Résumé
Intermediate mesoderm or intermediate mesenchyme is a narrow section of the mesoderm (one of the three primary germ layers) located between the paraxial mesoderm and the lateral plate of the developing embryo. The intermediate mesoderm develops into vital parts of the urogenital system (kidneys, gonads and respective tracts). Factors regulating the formation of the intermediate mesoderm are not fully understood. It is believed that bone morphogenic proteins, or BMPs, specify regions of growth along the dorsal-ventral axis of the mesoderm and plays a central role in formation of the intermediate mesoderm. Vg1/Nodal signalling is an identified regulator of intermediate mesoderm formation acting through BMP signalling. Excess Vg1/Nodal signalling during early gastrulation stages results in expansion of the intermediate mesoderm at the expense of the adjacent paraxial mesoderm, whereas inhibition of Vg1/Nodal signalling represses intermediate mesoderm formation. A link has been established between Vg1/Nodal signalling and BMP signalling, whereby Vg1/Nodal signalling regulates intermediate mesoderm formation by modulating the growth-inducing effects of BMP signalling. Other necessary markers of intermediate mesoderm induction include the odd-skipped related gene (Osr1) and paired-box-2 gene (Pax2) which require intermediate levels of BMP signalling to activate Markers of early intermediate mesoderm formation are often not exclusive to the intermediate mesoderm. This can be seen in early stages of intermediate mesoderm differentiation where higher levels of BMP stimulate growth of lateral plate tissue, whilst lower concentrations lead to paraxial mesoderm and somite formation. Osr1, which encodes a zinc-finger DNA-binding protein, and LIM-type homeobox gene (Lhx1) expression overlaps the intermediate mesoderm as well as the lateral plate. Osr1 has expression domains encompassing the entire length of the anterior-posterior (AP) axis from the first somites.
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