Amphibians Regional differentiation

1 amphibians

1.1 dorsal/ventral axis , organizer
1.2 organizer function
1.3 anterior/posterior axis

amphibians
dorsal/ventral axis , organizer

between fertilization , first cleavage in xenopus embryos, cortical cytoplasm of zygote rotates relative central cytoplasm 30 degrees uncover (in species) gray crescent in marginal or middle region of embryo. cortical rotation powered microtubules motors moving along parallel arrays of cortical microtubules. gray crescent marks future dorsal side of embryo. blocking rotation prevents formation of dorsal/ventral axis. late blastula stage, xenopus embryos have clear dorsal/ventral axis.

in gastrula, of tissue in embryo not determined. 1 exception anterior portion of dorsal blastopore lip. when tissue transplanted part of embryo, developed would. in addition, tissue able induce formation of dorsal/ventral axis. hans spemann named region organizer , induction of dorsal axis primary induction.

the organizer induced dorsal vegetal region called nieuwkoop center. there many different developmental potentials throughout blastula stage embryos. vegetal cap can give rise endodermal cell types while animal cap can give rise ectodermal cell types. marginal zone, however, can give rise structures in embryo including mesoderm. series of experiments pieter nieuwkoop showed if marginal zone removed , animal , vegetal caps placed next each other, mesoderm comes animal cap , dorsal tissues adjacent dorsal vegetal cells. thus, dorsal vegetal region, named nieuwkoop center, able induce formation of organizer.

twinning assays identified wnt proteins molecules nieuwkoop center specify dorsal/ventral axis. in twinning assays, molecules injected ventral blastomere of four-cell stage embryo. if molecules specifies dorsal axis, dorsal structures formed on ventral side. wnt proteins not necessary specify axis, examination of other proteins in wnt pathway led discovery β-catenin was. β-catenin present in nuclei on dorsal side not on ventral side. β-catenin levels regulated gsk-3. when active, gsk-3 phosphorylates free β-catenin, targeted degradation. there 2 possible molecules might regulate gsk-3: gbp (gsk-3 binding protein) , dishevelled. current model these act inhibit gsk-3 activity. dishevelled able induce secondary axis when overexpressed , present @ higher levels on dorsal side after cortical rotation (symmetry breaking , cortical rotation). depletion of dishevelled, however, has no effect. gbp has effect both when depleted , overexpressed. recent evidence, however, showed xwnt11, wnt molecule expressed in xenopus, both sufficient , necessary dorsal axis formation.

mesoderm formation comes 2 signals: 1 ventral portion , 1 dorsal portion. animal cap assays used determine molecular signals vegetal cap able induce animal cap form mesoderm. in animal cap assay, molecules of interest either applied in medium cap grown in or injected mrna in embryo. these experiments identified group of molecules, transforming growth factor-β (tgf-β) family. dominant negative forms of tgf-β, experiments able identify family of molecules involved not specific member. recent experiments have identified xenopus nodal-related proteins (xnr-1, xnr-2, , xnr-4) mesoderm-inducing signals. inhibitors of these ligands prevents mesoderm formation , these proteins show graded distribution along dorsal/ventral axis.

vegetally localized mrna, vegt , possibly vg1, involved in inducing endoderm. hypothesized vegt activates xnr-1,2,4 proteins. vegt acts transcription factor activate genes specifying endodermal fate while vg1 acts paracrine factor.

β-catenin in nucleus activates 2 transcription factors: siamois , twin. β-catenin acts synergistically vegt produce high levels of xnr-1,2,4. siamois act synergistically xnr-1,2,4 activate high level of transcription factors such goosecoid in organizer. areas in embryo lower levels of xnr-1,2,4 express ventral or lateral mesoderm. nuclear β-catenin works synergistically mesodermal cell fate signal create signaling activity of nieuwkoop center induce formation of organizer in dorsal mesoderm.

organizer function

there 2 classes of genes responsible organizer s activity: transcription factors , secreted proteins. goosecoid (which has homology between bicoid , gooseberry) first known gene expressed in organizer , both sufficient , necessary specify secondary axis.

the organizer induces ventral mesoderm become lateral mesoderm, induces ectoderm form neural tissue , induces dorsal structures in endoderm. mechanism behind these inductions inhibition of bone morphogenetic protein 4 signaling pathway ventralizes embryo. in absence of these signals, ectoderm reverts default state of neural tissue. 4 of secreted molecules organizer, chordin, noggin, follistatin , xenopus nodal-related-3 (xnr-3), directly interact bmp-4 , block ability bind receptor. thus, these molecules create gradient of bmp-4 along dorsal/ventral axis of mesoderm.

bmp-4 acts in trunk , tail region of embryo while different set of signals work in head region. xwnt-8 expressed throughout ventral , lateral mesoderm. endomesoderm (can give rise either endoderm or mesoderm) @ leading edge of archenteron (future anterior) secrete 3 factors cerberus, dickkopf, , frzb. while cerberus , frzb bind directly xwnt-8 prevent binding receptor, cerberus capable of binding bmp-4 , xnr1. furthermore, dickkopf binds lrp-5, transmembrane protein important signalling pathway of xwnt-8, leading endocytosis of lrp-5 , inhibition of xwnt-8 pathway.

anterior/posterior axis

the anterior/posterior patterning of embryo occurs sometime before or during gastrulation. first cells involute have anterior inducing activity while last cells have posterior inducing activity. anterior inducing ability comes xwnt-8 antagonizing signals cereberus, dickkopf , frzb discussed above. anterior head development requires function of igfs (insulin-like growth factors) expressed in dorsal midline , anterior neural tube. believed igfs function activating signal transduction cascade interferes , inhibits both wnt signaling , bmp signaling. in posterior, 2 candidates posteriorizing signals include efgf, fibroblast growth factor homologue, , retinoic acid.