lec 14- early development in ascidians Flashcards
What are ascidians?
-they are invertebrates that belong to the same phylum as vertebrates, the chordata
if adult ascidians lack a backbone, why are they still considered chordata?
because their larvae are very similar to the tailbud stage of vertebrate embryo, they have a notochord, neutral tube, and muscles
why do they study development in ascidian embryos?
-because the ascidian larva is similar to vertebrates in terms of its development and morphological structures
-embryos are much similar
-they have a small genome
what is the ascidian life cycle?
-eggs and sperm are released into the seawater
-embryonic development occurs in sea water, free from mothers body
-the embryo develops into a tadpole
-the tadpole hatches into a larva
-after 20 days, larva undergoes metamorphosis into a juvenile, it sticks to the sea floor during metamorphosis and its neural tube and notochord degenerate
-the juvenile matures into an adult
what type of development are ascidian embryos an example of?
mosaic development
what are localized cytoplasmic factors that specify cell fates?
localized cytoplasmic factors that specify cell fates are regulators of development
how do ascidian embryos develop and what occurs during fertilization?
-ascidian eggs are polarized along AV axis
-maternal mRNAs are distributed in a gradient of increasing density along the axis
-at fertilization, sperm entry triggers a reorganization of the cytoplasmic components in the egg
-at this time different regions of the cytoplasm are destined to form particular cell types
what happens during the cleavages in the ascidian embryos?
-the first cleavage occurs parallel with the AV axis, it bisects the embryo into left and right halves
-the two cells are developmentally equivalent, if separated they can each give rise to a half-sized tadpole
-the second cleavage is in the same plane, at the right angles to the first
-the anterior posterior regions of the embryo are now delimited
-the third cleavage separated the animal and vegetal halves of the embryo
-the second and third cleavages segregate cytoplasmic determinants differently and the molecules that each cell inherits direct their development
what happens to the blastomeres if they are separated from each other at the 8-cell stage and cultured in isolation?
they adopt the fates that they would have normally acquired if they had been left in an intact embryo
what is one tissue that is specified by localized cytoplasmic factors?
muscle
what gives rise to muscle cells?
-myoplasm, redistribution of it to other cells causes muscle cells to develop
-there must be something in the myoplasm that gives rise to muscle cells
what is the maternally supplied mRNA called in myoplasm that encodes a transcription factor?
macho-1 mRNA
does macho-1 specify muscle formation?
yes, when there is no macho-1, there is a failure in producing muscles in embryos
what happens when extra macho-1 is added to embryos?
a lot of muscle is formed in areas where it does not normally form like the head and tail
if a zygote is depleted of macho-1 mRNA and then transplanted onto another part of the cell, does it form muscle cells?
no
are ascidian embryos examples of a purely mosaic type of development?
no, because the notochord is induced by signals from neighbouring cells
what is the notochord derived from and what is produced?
an asymmetric division of the A6.2 and A6.4 cells at the 32 cell stage which produce the notochord precursor and neutral precursor
If notochord precursor is isolated, does the notochord cell fate form?
no
if notochord is isolated with the addition of the signalling molecules Fibroblast Growth Factor (FGF), does the notochord fate form?
yes
is FGF pathway for the notochord cell fate?
yes
explain the FGF pathway:
-FGF binds to its receptors which causes the receptor to dimerize, activating the intracellular kinase domains and they then phosphorylate each other
-phosphorylated receptors recruit adaptor proteins that activate Ras as the plasma membrane, Ras then activates the ser/thre kinase
-The ser/Thre kinase phosphorylates MAPKK which can then phosphorylate MAPK
-phosphorylated MAPK enters nucleus where it phosphorylates and activates transcription factors
do FGF signals from endodermal tissue in combination with macho-1 specify notochord and mesenchyme?
yes
