The yellow cytoplasm specifies the anterior/posterior axis. The cytoplasm in the vegetal pole specifies this axis and removing this cytoplasm leads to a loss of axis information. The cytoplasm of the egg not only determines cell fate, but also determines the dorsal/ventral axis. Anterior cells respond to FGF to become notocord while posterior cells (identified by the presence of macho-1) respond to FGF to become mesenchyme. The endoderm sends a fibroblast growth factor (FGF) signal to specify the notocord and the mesenchyme fates. Two more cell fates are determined by conditional specification. Similar to Sea urchins, the accumulation of b-catenin in the nuclei was identified as both necessary and sufficient to induce endoderm. The cytoplasmic determinant macho-1 was isolated as the necessary and sufficient factor for muscle cell formation. This cytoplasm was segregated to future muscle cells and if transplanted could induce the formation of muscle cells. Tunicates have been a popular choice for the study of regional specification because tunicates were the first organism in which autonomous specification was discovered and tunicates are evolutionary related to vertebrates.Įarly observations in tunicates led to the identification of the yellow crescent (also called the myoplasm). Nodal also has a role in left/right axis formation. Experiments confirmed that nodal is both necessary and sufficient to promote development of the oral fate. This nodal was localized on the future oral side of the embryo. The aboral/oral axis (analogous to the dorsal/ventral axes in other animals) is specified by a nodal homolog. This gene represses a repressor to allow micromere genes to be expressed. The only important role of β-catenin is to activate the transcription of the gene Pmar1. This allows β-catenin to escape degradation and enter the nucleus. disheveled inactivates GSK-3 and prevents the phosphorylation of β-catenin. The high nuclear levels of b-catenin results from the high accumulation of the disheveled protein at the vegetal pole of the egg. The micromeres express the ligand for Notch, Delta, on their surface to induce the formation of SMCs. Notch is both necessary and sufficient for determination of the SMCs. Transplants of micromeres lacking nuclear accumulation of β-catenin were unable to induce a second axis.įor the molecular mechanism of β-catenin and the micromeres, it was observed that Notch was present uniformly on the apical surface of the early blastula but was lost in the secondary mesenchyme cells (SMCs) during late blastula and enriched in the presumptive endodermal cells in late blastula. Reduction of expression of β-catenin in the nucleus correlated with loss of vegetal cell fates. Treatments of lithium chloride sufficient to vegetalize the embryo resulted in increases in nuclearly localized b-catenin. Through a series of experiments, one study confirmed the role of β-catenin in the cell-autonomous specification of vegetal cell fates and the micromeres inducing ability. It was observed that β-catenin was present in the nuclei at the vegetal pole of the blastula. The animal cells are not determined because the micromeres can induce the animal cells to also take on mesodermal and endodermal fates. The micromeres induce the nearby tissue to become endoderm while the animal cells are specified to become ectoderm. The anterior/posterior axis lies along the animal/vegetal axis set up during cleavage. Specification in sea urchins uses both autonomous and conditional mechanisms to determine the anterior/posterior axis. ![]() Another type of specification is syncytial specification, characteristic of most insect classes. A cell specified conditionally will develop into a specific fate based upon other surrounding cells or morphogen gradients. A cell specified autonomously will develop into a specific fate based upon cytoplasmic determinants with no regard to the environment the cell is in. There are two main types of specification: autonomous and conditional. A cell that is specified can have its commitment reversed while the determined state is irreversible. Specification is the first stage in differentiation. In terms of developmental commitment, a cell can either be specified or it can be determined.
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