building a skeleton

Question 1

5 key developmental signalling pathways

Answer 1

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

Question 2

pathologies of aberrant segmentation

Answer 2

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

Question 3

somites

Answer 3

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

Question 4

how is it known that long term axial progenitors in node streak border contribute to neural tube somites and notochord

Answer 4

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

Question 5

how is somitogenesis closely associated to axis elongation

Answer 5

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

Question 6

what is somite formation governed by

Answer 6

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

Question 7

what do gastrulation movements require

Answer 7

epithelial to mesenchymal transition

Question 8

e-cadherin and EMT

Answer 8

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

Question 9

periodic expression of cHairy1 in chick presomitic mesdoerm

Answer 9

dynamic caudal to rostral wave across non-segmented mesoderm

pattern of expression is reminiscent of oscillator predicted by clock and wavefront model

Question 10

analysis of oscillatory mechanism implicated in somitogenesis

Answer 10

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

Question 11

vertebrate cycling genes are linked to notch pathway

Answer 11

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

Question 12

what do clock gene oscillations rely on

Answer 12

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

Question 13

protein products of clock genes

Answer 13

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

Question 14

what is FGF8s function when expressed as in a caudal-rostral gradient in PSM

Answer 14

maintains cells in an immature mesenchymal cell state

regulates somite size

Question 15

how is morphological boundaries introduced

Answer 15

by ectopic activation of notch activity

Question 16

early specification of anterior and posterior identity

Answer 16

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

Question 17

expression of hox gene provides basis for antero-posterior specification

Answer 17

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

Question 18

what does loss of hoxc8 lead to

Answer 18

transformation of 1st lumbar vertebra into a more anterior structure

Question 19

signals surrounding tissues govern somite differentiation

Answer 19

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

Question 20

human segmentation defects

Answer 20

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

Question 21

mutation in DLL3 leads to …

Answer 21

abnormal vertebral segmentation

Question 22

mutation in MESP2 leads to..

Answer 22

thoracic vertebrae severely affected, lumbar vertebrae only mildly affected

Question 23

mutation in LFNG leads to..

Answer 23

short neck and trunk, severe rib abnormalities and multiple hemiveribrae

Question 24

mutation in HES7 leads to..

Answer 24

shortening of spine, multiple segmentation defects mainly in thoracic region

Question 25

what inhibits the notch pathway

Answer 25

cell autonomous interaction between notch and delta

Question 26

how does gene activity provide positional information

Answer 26

code expression defines distinct regions

Question 27

what are hox genes involved in the controlling of

Answer 27

regional identity

Question 28

what do homeobox genes encode

Answer 28

transcription factors