cell migration during tissue formation

mesenchymal cells - at real, myoli is located in stress fires composed of actin and myosin bundles GTPare RhoA needed to build stress fibres, RhoA activated by down stream effector ROCK ROCK inhibition reduces stress fibre tension contractility of the rear mediated by local phosphorylation of myoll by RhoA-ROCK pathway epithelial cells - morphogenic process dependent on myoll distribution and tensile activity in the cell activation of myoll relies on activation of RhoA to apical domain apical localisation requires RhoA and ROCK

actin extension of pseudopadoia in the direction of signal, cells polarise and move directionally

cells detect changing SDF1 signal 2hrs after fertilisation through serpentine CXCR4 receptors cells move by blebbing cells correct motion during migration using a run and tumble mechanism SDF1 is secreted into extracellular matrix by tissue cells in areas which germ cells migrate to in sebrafish CXCR4 is sdf1s receptor, found on the surface of primordial germ cells SDF1 expresssion causes PGC to migrate posteriorly PCG polarisation is a response to chempkine signalling during apolar tumbling stage, stochastic loacal increase in calcium at various points at the cell periphery induces contraction of myosin associated with the actin nerve

mesoderm induction

epiblast cells converge on the primitive streak and pass through it as individual cells, spreading out underneath the surface forming the bottom layer of the endoderm and middle layer of mesoderm cells become specified as endo and meso during passage through the streak, cells remaining on the surface stay ectoderm

mesoderm cells migrate out of the streak at various anterior and posterior positions in the cells cells move out of the streak under influences of a repulsive FGF8 signal anterior mesoderm cells move anteriorly in response to FGF4 produced by the forming head process and notochord, cells in the posterior streak move into the extra embryonic area to form blood islands in response to VEGF PDGF signalling in the epiblast controls N-cadherin expression of migrating mesoderm cells and lays out a migration domain for mesoderm cells

CXCR4 and CXCR7 receptors bind SDF1 but mainly CXCR4 transduces a movement signal CXCR4 bind at the end closest to the direction of movement

in epithelium, adherens junctions hold cells tightly together to form a continuous sheet mesenchyme has a much looser arragnement - some cases lack mature adhesive junctions - offer partly surrounded by extracellular matrix and contact each other through thin contacts - becomes connected by gap junctions - protein pores in adjacent cell membranes epithelia are present within early embryos as dynamic and transient structures epithelia are the starting tissue from which other cells emerge EMT = dissolution of adherens junctions between epithelial cells

neural crest cells are induced at the neural plate border by intermediate levels of BMP signalling and high levels of Wnt, FGF and RA signalling signals induce several transcription factors whose co-expression defines the neural crest cell territory and controls delamination neural crest cells differentiate into a plethora of different cell types - no other embyonic cell population can generate such a diversity of cells neural crest cells forming in the hind brain innervate the branchial arches - migration is confined by repulsive signalling via Semaphorins detected by neutrophilins - dispersal of cells controlled by contact inhibition of locomotion and chemotaxis

cell to cell interactions and external signals

cells are mostly in a mesenchymal state cells move in cohorts in close contact with other cells movement can be influenced by chemotaxis and chemokines cell dependent destruction of ligand may create gradients that help disperse cells cells may disperse as a result of contact inhibition of locomotion mechanisms are complex and can only be studied in vivo