Mesodermal Induction

Further development of the dorso-ventral axis involves transcription factors which are located in the vegetal pole of the embryo. In xenopus the main vegetal transcription factor is VegT. VegT induces Vg1 (a member of the TGFβ superfamily), which induces another member of the TGFβ superfamily Nodal.

Nodal activation occurs Vg1 binding to TGFβ receptors and stimulating the intracellular Smad mechanism (FigIIIa). Nodal is expressed in a concentration gradient steming from the vegetal pole. This causes different cell fates, cells with the highest Nodal concentrations eventually differentiate into definitive endoderm. Cells that recieve a medial concentration become mesoderm, which is located as in band around the equator of the embryo (FigIIIb)

In this band around the blastula start to express the gene known as Brachyury, the cause of this expression is still unknown.

FIG IIIa: TGF Signalling.

FIG III: TGFbeta signalling, and Smad recruitment.

 

There are several other signalling molecules such as BMP-4 expressed uniformly across the blastula, which originate from the unfertilized oocyte. However these signalling molecules have no known role at this stage (see Spemann Organiser), but BMP-4 signalling is involved in determining mesodermal cell fates of the later developing blastula.

FigIIIb: Mesoderm Induction at Medial Nodal Concentration.