The formation of the basic body plan is the result of a series of inductive interactions between different parts of the embryo.These interactions are mediated by secreted signaling molecules.

The growth factor b (TGFb) family plays a key role in signaling inductions. The bone morphogenetic protein (BMP) subgroup is especially important. BMP-related growth factors cause both mesoderm induction in the blastula stages and pattering of ventral mesoderm in the gastrula stages (73). Activin and fibroblast growth factor (FGF) also play a huge role in induction (71).

BMP-4 is also an important factor in neural development.

Once inducted, mesoderm then differentiates into its many derivatives.

Fate map of frog blastula (modified 84)

The induction and development of mesoderm can also be carried out in vitro for further study of differentiation factors. Different strategies can be used to induce differentiation.

Stem cells are cultured in a medium that contains:

• leukemia inhibitory factor (LIF) - to prevent differentiation of embryo stem (ES) cells
• basic fibroblast growth factor (bFGF) - to enhance stem cell multiplication
• fetal serum (optional) with small amounts of growth and differentiation factors.

When LIF is withheld, the cells aggregate to form differentiating embryoid bodies (EBs). Within the first couple of days, primitive endoderm and mesoderm marker genes can be observed on the embryoid bodies. (71) By controlling the culture environment and the signaling molecules available, the formation of EBs can be studied to understand induction and differentiation methods.

(81)

BMP-4 AND VENTRAL MESODERM INDUCTION

BMP signaling can be controlled by inhibitory binding proteins chordin (Chd) and noggin. Chd and noggin work by directly binding to BMP-4 and in doing so prevent it from activating its receptor. Experiments by Leslie Dale suggest a model in which extracellular proteinases control the amount of active BMPs available by regulating the levels of these inhibitory binding proteins. (73)

When a Xenopus stage embryo is injected with BMP-4, it results in a ventralization of dorsal mesoderm. Dorsal structures like notochord and muscle are replaced by ventral structures such as blood. (67, 73).

In contrast, when BMP-4 is inhibited by injection of mRNA that encodes Chd or noggin, ventral mesoderm is transformed into dorsal fates. (73)
Thus BMP-4 is the major signaling molecule responsible for the induction of ventral mesoderm in the Xenopus gastrulae, the site where hematopoietic induction occurs and all hematopoietic stem cells are derived (67).