PITX2

Paired-like homeodomain transcription factor 2 (pituitary homeobox 2) is a protein that in humans is encoded by the PITX2 gene.

: acts as a transcription factor and regulates procollagen lysyl hydroxylase gene expression. Also involved in the development of the eye, tooth and abdominal organs.

Pitx2 is responsible for the establishment of the left-right axis, the asymmetrical development of the heart, lungs, and spleen, twisting of the gut and stomach, as well as the development of the eyes. Once activated Pitx2 will be locally expressed in the left lateral mesoderm, tubular heart, and early gut which leads to the asymmetrical development of organs and looping of the gut. When Pitx2 is deleted, the irregular morphogenesis of organs results on the left hand side.

Nodal

Nodal is a secretory protein that in humans is encoded by the NODAL gene. It is located on chromosome10q22.1. Besides, it belongs to TGF-beta superfamily. It is involved in cell differentiation in early embryogenesis, playing a key role in signal transfer from the node, in the anterior primitive streak, to lateral plate mesoderm (LPM). Nodal signaling is important very early in development for mesoderm and endoderm formation and subsequent organization of left-right axial structures.

Nodal signaling regulates mesoderm formation in a species-specific manner. Thus, in Xenopus, Xnr controls dorsoL-ventral mesoderm formation along the marginal zone. In zebrafish, are responsible for animal-vegetal mesoderm formation. In chicken and mouse, Vg1 and Nodal promote primitive streak formation in the epiblast. In chick development, Nodal is expressed in Koller's sickle.

Sonic hedgehog (shh)

Sonic hedgehog is a protein that in humans is encoded by the SHH gene.

: One of the mammalian signaling pathway family called hedgehog, the others: desert hedgehog (DHH) and Indian hedgehog (IHH).

 SHH plays a key role in regulating vertebrate organogenesis, such as in the growth of digits on limbs and organization of the brain. Sonic hedgehog is the best established example of a morphogen as defined by Lewis Wolpert's French flag model—a molecule that diffuses to form a concentration gradient and has different effects on the cells of the developing embryo depending on its concentration.

SHH in the adult: controls cell division of adult stem cells and has been implicated in the development of some cancers.

SHH is secreted at the zone of polarizing activity, which is located on the posterior side of a limb bud in an embryo.

            The sonic hedgehog transcription pathway has also been linked to the formation of specific kinds of cancerous tumors.

SHH that is expressed in a developing embryo, a related morphogen called Fibroblast growth factors must be secreted from the apical ectodermal ridge. 

http://en.wikipedia.org/wiki/Sonic_hedgehog

http://www.mun.ca/biology/desmid/brian/BIOL3530/DB_03/DBNVert1.html

http://www.cell.com/developmental-cell/abstract/S1534-5807(13)00449-8