lec 13- early development of sea urchin embryos Flashcards
why are sea urchins good model systems?
-gametes released in water, s easy to get
-transparent, so easy to see development
what are the four steps in sea urchin life cycle?
-fertilized egg and early embryo develop inside the fertilization envelope
-the blastula hatches from the fertilization envelope and undergoes gastrulation
-after gastrulation, the embryo hatches into bilaterally symmetrical larvae called pluteus
-the pluteus larva undergoes metamorphosis into an adult
what did Han Driesh conclude when he worked with the two celled sea urchin embryo and one of the cells were removed, resulting in one to develop into a complete larva?
he concluded that the cell fate was determined by cells detecting their surroundings in developing embryo
what is radial cleavage pattern and what does it produce?
a pattern of cleavages in which cell divisions occur at right angles to the previous divisions, it produces tiers of blastomeres that sit on top of one another
what happens to the embryo during the first, second, third, fourth, and further cleavages?
-asymmetrical along the animal-vegetal axis are present in the unfertilized egg
-first two cleavages divide the embryo along AV axis, third is equatorial and divides the embryo into animal and vegetal halves
-fourth cleavage causes the cells to at the animal pole to divide symmetrically and the vegetal pole to divide asymmetrically which creates 4 large macromeres and 4 micromeres
-further cleavages produce a hollow blastula
what are the two axes that form in the embryo during gastrulation?
-animal-vegetal axis
-oral-aboral axis
what are the four steps to gastrulation?
-primary mesenchyme (mesoderm) cells migrate inward
-the mesenchymal cells migrate along the inner wall of the embryo and the gut invaginates
-the mouth invaginates from the opposite side and the two fuse, the two invagination sites define the oral/aboral axis, this is the main axis of symmetry of the larvae
-the mesenchyme lays down skeletal rods that grow to extend the four arms of the larvae
when is oral-aboral axis established?
later in development during gastrulation
what do the cells in the blastula give rise to?
-the cells above the vegetal plate give rise to ectoderm
-the vegetal plate gives rise to ectoderm, endoderm, and some secondary mesenchyme
-the micromeres give rise to mesenchyme
what is already present along the AV axis in the unfertilized egg?
asymmetries
when patterning the AV axis, The first two cleavages are parallel with the AV axis, all 4 blastomeres inherit what?
prospective ectoderm, endoderm, and mesoderm
what does the 3rd cleavage do when patterning the AV axis?
bisects the embryo into animal and vegetal halves
what does the 4th cleavage do when patterning the AV axis?
its asymmetric, it produces the micromeres
what do the micromeres give rise to and how are they specified?
they give rise to primary mesenchyme, they are specified by cytoplasmic factors that are localized at the vegetal pole of the egg
when is the fate of micromeres determined?
at the moment they are formed, even when grafted they only form primary mesenchyme
what do the 4 cell stage cells turn into when separated?
normal larvae, normal for regulative development
what do the 8 cell stage cells turn into when separated into animal and vegetal halves?
-animal half turns into a hollow sphere of ciliated ectoderm, characteristic of animal half
-vegetal half develops structures that are normally formed by the cells on the vegetal side
if the animal half of a 32 cell embryo is removed and cultured on its own, what does it form?
ectoderm, this is what normally forms in an intact embryo (the cells are specified)
what is produced when micromeres from a 16 cell embryo are combined with the animal half of a 32 cell embryo?
a normal larvae is produced
if the micromeres are transplanted onto the side of the 32 cell embryo,, what forms?
another gut forms at the site where the micromeres were planted
what do the micromeres act as?
inducers and the cells from the animal half of the embryo are capable of responding
what does lithium chloride do in the Wnt signalling pathway?
-it inhibits B-catenin degredation, preventing its degredation when no Wnt is present
-meaning B-catenin will work when Wnt is present and not present
what happens to an embryo if B-catenin is prevented from entering the nucleus, LiCl treated, and an untreated normal embryo?
-endoderm and mesoderm fail to form, the entire embryo is ectoderm
-embryo vegetalized, B-catenin accumulates in nuclei of all the cells
-normal larva, B-catenin accumulates only in nuclei of micromeres and not in other cell types
how does Wnt/B-catening signalling from the vegetal-most cells diffuse, and what does it specify?
they diffuse throughout the cell and specify endoderm and mesoderm
what are the most distant tiers of cells at the animal pole specified as?
neuroectoderm
