Lecture 7: Somites Flashcards
what is the tissue organisation of the germ layers in triploblastic animals
cells from the epiblast/primitive ectoderm move towards a blastopore/primitive streak = generates endoderm first which replaces the extrabonic hypoblast
cells leaving epiblast after settle between the endoderm and ectoderm = mesoderm
cells that don’t become internalised settle on the outside and become the definitive ectoderm
epithelium - simple/non-layered/stratified mono-layered
endothelia - cuboidal/long with diff thickness
transitional - changes from one layer to the next
polarised - basal side anchored on extracellular matrix basement membrane - cells linked with demosomes
cilia on opposite side
mesynchime - loosely held together cells
epithelial and mesenchyme cells can occur in any of the 3 germ layers
what will become of the ectoderm endoderm
ectoderm
neural tube w forebrain, midbrain, hindbrain + spinal cord
neural crest cells
epidermis
endoderm
alimentary gut
canal
air sacs/lungs
how does the axial mesoderm signal to the surrounding tissues
produces Shh and noggin
the dorsal neural tube and surface ectoderm release Wnt signals, the lateral mesoderm produces BMP signals
how does trunk paraxial mesoderm segment into somites
metameric segmented organisation
somites form caudal to inner ear
segmentation is visible in adult in vertebral column
human spondylocostal dysostosis = malformed vertebral column
initially the paraxial mesoderm is laid down as a mesenchyme on either side of notochord
paraxial mesoderm forms epithelial balls of cells
somite formation progresses in rotro to caudal or anterior to posterior direction
somites appear at regular intervals of 90 mins in chicken or 2hrs in mice 30 mins in zebrafish
regular segmentation is regulated by a negative feedback loop = molecular clock oscillator and a signal to tell cells to go ahead and organise themselves int a discrete unit or a wave front
auto regulatory loop of the notch delta signalling between cells that touch
Fibroblast growth factor Fgf8 levels fall below a threshold = signal
high fgf = immature state = inhibits segmental boundary formation
low threshold = determination front
segment boundaries are formed when cells in a certain phase of oscillation encounter the determination front
new cells are added to the caudal end
somites bud off rostral end all the time
basic function of somites
making segmented muscle myotomes for left right undulating movements
dorsal dermis
formation of somites
specification &proliferation of mesoderm precursors
segmentation of the paraxial mesoderm
delta notch fgf8
epithelial somite formation
wnt 6 inclusing paraxis
patterning & differetiation
epithelialisation - cells must condense and undertake a mesenchyme to epithelium transition requiring the increase of cel adhesion formation of tight junctions
epithelial somite formation is regulated by Wnt6 signalling from the surface ectoderm activating the transcription factor Paraxis and the two related transcription factors Pax 7 and 3
somite patterning
induction of dermomyotome (holds the precursors for the dorsal dermis and muscle) and the myotome
myotome (contractile muscle) = subdivided into deep muscles of the back called epaxial muscles from the medial lip
from the lateral lip form hypaxial muscles abdominal and limb muscles
cells from the centre contribute to epaxial and hypaxial muscle
sclerotome (delivers the vertebral column and ribs
pstterning depends on extrinsic cues = appositional induction
how is the mesoderm subdivided along the mediolateral axis
= dorso-ventral axis into axial, paraxial, intermediate and lateral mesoderm
primitive streak will form the neural plate = neural tube and axial mesoderm the notochord
paraxial mesoderm is the source of the axial skeleton
segments into somites and give rise to skeletal muscle, dorsal dermis + vertebrae and ribs
intermediate mesoderm - anlagen kidney and gonads
lateral mesoderm delivers peritoneum, skeleton outer muscular layer of gut, heart, blood
somites
reiterated morphologically alike units
epithelial balls of cells
muscle formation in the embryo from paraxial mesoderm = epithelial somites by mesenchymal-epithelial transformation
sorsal smite = epithelial dermomyotome
myotome forms first from the lips of the dermomyotome later from ingressing cells from the centre
remaining cells from dermomyotome = dermomyatome forms the dorsal dermis
cells from the medial wall of the somite detach and differentiate = primary myotome = scaffold
express specific muscle regulatory factors
cells from all 4 libs contribute = secondary myotome
post mitotic
cells from centre = tertiary myotome
act as stem cells
self renew into satellite cells or differentiate and make the bulk of fetal and adult muscle
tissues interactions in somite patterning to secrete patterning signals
dorsally
surface ectoderm and neural tube
ventrally notochor floor plate and endoderm
laterally intermediate and lateral msodem
all secrete signal
molecular markers for somite compartments
Pax3 - dermomyotome lips
MyoD - myotome
Pax1 - sclerotome
Labx1 - lateral dermomyotom
can be used as early molecular
markers for somite differentiation
molecular markers for somite compartments
Pax3 - dermomyotome lips
MyoD - myotome
Pax1 - sclerotome
Labx1 - lateral dermomyotom
can be used as early molecular
markers for somite differentiation
the dorsal neural tube and surface ectoderm release Wnt signals, the lateral mesoderm produces BMP signals
Dorsal patterning Wnt
the dorsal neural tube and surface ectoderm release Wnt signals
removal of neural tube and ectoderm leadds to loss of Pax3 and MyoD and expansion of Pax1 expression
Loss of Wnt signalling results in reduced muscle formation
Wnts from dorsal neral tune and ectoderm are require for dermomyotome and myotome formation and the restriction of the sclerotome
Antagonistic action of signals
non co operatiely
Shh vs Wnt
controls the subdivision of the epithelial somite into ventral sclerotome and dorsal dermomyotome
notochord and floor plate secrete ventralising
Shh
induces sclerotome Pax1 and suppresses dermomyotome
\Dorsal neural tube and epidermal ectoderm secrete dorsalising wnts
induces dermomyotome Pax3 and supresses sclerotome
