Laying Down A Body Plan Flashcards
(26 cards)
Hypoblast
The cells of the inner cell mass that are facing the free/fluid space
Lining of the yolk sac
Second layer under the trophectoderm made by hypoblasts invading
Epiblast cells
Inner mass cells which do not directly contact the hypoblast cells
Amniotic cavity
Cavity created by epiblast
Yolk sac
Cavity created under the hypoblast cells
Dizygotic twins
Two oocytes released and both fertilised by different sperm
Genetically dissimilar
Monozygotic twins
Both from same zygote -> one oocyte, one sperm
Made 2 bodies instead of one each of which developed into an individual
Genetically identical
Cells separate in zona pellucida at two cell stage
Cells dont stick together properly at two cell stage -> both think they are individual cells so divide separately and make own set of inner cell mass and trophectoderm cells
Result is two placentas each containing independent embryos which develop in the same uterus
Two inner masses occur in the same blastocyst
Most common
Share trophectoderm layer so therefore the same placenta - have distinct yolk sacs and amniotic cavities
Risk of foetal transfusion syndrome
Foetal transfusion syndrome
One foetus uses the others nutrients due to how the blood vessels connect to the maternal supply via the placenta
Results in one foetus being smaller than the other
Affects metabolism - hoard fat and affect kidneys and the vascular system, to optimise the foetus for low food after birth but can result in obesity and cardiovascular problems
Two primitive streaks form in epiblast
Two embryos which share common amniotic cavity and chorionic cavities and placentas
Larger problem as no barriers (definitive membranes) to differentiate embryos -> resulting in conjoined twins
Partial axis duplication
Two head organising areas on primitive streak to one tail site
Very rare type of monozygotic twinning which results in one body forming two heads
Usually lethal pre or peri-natally
Hex protein
Switched on in the centre of hypoblast cells
Causes cells cling to each other and move to the edge of the embryo
Cells then secrete proteins up to the epiblast layer which inhibit the epiblast cells
At a point far away the epiblast cells escape inhibition and form the primitive streak
Primitive streak
Forms the trunk of the body
Hex cell area
Forms head
Ectoderm
Forms the outer skin
Mesoderm
Middle layer - forms muscle, bone, etc
Ectoderm
Inner skin - forms the gut lining
Gastrulation
How the bilayer flat disc blastocyst forms a 3 layered structure
First step of gastrulation
Epiblast cells converge and drop through layer via the primitive streak towards the hypoblast allowing more cells to follow
The first epiblast cells
Attach to hypoblast cells and start to push them aside
-> forming the endoderm
The second epiblast cells to fall through
Do not attach to hypoblast -> they spread out in the free layer between epiblast and hypoblast
—> form the mesoderm
Some epiblast cells
Never drop through
—> form the ectoderm
Notochord
Rod of cells that run down trunk of embryo
