In the field of developmental biology, regional differentiation is the process by which different areas are identified in the development of the early embryo.
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.
[4] Treatments of lithium chloride sufficient to vegetalize the embryo resulted in increases in nuclearly localized b-catenin.
Transplants of micromeres lacking nuclear accumulation of β-catenin were unable to induce a second axis.
The micromeres express the ligand for Notch, Delta, on their surface to induce the formation of SMCs.
Similar to Sea urchins, the accumulation of b-catenin in the nuclei was identified as both necessary and sufficient to induce endoderm.
The endoderm sends a fibroblast growth factor (FGF) signal to specify the notocord and the mesenchyme fates.
Bicoid works as a transcriptional activator of the gap genes hunchback (hb), buttonhead (btd), empty spiracles (ems), and orthodentical (otd) while also acting to repress translation of caudal.
Both positive and negative regulatory interactions by both maternal and gap genes and a unique combination of transcription factors work to express even-skipped in different parts of the embryo.
The means to which the genes accomplish this is believed to involve a wingless and hedgehog graded distribution or cascade of signals initiated by these proteins.
The continued expression of the segment polarity genes is maintained by a feedback loop involving hedgehog and wingless.
Molecular mapping revealed that the HOX gene cluster has been inherited intact from a common ancestor of flies and mammals which indicates that it is a fundamental developmental regulatory system.
The follicle cells secrete windbeutel, nudel, and pipe, which create a protease-activating complex.
Once phosphorylated, cactus no longer binds to dorsal, leaving it free to enter the nucleus.
Thus, this dorsal vegetal region, named the Nieuwkoop center, was able to induce the formation of the organizer.
Twinning assays identified Wnt proteins as molecules from the Nieuwkoop center that could specify the dorsal/ventral axis.
In twinning assays, molecules are injected into the ventral blastomere of a four-cell stage embryo.
Recent evidence, however, showed that Xwnt11, a Wnt molecule expressed in Xenopus, was both sufficient and necessary for dorsal axis formation.
With dominant negative forms of TGF-β, early experiments were only able to identify the family of molecules involved not the specific member.
Inhibitors of these ligands prevents mesoderm formation and these proteins show a graded distribution along the dorsal/ventral axis.
Siamois will act synergistically with Xnr-1,2,4 to activate a high level of the transcription factors such as goosecoid in the organizer.
Goosecoid (which has a homology between bicoid and gooseberry) is the first known gene to be expressed in the organizer and is both sufficient and necessary to specify a secondary axis.
The mechanism behind these inductions is an inhibition of the bone morphogenetic protein 4 signaling pathway that ventralizes the embryo.
Four of the secreted molecules from the organizer, chordin, noggin, follistatin and Xenopus nodal-related-3 (Xnr-3), directly interact with BMP-4 and block its ability to bind to its receptor.
The endomesoderm (can give rise to either endoderm or mesoderm) at the leading edge of the archenteron (future anterior) secrete three factors Cerberus, Dickkopf, and Frzb.
Noggin and chordin homologue Chordino, binds to a BMP family member, BMP2B, to block it from ventralizing the embryo.
This axis is defined by the creation of a pH difference "inside" and "outside" of the blastoderm between the subgerminal space and the albumin on the outside.
Similar to the amphibians and fish, the organizer does secrete Chordin, Noggin and Nodal proteins that antagonize BMP signaling and dorsalize the embryo.
The anterior/posterior patterning of the embryo requires signals like cerberus from the hypoblast and the spatial regulation of retinoic acid accumulation to activate the 3' Hox genes in the posterior neuroectoderm (hindbrain and spinal cord).
The discovery of the homeobox in Drosophila flies and its conservation in other animals has led to advancements in understanding the anterior/posterior patterning.