lec 22- patterning the nervous system Flashcards
do germ layers move into the positions they would occupy in the organism during gastrulation?
yes
what is ectoderm patterned into?
nervous system
what is the first tissue to be internalized during gastrulation?
the mesoderm
what does the mesoderm form into in Xenopus?
the prechordal plate
what does the prechordal plate do?
gives rise to the mesoderm on the ventral side of the head
what is next to the prechordal plate?
the notochord which runs along the dorsal midline of the embryo
what is the notochord?
a transient structure its cells become incorporated into the vertebrae and spinal column
what is on top of the notochord and what does it do?
the neural tube, it begins neurulation while gastrulation is still occuring
what does she mesoderm on each side of the notochord give rise to?
somites
what happens during neurulation in the Xenopus embryo?
-the ectoderm overlying the notochord folds upwards
-cells called neural crest cells are specified at the sides of the neural plate
-as the neural plate folds form, the neural crest cells are brought together to form the dorsal portion of the neural tube
-the neural tube differentiates into the central peripheral nervous system
-it also gives rise to some non-neural tissues, mainly the face and jaws
-the neural crest cells are released from the neural tube and migrate throughout the body to give rise to the peripheral nervous system
what happens when neurulation is complete?
-the germ layers are all in place
-the neural folds have completely closed to form the neural tube, lying over the notochord
-patterning along the AP axis has already begun
what happens to the three set of internalized cells during the early gastrula?
-the first cells give rise to anterior endoderm of the neurula stage embryo
-the second cells give rise to the prechordal plate mesoderm which lies just anterior to the notochord, which forms the ventral region
-the final cells forms the notochord
what must the organizer be to give rise to different structures at different points during development?
-it must be a complex signaling center endowed with the capacity of changing its inductive capacities and capable of expressing different genes at different times
what happens to the cells that pass through the organizer first and a little later during early gastrulation?
-first cells in the Spemann-Mangold Organizer become the prechordal plate and express different genes than the later cells which develop into the notochord
what is the nervous systems development coordinated with?
other body structures particularly the skeleton and musculature, which are mesoderm derivatives
what accomplishes the nervous system to be coordinated with the mesoderm derivatives?
the organizer
when the organizer is transplanted to the ventral side of an embryo near the beginning of gastrulation, what occurs?
the organizer induces a twinned embryo with an extra head when it is transplanted
when the organizer is transplanted to the ventral side of an embryo at the late gastrula stage, what happens?
the organizer induces the formation of an additional tail
what are the results of the two organizer experiments?
the inductive properties of the organizer change with time
what does ectoderm near the organizer and ectoderm on the opposite side give rise to?
ectoderm near organizer: neural plate
ectoderm on opposite side: epidermis
what happens to the epidermal ectoderm cells when transplanted in a place where neural plate ectoderm forms? what does this mean?
-the transplanted cells take on the fate of the new position, they become neural plates
-this means that the transplanted tissue is not determined at the time of transplantation, it must be induced during gastrulation
how does neural plate induction occur?
-in xenopus default dorsal ectoderm turns into neural tissue
-development into neural tissue is blocked by BMP signaling, BMP signals are expressed throughout the early blastula (they block neural fate and promote epidermal fate)
-the organizer produces BMP antagonists to get rid of block
-BMP antagonists inhibit BMP signalling in cells that give rise to all 3 germ layers
-the region of ectoderm that comes under the infleuence of the organizer (and BMP antagonists) develops as neural ectoderm
-as gastrulation proceeds, internalized cells continue to secrete the antagonists and they act on the ectoderm that lies above them
is BMP antagonists alone enough to induce neural tissue?
no, earlier FGF signaling from cells in the blastula is also needed
what is the evidence behind BMP signalling inhibiting neural tissue formation?
- BMPs are expressed throughout the early blastula embryo and their expression is subsequently lost in the neural plate, correlation between loss of BMP and neural plate development
- when BMP expression is removed, embryo is covered in neural tissues, BMP must be inhibiting neural tissue
- in the early gastrula, BMP antagonists are secreted from the organizer
- elimination of any one of the antagonists has a small affect on neural induction, but when a huge amount is depleted, neural tissue fails to form
