Lecture 8: The Pharyngeal arches Flashcards
neural induction
neural and surface ectoderm become different
neurulation
process that delivers the neural tube when neural plat rolls up into a neural groove then dorsal lateral hinge points appear and at junction to the surface ectoderm you have the neural folds = neural tube
neural crest cell emigration
msynchymal cells under surface ectoderm - crawl over neural tube then move away from it
= neural crest cells
come from crest of the fold at the border between the surface ectoderm and neural tube
placodes
ectodermal thickenings that give rise to sense organs and nerves belonging to the PNS
otocyst - cup - pit - ectodermal placode
pharyngeal branchial gill arches
ventral side of embryo confined to head segmented 5-6 weeks human embryos caudal to the mouth jawwed= 7 jawless - 9 hagfish - 15
swellings around lateral and ventral side of pharynx
first arch - maxillary
second - mandibular
structure
endoderm - pouch
endoderm - cleft/groove invagulates and meets endoderm
aortic arch endothellum and mesynchyme filling space between those tissues
parts of pharyngeal arches
ECTODERM
inner ear vesicle lines up with second pharyngeal arch
distinct thickenings in ectoderm = epibranchial placodes = deliver sensory of facial 7 glossopharyngeal 8 and vagal nerves 9
ENDODERM
arch mesenchyme surrounds blood vessel fills space between endoderm and ectoderm is not homogenous
MyoD expressed in the core of pharyngeal arches and close to the aortic arch blood vessel
which cells contribute to the arch mesenchyme and what do the cells give rise to
do fate mapping experiments
see what the neural crest cells do= deliver pigments cells to skin nodes of entry in the sympathetic and parasympathetic
neural crest cells deliver the arches connective tissue and the skeleton inc. the jaws
head mesoderm
makes head skeletal muscle
neural crest cells deliver connective tissue n tendons for the muscle and pharyngeal arch cartilage and bone
what are the cones the arch neural crest give rise to
midbrain + forebrain neural crest migrate rostrally = frontal nasal facial bones
caudal midbrain to caudal rhombomeres of hindbrain there are streams of neural crest cells that migrate laterally into the pharyngeal arches
1st archL macels cartilage - mandible. malleus
2ndstapes, hyoid
3rd
layout
surface ectoderm - delivers placodes - sensory nerves
pharygneal endoderm- pharyngeal glands
neural crest cells = skeleton cartilage
mesoderm delivers blood vessels and craniofacial skeletal muscle
aortic arch
stage vertebrates look most similar
= phylotypic stage
are they homologous
develop in same way
give rise to same structures
only in vertebrates
how far down the chorsate tree can we go and find pharyngeal arches with a neural crest derived skeleton
neural crest cells were there to give pigment to the skin only in vertebrates do they build skeleton
not present in non vertebrates
ecto-mesenchymal crest = derivatives on neural crest cells: gilia cells pigments cells connective tissue fibroblasts cartilage ost - bones odontoblasts - make dentine of our teeth
jawed vertebrates
skeletal elements in the arches are specialised
1st arch elements form the jaws
sensorimotor nerves epibranchial ganglia skeletal muscle skeletal elements gill slits
the jaw is mobailsed by head mesoderm skeletal muscle
each arch has its placodes
sensor neurons stimuate muscle contraction
how is the visceral skeleton pharyngeal arch skeleton made from ecto-mesechymal neural crest cells
how do cells know they should be skeletogenic neural crest cells
AP2 alpha neural crest marker markers cells as they emigrate
Sox10 marks newly formed
Foxd3 = marks neurogenic neural crest cells
Sox10 and Foxd3 marks are turned off when the cells enter the pharyngeal archs
therefore migrating neural crest cells express AP2 alpha all the time
immature express sox
neurogenic express dlx2
as neural crest cells enter the pharyngeal arch
fgf8 expressed in the pouches trigger ectomesynchymal fate and should start to express dlx2
when neural crest cells are made insenstive to energy they are harbouring the dominant negative
they migratepst the fgf pouches wihtout giving up their neurogenic program they do no become ecto mesynchymal but they keep on being naive bias towards being neurogenic
the normal wild type cells are able to active fgf cascade when passing fgf pouches they become specified
what tells cells how far dorsal or ventral they are to make articulated bony elements reaching round the pharynx
dorsal cells will only have expression dlx 1 and 2
cv
entrals will also have dlx 5 and 6
overlapping expression of dlx 1 and 2 is nnecessaryfor the sspecificationof the lateral
tells 1st arch from the other archs to make jaws - rostrocaudal positional info
Hoxa2 genes
the further away from the 3 prime end the 7th pharygeal arch
the later the more caudal expression of the gene
the first arch doesnt express the hox gene
the rostral region of the brain and first arch express the transcription factor Dlx2
hypothesis :
overlapping hox gene expression specifies the caudal an posterior arches and absense of hox genes specifies the first arch
hox genes specify the posterior arches
absence of hox gene expression specifies the first arch
test: knock out hox genes, expect anterior transformation misexpress Hox, expect posterior transformation
in mice
hox genes as hox a2 are not expressed in the first arch = mandibular arch
loss of hoxa2 leads to
1. duplication of first arch structres
partial loss of 2nd arch sturctures
conc hox proteins make the posterior arches different from the first
overlapping hox gene expression specifies he second and the more posterior
Otx2 transcription factor gives rise to the maxillary and mandiular
neurocranium the brain case
crown grathosome
compound structure
made up of:
1. from ecto- mesynchymal cells = fronto-nasal neural crest cells - facial bones
- head mesoderm
- occupital somites / modified vertebrae
skull development
primary palate
maxillary competent of first pharyngeal arch and the frontal nasal
maxillary fuses w frontonasal elements of neurocranium to form upper part of face
failure of fusion between maxillary and medial nasal swelling lateral + medial nasal swelling = cleft lip
only in mammals a secondary palate will form
secondary palate = part of skull vault that separates the oral and nasal cavity
swelling on either size of the maxillary - grow out n fuse = palatal shelves
failure of the fusion of the shelves = cleft palate
fusion of skull plates
areas between bony plates
fgf signalling promote osteogenic differentiation
immature ostoblaston the outer edge of the mineralised osteoid express fgf1
bone maturation proliferation recruitment diferentiation apoptosis
signalling through receptor 2 and 3 keeps cells mitotically active and undifferentiated keeping suture open
eventually fgf levels rise above a threshold = all the cells complete differentiation = sutures fuse = skull bone differentiation
mesoderm an neural crest cells derived portions of the skull vault do not fuse for a prolonged period of time to accomodate for brain growth
gain of function mutation
fgf related craniosynostosis
premature fusion of sutures = skull defects craniosynostosis
mutations cause fgfr recptior to initiate the signalling system without fgf ligand being present or overactivate the fgf pathway = more than normal fgf signalling
how does notch delta signalling create a negative feedback loop
what molecular oscillations creates the on off state of notch delta signalling
what in the segmentation of the paraxial mesoderm in the trunk creates the molecular clock
how do you create this on off state of notch delta signalling
when lunatic fringe is high it suppresses notch because it inactivates notch on the cell membrane and even if not were to interact with delta
notch will not be in the position to become processed and enter the nucleus there is nothing to activate lunatic levels goes down
as lunatic levels goes down the suppression of notch is relieved - notch when it touched delta it can engage in this cascade where it will activate lunatic and bring about repression again
