Practice question

When Activin concentration minimal, Smad2 activation weak inadequate for organizer or pan-mesodermal genes, but sufficient for low-threshold ventral mesoderm program. Cells activate transcription factors Vent1/2, GATA2 and genes for hematopoiesis, vasculogenesis and cardiac muscle specification including SCL, GATA4 and Nkx2.5. Animal cap assays show such cells differentiate into blood cells, endothelial tubes and beating heart tissue. Zero Activin leads to atypical epidermis, slight Nodal induces ventral mesoderm, confirming low concentration defaults to blood vessels and heart rather than epidermis or muscle. This illustrates conserved developmental logic of morphogen gradients patterning embryonic axes through Wnt and BMP antagonism.