embryoblast, associates with the amniotic cavity. Amnions are derived from the epiblast
Hypoblast
Derived from the embryoblast, adjacent to exocoelomic cavity
Amniotic Cavity Formation
As implantation of the blastocyst progresses, a small space appears in the embryonic cavity
Embryonic disc formation
Morphologic changed occur in the embryoblast that result in a flat, almost circular, bilaminar plate of cells
Amniotic cavity
new cavity that appears between the nner cell mass and the cytotrophoblast
What does the umbilical vesicle form from
blastocyst cavity
what is the end of the second week characterized by
appearance of the primary chorionic villi
What does the extraembryonic coelom split into
extraembryonic somatic mesoderm and extraembryonic splanchnic mesoderm
Extraembryonic somatic mesoderm
Derived from the extraembryonic coelom, lines the trophoblast and covers the amnion
Extraembryonic Splanchnic Mesoderm
Derived from the extraembryonic coelom, surrounds the umbilical vesicle (Yolk sac)
Chorion
Formed from the extraembryonic somatic mesoderm and the 2 layers of the trophoblast. Forms the wall of the chorionic sac
Chorionic sac
Contains the embryo and its amniotic sac and umbilical vesicle, which are surrounded by the connecting stalk
Chorionic Cavity
Extraembryonic cavity once the chorion has formed
Extraembryonic Mesoderm
Develops between the trophoblast and the amnion and umbilical vesicle. Spaces appear within the mesoderm which forms the extraembryonic coelom which splits in two
Prechordal plate
Localized thickening of the hypoblast, indicates the future cranial region of the embryo and the future site of the mouth
hCG
Human chorionic gonadotropn, secreted by the blastcyst to stimulate the corpus luteum to produce estrogen and progesterone
When does the placenta take over hormone production
Week 20
Implantation of Blastocyst
Occurs in the endometrium of the uterus, can be detected by ultrasonography and highly sensitive radioimmunoassays of hCG as early as the second week
Placenta previa
When implantation occurs in the lower segment of the uterus
Ectopic pregnancies
When implantaton doesn’t occur in the uterus
Summary of implantation
- Day 5: Zona pellucida disappears due to blastocyst enlargement and enzymatic lytic degradation
- Day 6: Blastocyst adheres to the endometrial epithelium
- Day 7: Trophoblast differentiates
- Day 8: Syncitiotrophoblast erodes endometrial tissues, blastocyst embeds in the epithelium
- Day 9: Blood-filled lacunae appear in the syncitiotrophoblast
- Day 10: Blastocyst sinks below epithelium, defect is filled by a closing plug
- Days 10/11: Lacunar networks form by fusion of adjacent lacunae
- Days 11/12: Syncitiotrophoblast erodes blood vessels, allowing maternal blood to seep in/out of lacunar networks, establishing uteroplacental circulation
- Days 12/13: Defect in epithelium is repaired
- Days 13/14: Primary chorionic villi develop
Uteroplacental Circulation
Established by the erosion of blood vessels by the syncitiotrophoblast, allowing maternal blood to seep in/put of lacunar networks
Summary of the second week
- Rapid proliferation and differentiation of the trophoblast
- Primary umbilical vesical forms, extraembryonic mesoderm develops, extraembryonic coelom forms from spaces that develop in the mesoderm, coelom becomes the chorionic cavity
- Primary umbilical vesicle disappears are the secondary vesicle appears
- Amniotic cavity appears as a space between the cytotrophoblast and the embryoblast
- Embryoblast differentiates into the bilaminar embryonic disc: epiblast and hypoblast
- Prochordal plate develops
