Development of bilaminar germ disc Flashcards
What 2 distinct lineages do embryoblasts split into to form the bilaminar germ disc?
Epiblasts, Hypoblasts
What type of cells are in the epiblast/primitive ectoderm?
Lineage of pseudostratified columnar epithelial cells
What does the epiblast/primitive ectoderm develop into?
Embryo itself and some extraembryonic mesoderm
What type of cells are in the hypoblast/primitive endoderm?
Lineage of cuboidal epithelial cells that are extraembryonic
What does the hypoblast/primitive endoderm develop into?
Extraembryonic cells structures eg. umbilical cord, placenta
What are extraembryonic cells?
Cells that give rise to extraembryonic structures that support embryonic development and aren’t retained after birth
Are epiblasts located above or below hypoblasts in bilaminar germ disc formation?
Epiblasts above hypoblasts
How do the hypoblasts organise themselves when forming the bilaminar germ disc?
Monolayer
How do epiblasts organise themselves when forming the bilaminar germ disc?
Pseudostratified columnar epithelium structure
What structure separates epiblasts and hypoblasts in the bilaminar germ disc?
Basement membrane
What is the completed structure of the bilaminar germ disc?
Epiblasts in pseudostratified columnar epithelium structure separated from monolayer of hypoblasts below by basement membrane
How does the organisation of epiblasts change to form the primordial amniotic cavity?
Epiblasts stretch and cavitate to form internal empty space, which is the primordial amniotic cavity
How does amniotic epithelium complete the amniotic membrane/amnion?
Roof of primordial amniotic cavity opens, so amniotic epithelium forms solid roof to complete amnion
What is the name of the cavity formed inside the amnion?
Amniotic cavity
What is amniotic epithelium derived from?
Amnioblasts, which are epiblasts located adjacent to the amniotic cavity
After the amnion has been formed, what structure is developed next?
Primary yolk sac
What does the hypoblast do to form the primary yolk sac?
Send migratory endodermal cells
What is the name of the membrane that surrounds the primary yolk sac?
Heuser’s membrane
Where does the extraembryonic/chorionic cavity develop?
Extraembryonic mesoderm
How does the extraembryonic mesoderm form the extraembryonic/chorionic cavity?
Splits into 2 layers so that inner layer coats Heuser’s membrane of primary yolk sac and outer layer coats cytotrophoblasts to leave chorionic cavity in between
What is the function of the chorionic cavity?
Separates amnion and primary yolk sac from blastocyst wall
How does the hypoblasts initiate change from primary to secondary yolk sac?
Send out more migratory endodermal cells
How do the migratory endodermal cells change the primary yolk sac into the secondary yolk sac?
Migrate through extraembryonic mesoderm and pushes primary yolk sac in front of them until it reaches other side of chorionic cavity
Endodermal cells then become membrane of secondary yolk sac
What tissue forms the connective stalk/primitive umbilical cord?
Extraembryonic mesoderm
What system develops after formation of the connective stalk?
Uteroplacental circulation
What is uteroplacental circulation?
System that allows maternal and foetal blood to flow close together and exchange gases and metabolites
Why is uteroplacental circulation important in the developing embryo?
Simple diffusion now insufficient for embryo nutrition needs and waste excretion
To develop uteroplacental circulation, what 2 structures anastomose?
Trophoblastic lacunae, maternal sinusoids
What are trophoblastic lacunae and where are they found?
Vacuoles found in syncytiotrophoblast
What are maternal sinusoids and where are they found?
Expanded maternal capillaries in syncytiotrophoblast
What are primary chorionic stem villi, and how do they facilitate uteroplacental circulation?
Protrusions that grow from cytotrophoblast through syncytiotrophoblast and inject into blood-filled trophoblastic lacunae
What is the appearance of primary chorionic stem villi?
Pointy extension of cytotrophoblast covered by syncytiotrophoblast layer
