14 Flashcards
What is the organizer in amphibians?
The dorsal lip of the blastopore
What happens to cells that gastrulate through the organizer?
They become the dorsal mesoderm (prechordal plate and notochord and portions of anterior endo in some spp), and secrete BMP inhibitors (noggin, chordin, follistatin) to induce dorsal structures in the ectoderm above, especially the nervous system
Which pole does fertilization occur at in the frog egg? Why?
Animal. Spatially restricted compatibility b/w sperm’s surface glycoproteins and vitelline envelope proteins.
How does cortical rotation occur in the frog egg? What is the result?
Cortical rotation occurs after fertilization. The centriole from the sperm organizes microtubules, causing the pigmented outer cytoplasm to rotate which causes an overlap between clear cortical cytoplasm and diffuse black “gray” pigment, creating a gray crescent where the organizer is located.
How long does the first cell division cycle take in frog eggs? Is this shorter, longer, or the same as future cleavage cycles? What happens to the gray crescent?
75 minutes. Longer. Split in two.
What type of cleavage does a frog egg experience?
Holoblastic mesocleithal, with faster divisions in the animal than vegetal pole due to denser yolk.
Name the five key movements experienced during Xenopus gastrulation.
Epiboly - Surface ectoderm moves in and fully encloses the embryo
Vegetal rotation - the vegetal pole rotates into the blastocoel (because of animal pole extension during epiboly, extending to side opposite sperm entry)
Invagination - bottle cells from the outside pinch inward
Involution and migration - endoderm and mesoderm migrate into the blastocoel (started by apical constriction of bottle cells)
Convergent extension - layers of cells combine at the midline and cause an axial extension
What is the archenteron? What does it become?
A cavity formed during the involution of cells at the blastoporal lip in amphibians. It will become the interior of the primitive gut.
What is the order of involution in Xenopus gastrulation?
- Pharyngeal endoderm
- prechordal plate (future head mesoderm)
- Chordal mesoderm (future notochord)
- Somitic mesoderm
Do the cells of the dorsal blastopore lip change during gastrulation?
Yes. Constantly
What happened to a Xenopus embryo when a blastomere gets split in a lab after the gray crescent was divided evenly between the halves? What about unevenly?
When divided evenly and then split, two normal embryos formed because each received part of the organizer.
When divided unevenly and then split, one normal embryo formed because it had all of the organizer and one belly piece formed because it lacked the organizer.
What was the result of the primary embryonic induction experiment? What did it show the importance of?
Part of the dorsal lip of the blastopore was transplanted to a different non-dorsal region of a different species blastomere. A secondary invagination formed at this transplant followed by the formation of induced secondary structures along with normal primary development, leading to a secondary axis. It shows the importance of the dorsal blastopore lip tissue in gastrulation and future development.
What is the basic paradigm for all organizer function in vertebrates?
BMP inhibitors produced by the organizer are necessary to form dorsal structures like the notochord and CNS.
How do BMP inhibitors contribute to the formation of the notochord and central nervous system? What are these inhibitors? Where are they produced?
They bind BMPs and prevent them from binding their own receptors, and thus formation is restricted dorsally. These inhibitors are noggin, chordin, and follistatin and they are produced by the organizer.
What are the signals that induce the formation of the organizer in a specific location?
It occurs where mesoderm inducing signals from the vegeral pole intersects with dorsal mesoderm inducing signals from the Nieuwkoop centre. The organizer forms just above the Nieuwkoop centre, and nuclear B-catenin is stabilized in both of these sites.
