Gastrulation Flashcards
Describe the blastocyst/blastula stage of development.
Embryos undergo a series of cleavage divisions to divide the cell into smaller blastomeres arranged around a fluid filled capsule called the blastocoel.
Define gastrulation.
A coordinated series of cell and tissue movements tat result in reorganisation of the blastula. Literally ‘gut formation’ but it also includes the mesoderm.
Define invagination.
The infolding of the epithelium of the blastula at the vegetal pole to form an internal tube called the archenteron (like sticking your finger into a balloon until you get to the other side). Sea urchins
Define involution
Similar and forms an archenteron from the blastopore to an animal pore. However, involting cells fold back to the inner surface and use this to migrate, namely the mesenchyme cells. Amphibians.
Define ingression
Migration of cells from the surface epithelium to the interior of the embryo. They break away from the epithelium, push through the basal lamina and migrate away. Cells ingress to primitive streak. Chicks and mice.
Name the three types of gastrulation movement.
Invagination, involution and ingression.
Summarise in a sentence gastrulation in sea urchins
Small rapidly developing transparent isolethical eggs that undergo holoblasic cleavage. Good for studying small gastrulation movements.
What does the blastula stage of sea urchins look like?
The blastula is surrounded by a monolayer of cuboidal cells. The blastula contains protein fibres required for cell migration. The basal (inside) surface of the cells is attached to the basal lamina (basement membrane) and the ciliated apical surface is attached to an outer hyaline membrane.
When does gastrulation in sea urchins begin?
When the cells at the vegetal pole elongate and form the vegetal plate. Bottle cells are formed which ingress into the blastocoel to form precursors of the mesoderm (primary mesenchyme cells). Requires changes in adhesion.
What is the 2nd step in sea urchin gastrulation?
Inward buckling of monolayer at vegetal plate. Site of invagination = blastopore.
What is convergent extension?
Directed intercalculation. Mechanism required for elongation of invaginating monolayer. Cells elongate perpendicular to the axis and form lamellipodia rich in actin + myosin. Used to exert traction on each other to narrow the invagination tube and its elongation towards the animal pole. Forms archenteron which will eventually form teh gut tube.
What else is needed to help convergent extension?
Secondary mesenchyme cells (SMCs) to form the invaginating tup of the archenteron and produce filopodia which search for the basal surface of the animal pole in order to pull it towards it. So the archenteron fuses with the ventral surface to form the mouth.
What happens in mammals at the 16-32 cell stage?
Cells increase adhesion.
Form trophoblast and inner cell mass.
Trophoblast pumps Na+ into the centre followed by water to form the blastocoel.
Embryo now called a blastocyst.
What does the trophoblast layer become?
The chorionic layers of the placenta
What does the inner cell mass become? (ICM)
The tissues, amniotic membrane, yolk sac, epithelium ad vasculature of placenta and umbilical cord.
What is GFP?
Green flourescent protein from jellyfish often used in transgenic mice
How is Heuser’s membrane formed?
From the cells of the hypoblast spreading out around the trophoblast lining the blastocoel forming the extraembryonic endoderm Heuser’s membrane. (Enclosed cavity now called yolk sac)
What happens to the yolk sac at birth?
It is lost but about 2% of the population will keep a remnant called Meckel’s diverticulum (a small bulge in the small intestine)
What does the epiblast form?
Amniotic membrane (layer of flattened epithelial cells lining the amnion cavity). Cavity surrounds embryo at later stages to act as protection.
Why is implantation in mammals crucial?
Don’t have enough nutrition to sustain them so the blastocyst burrows into the moher’s uterine wall.
Where does implantation occur?
In the uterus as the egg is held within the zona pelluicda. Ectopic pregnancies occur in the fallopian tubes.
How does implantation occur?
Trophoblast rapidly proliferates
Cells fuse to form syncytiotrophoblast tissue which invades the uterine wall by secreting digestive enzymes which break down the exrtacelluar matrix.
Trophoblast cells at blastocoel and in contact with ICM stay and form the cytotrophoblast.
What are lacunae?
Cavities which form in the syncytiotrophoblast and fill with maternal blood from the capillaries ruptured by the invasion. This is the beginning of the maternal-placental circulation.
