Lineage Specification Proteins

 Creative BioMart Lineage Specification Proteins Product List
 Lineage Specification Proteins Background

The onset of lineage specification and loss of pluripotency during embryonic development occurs during gastrulation, the process by which the three germ layers are established from the pluripotent epiblast. The hallmark of gastrulation is the formation of the primitive streak (PS) a transient structure that defines the posterior of the epiblast and the basic embryonic body plan. Prior to PS formation, the epiblast is essentially symmetrical; however, localized gene expression in the surrounding primitive endoderm establishes a gradient of signaling activity that breaks this symmetry and induces PS formation. At ~e5.5, a thickened patch of visceral endoderm cells at the distal tip of the embryo migrates proximally to the prospective anterior side of the epiblast. This anterior visceral endoderm (AVE) serves as a signaling center and expresses a cohort of signaling antagonists including Cer1, Lefty1, and Dkk. The AVE signaling center functions in opposition to a second signaling center in the extra-embryonic ectoderm (EXE) adjacent to the proximal epiblast. The EXE signaling center is characterized by the expression of signaling ligands such as Nodal, Wnt3, and BMP4. The signaling antagonists expressed in the AVE function in opposition to the ligands expressed by the EXE and result in a gradient of signaling activity in the proximal epiblast, with low activity in the anterior epiblast near the AVE and high activity in the prospective posterior epiblast.

The signaling pathways activated by the ligands expressed in the EXE signaling center function to promote the formation of the PS. Genetic ablation of Wnt3, Betacatenin, or the Wnt receptors Lrp5/6 completely blocked PS formation. Conversely, over-activation of Wnt signaling by forced expression of chicken Wnt8C caused ectopic PS formation and axis duplications. Ablation of Nodal or its cofactor Cripto blocked PS formation. Double knockout of the AVE-expressed Nodal antagonists Cer1 and Lefty1 resulted in an expansion of the PS or formation of multiple PSs. Finally, inhibition of BMP signaling by genetic ablation of the BMP receptor Bmpr1 prevented PS formation. Thus, each of the Wnt, Nodal, and BMP signaling pathways were required for induction of PS formation and sufficient to induce ectopic/expanded PS formation. As such, the opposition of the AVE and EXE signaling centers localized the activity of PS-inducing signaling pathways to the proximal posterior epiblast.

At approximately e6.5, this localization of Wnt, Nodal, and BMP signaling activity results in the induction of PS formation at the proximal posterior region of the epiblast, which defines the embryonic posterior. At this site, both morphological changes and lineage specification are induced in epiblast cells. Epiblast cells in the PS begin to express markers of mesoderm lineage specification and undergo an epithelial-to-mesenchymal transition. The result of this is the formation of a new layer of cells: the mesoderm. As gastrulation proceeds, epiblast cells rapidly proliferate and migrate towards the PS. Subsequently, the mesoderm layer expands laterally around the epiblast. Cells from the PS also contribute to the outer layer of definitive endoderm by intercalating into existing endoderm layer. Finally, anterior epiblast cells that do not enter the PS undergo neurectoderm lineage specification. As such, by e7.5, the three germ layers are specified and the basic embryonic body plan is established.

During PS formation, the pluripotency of the epiblast is progressively lost as cells undergo lineage specification. Cell labeling and transplantation experiments showed that up until the early stages of PS formation (~e6.5), the epiblast was uniformly pluripotent and cells from any part of the epiblast could contribute to all three germ layers. However, as PS formation progressed, the developmental potential of epiblast cells was restricted. In embryos with a fully formed PS (~e7.5), cells from the posterior epiblast predominantly contributed to mesodermal cell types and cells from the anterior contributed to neural and ectodermal cell types. In teratoma formation assays, embryos could give contribute to all three germ layers up until e7.5, suggesting that the pluripotency of embryos was lost after PS formation. In a more recent series of experiments, the Chambers and Wilson groups examined epiblast pluripotency by deriving EpiSC from embryos at various stages of PS formation. They found that EpiSC could only be derived from embryos up until ~e8.0, validating the previous findings that epiblast pluripotency was lost following PS formation.