Blastocyst Flashcards
how do cells from inner cell mass differentiate?
cells facing free surface become hypoblasts, these stream down the edge of the trophectodome and form the yolk sac
what do cells touching the hypoblast differentiate into?
epiblasts - these separate from the rest of the cells and an amniotic cavity forms be tween the epiblast and the rest of the cells
when can monozygotic twinning occur?
early on during development
what are the 3 forms of monozygotic twinning?
inner cell mass separates and both masses go on to form their own blastocysts, placenta etc (early development)
inner cell mass splits after trophective layer has already formed, each have their own amniotic sac but share a placenta, can lead to one twin taking more of the blood supply (slightly later development)
two primitive streaks form, - these embryos share everything, including amniotic sac, there is nothing to separate the two embryos, can lead to conjoined twins
what do cells in the middle of the hypoblast do?
differentiate and migrate towards the edge, when they reach the edge they go on to secrete hormones which produce the tail end of the body
what is the primitive streak?
structure which extends from the edge (where the tail end begins) into the centre and dictates where the future head and trunk will be
what is gastrulation?
transformation of the epiblast from a flat disc to a 3-layered structure
what forms during gastrulation?
endoderm
mesoderm
ectoderm
how does the endoderm form?
many epiblast cells converge on the streak and push aside the hypoblast, they drop down into the amniotic cavity, the hypoblast continues to be pushed away until it reaches the edge and the endoderm is formed
how does the mesoderm form?
after endoderm forms more cells drop down into the amniotic cavity, these cells spread out and form the mesoderm
how does the ectoderm form?
the remainder of cells in the epiblast become the ectoderm (cells that have not dropped into the amniotic cavity)
what happens once the endoderm forms?
middle part of endoderm rises to make notochord plate and detaches do become the notochord
the middle part of the endoderm then drops down even further and is pulled into a tube
where is the CNS derived from?
the ectoderm
how does the CNS form
CNS formation begins when the ectoderm folds inwards and makes a valley (this inward push is due to cell proliferation) this forms a sealed ectoderm and the neural tube forms underneath the ectoderm
what happens if the sealing of the ectoderm fails?
spina bifida - spinal cord is exposed, vertebrae do not form fully
anencephaly - this is when the spinal cord is exposed at the head, this stops brain growth and is fatal
folic acid is required to prevent this
