building a skeleton

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

spondylocostal dystosis is characterised by skeletal malformations only linked mutation is to notch signalling pathway notch is essential for somitogenesis in mouse and chick

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

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

new pre-somitic mesoderm cells are recruited from primitive streak progress of incorporation of cells into somites at rostral/head end of presomitic mesoderm

an oscillatory mechanism ETM ‘clock and wave front’ models to account for periodic formation of somites

epithelial to mesenchymal transition

as cells lose e-cadherin they change from columnar shape to bottle neck to mesenchymal

dynamic caudal to rostral wave across non-segmented mesoderm pattern of expression is reminiscent of oscillator predicted by clock and wavefront model

dynamic expression is not associtated with cell movement maintained in synchrony in small fragments relies on cell to cell contact

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

negative feedback of unstable inhibitors - predominantly in notch signalling and short half lived of key components

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

maintains cells in an immature mesenchymal cell state regulates somite size

by ectopic activation of notch activity

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

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

transformation of 1st lumbar vertebra into a more anterior structure

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

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

abnormal vertebral segmentation

thoracic vertebrae severely affected, lumbar vertebrae only mildly affected

short neck and trunk, severe rib abnormalities and multiple hemiveribrae

shortening of spine, multiple segmentation defects mainly in thoracic region