building a skeleton Flashcards
5 key developmental signalling pathways
hedgehog, notch, wingless, fgf and tgfb
all are essential during embryonic development
all are highly conserved from flies to humans
aberrant activity is seen in a variety of cancers
pathologies of aberrant segmentation
spondylocostal dystosis is characterised by skeletal malformations
only linked mutation is to notch signalling pathway
notch is essential for somitogenesis in mouse and chick
somites
made from non-segmented mesoderm
adjacent to midline tissues, neural tube/ notochord
developmental gradient along antero-posterior cells
formation:
recruitment and maturation of precursor cells
periodicity driven by a molecular oscilator
segmentation
border formation and epithelialisation
specialised along anterior posterior axis
differentiation
how is it known that long term axial progenitors in node streak border contribute to neural tube somites and notochord
grafting experiments along with pharmacological inhibition of the notch pathway were performed
WT chick embyros and embryos ubiquitously expressing GFP were disected at HH4 and cultured in vitro until HH8 in presence of notch inhibitor or a biologically inert solvent DMSO
how is somitogenesis closely associated to axis elongation
new pre-somitic mesoderm cells are recruited from primitive streak
progress of incorporation of cells into somites at rostral/head end of presomitic mesoderm
what is somite formation governed by
an oscillatory mechanism
ETM
‘clock and wave front’ models to account for periodic formation of somites
what do gastrulation movements require
epithelial to mesenchymal transition
e-cadherin and EMT
as cells lose e-cadherin they change from columnar shape to bottle neck to mesenchymal
periodic expression of cHairy1 in chick presomitic mesdoerm
dynamic caudal to rostral wave across non-segmented mesoderm
pattern of expression is reminiscent of oscillator predicted by clock and wavefront model
analysis of oscillatory mechanism implicated in somitogenesis
dynamic expression is not associtated with cell movement
maintained in synchrony in small fragments
relies on cell to cell contact
vertebrate cycling genes are linked to notch pathway
notch pathways is an evolutionary conserved mechanism that functions in multiple cell determination processes
allows neighbouring cells to communicate through local short range intercellular interactions
what do clock gene oscillations rely on
negative feedback of unstable inhibitors - predominantly in notch signalling
and short half lived of key components
protein products of clock genes
Lfng protein is produced and degraded during each period of somite formation
misexpression of Lfng abolishes clock gene expression in PSM
dynamic Lfng is needed for correct somite formation
periodic modulation of notch signalling pathway by Hairy/Hes and lunatic fringe is implicated in the mechanism of the segmentation clock
what is FGF8s function when expressed as in a caudal-rostral gradient in PSM
maintains cells in an immature mesenchymal cell state
regulates somite size
how is morphological boundaries introduced
by ectopic activation of notch activity
early specification of anterior and posterior identity
sequence of somite formation from pre-somitic mesoderm is autonomous
presegmented mesoderm is committed to form somites with already fixed antero-posterior identity
exposure to ectopic environment has no effect
expression of hox gene provides basis for antero-posterior specification
homeobox genes are the most striking
it is widely accepted that these are common mechanisms underlying development of all animals
co-linearity = link between order of genes on chromosome and order of spacial and temporal expression along the anterior posterior axis
what does loss of hoxc8 lead to
transformation of 1st lumbar vertebra into a more anterior structure
signals surrounding tissues govern somite differentiation
surgical ablation/ grafting experiments in embyros identified tissues and molecules involved
positive signals: Wnt1 and 3a - skeletal muscles, Shh - vertebrae, cartillage and ribs, Wnt1 and NT3- dermis, Wnt and BMP4 and FGF5 - skeletal muscles
negative signals: Shh and BMP4 induction in sclerotome, noggin and BMP effect in medial dermomytome
human segmentation defects
spondylocostal dysostoses are vertebral malsegmentation disorders that arise from disruption of somitogenesis
muts in DLL3 and MESP2 cause autosomal recessive forms of the disease
mutations in Lfng identified in highly conserved phenylalanine close to the active site of the enzyme also lead to SCD
proper regulation of notch is an absolute requirement for correct patterning of axial skeleton
mutation in DLL3 leads to …
abnormal vertebral segmentation
mutation in MESP2 leads to..
thoracic vertebrae severely affected, lumbar vertebrae only mildly affected
mutation in LFNG leads to..
short neck and trunk, severe rib abnormalities and multiple hemiveribrae
mutation in HES7 leads to..
shortening of spine, multiple segmentation defects mainly in thoracic region
what inhibits the notch pathway
cell autonomous interaction between notch and delta
how does gene activity provide positional information
code expression defines distinct regions
what are hox genes involved in the controlling of
regional identity
what do homeobox genes encode
transcription factors
