Cell fate determination Regional differentiation



in terms of developmental commitment, cell can either specified or can determined. specification first stage in differentiation. cell specified can have commitment reversed while determined state irreversible. there 2 main types of specification: autonomous , conditional. cell specified autonomously develop specific fate based upon cytoplasmic determinants no regard environment cell in. cell specified conditionally develop specific fate based upon other surrounding cells or morphogen gradients. type of specification syncytial specification, characteristic of insect classes.


specification in sea urchins uses both autonomous , conditional mechanisms determine anterior/posterior axis. anterior/posterior axis lies along animal/vegetal axis set during cleavage. micromeres induce nearby tissue become endoderm while animal cells specified become ectoderm. animal cells not determined because micromeres can induce animal cells take on mesodermal , endodermal fates. observed β-catenin present in nuclei @ vegetal pole of blastula. through series of experiments, 1 study confirmed role of β-catenin in cell-autonomous specification of vegetal cell fates , micromeres inducing ability. treatments of licl sufficient vegetalize embryo resulted in increases in nuclearly localized b-catenin. reduction of expression of β-catenin in nucleus correlated loss of vegetal cell fates. transplants of micromeres lacking nuclear accumulation of β-catenin unable induce second axis.


for molecular mechanism of β-catenin , micromeres, observed notch present uniformly on apical surface of blastula lost in secondary mesenchyme cells (smcs) during late blastula , enriched in presumptive endodermal cells in late blastula. notch both necessary , sufficient determination of smcs. micromeres express ligand notch, delta, on surface induce formation of smcs.


the high nuclear levels of b-catenin results high accumulation of disheveled protein @ vegetal pole of egg. disheveled inactivates gsk-3 , prevents phosphorylation of β-catenin. allows β-catenin escape degradation , enter nucleus. important role of β-catenin activate transcription of gene pmar1. gene represses repressor allow micromere genes expressed.


the aboral/oral axis (analogous dorsal/ventral axes in other animals) specified nodal homolog. nodal localized on future oral side of embryo. experiments confirmed nodal both necessary , sufficient promote development of oral fate. nodal has role in left/right axis formation.








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