The mesodermal layer of the early embryo forms as a result of gastrulation. This mesodermal layer of cells initially constitutes an epithelium; after gastrulation the cells of this epithelium lose their close association with each other (undergo an epithelial-mesenchymal transformation).
|Axial Mesoderm||Most Dorsal||Forms the notochord.|
|Paraxial Mesoderm||Along Dorsal||Positioned on either side of the axial mesoderm. Gives rise to somites.|
|Intermediate Mesoderm||More Lateral||A mesenchyme that forms the ducts that will form the kidney and internal sexual organs.|
|Lateral Mesoderm||Most Lateral||Extends from either side of the embryo to the ventral midline. Gives rise to blood, blood vessels, smooth muscle and heart.|
Molecular Basis of Mesoderm Subdivision
In amphibians, certain growth factor antagonists are secreted by the most dorsal cells (i.e., Chordin, Noggin, Follistatin, Frzb). Mesoderm cells at different dorsal-ventral positions are committed to become different types of mesoderm based on their exposure to different levels of the dorsal-promoting Chordin, Noggin, Follistatin and Frzb proteins, which function by antagonizing the ventral-promoting growth factors BMPs and Wnts.
The most dorsal tissue (axial mesoderm, or notochord) forms from cells exposed to the highest levels of Chordin, Noggin, Follistatin and Frzb (and thus lowest effective levels of BMPs and Wnts) while the most ventral tissue (heart, which develops from lateral mesoderm) forms from mesoderm cells exposed to the lowest levels of Chordin, Noggin and Follistatin and hence the highest effective levels of BMPs and Wnts.