Neural Induction Flashcards
what is signal induction
signal originating in one tissue, causing another responsive tissue to differentiate into something else
from neural precursor to neuron
- competent neural precursor
ability to become neon if exposed to appropriate signals - specified neural precursor
has received signals to become neural precursor. still responds to signals that repress neural fate - committed neural precursor
received signals to become neural precursor. becomes neuron in presence of signals that repress neural character - differentiation into mature neuron
embryonic developmental stages of Xenopus embryo
egg –> blastula –> gastrula –> neurula
determining embryonic polarity
animal pole is pigmented, vegetal hemisphere is nit
grey crescent forms, where the pigmented half slides relative to egg opposite to point of sperm entry. first cell division always divides grey crescent in half
so have dorsal and ventral, and a anterior and posterior
what is the blastula
hollow ball of rapidly dividing cells
fate map
defined positions of where everything is and where all cell divisions happen
you know exactly all cells from certain position are going to be for certain fate
what happens during gastrulation
dorsal lip of the blastopore forms. this is the driver and first sign of invagination
cells migrate towards the inside of the mrbyo through a hole.
result: three apposed germ layers
outer: ectoderm
middle: mesoderm
inner: endoderm
fates of the three germ layers
ectoderm: CNS, PNS, skin, hair
mesoderm: muscle, blood, bone, kidney, gonads
endoderm: epithelial lining GI, resp tract,
axial mesoderm
lies along central axis under neural tube
gives rise to notochord
paraxial mesoderm
flanks neural tube
forms at same time as neural tube
gives tissue to somites
somites
blocks of tissue running along both sides of neural tube
gives rise to muscles and tissues of back
stages of neurulation
- neuroectoderm differentiates from ectoderm and thickens to become neural plate
- neural plate folds dorsally. two edges join with neural plate borders eventually touching and fusing. border cells become roof plate & neural crest
- closure of neural tube disconnects epidermis, neural crest, neural tube.
- notochord degenerates, persisting in the intervertebral discs.
what happens to neural crest cells
they migrate laterally to form DRG
spemann and mangold: dorsal lip transplantation 1924
took dorsal lip from donor embryo and transplanted it to a ventral site of host embryo
resulted in 2 neural tubes giving rise to 2 nervous systems w/o ventral tissue
conclusion: dorsal lip is the Spemmann organiser. it produces some sort of factor that induces formation of spatially distinct tissues
how does inducing signal move through tissues
- vertical induction
- planar induction
evidence for vertical induction
put in high salt water, cells migrate to outside instead. allowed to develop, couldn’t see any nervous tissue. conclusion –> planar induction not sufficient to induce nervous system
mesoderm transplant: neurula anterior mesoderm grafter into early gastrula induces a head. posterior mesoderm induces trunk and tail.
conclusion –> vertical induction required. mesoderm contains regionalised inducers
evidence for planar induction
homeobox genes
exogastrula embryos labelled with neural markers
keller sandwhiches: two bits tissue sandwiched together but are identical to only planar possible
is planar signalling sufficient
no.
no floor plate in developing spinal cord, so there are dorso-ventral patterning defects
can’t induce most anterior part CNS
both vertical and planar are required
neural default model
default state for ectoderm is neural tissue, not epidermis
therefore, the absence of an intracellular signal is required for neural differentiation
neural default model: spemann organiser
it spews out antagonists of epidermal inducer, to stop it interacting with receptor and inducing epidermal tissue
activin
induces mesoderm
when binds its receptor causes it to dimerise. Activates cytoplasmic serine-threonine kinase signalling. causing secondary mesoderm signal to convert ectoderm to neuronal fate
blocking activin signalling results in neural tissue formation without secondary mesoderm signalling. therefore, activin inhibit leads to formation of neural tissue
