Mesoderm Induction

The importance of MesodermThe patterning of the mesoderm from dorsal to ventral and anterior to posterior is crucial to the patterening of the rest of the embryo. Signals from neighbouring groups of mesodermal cells determine the specialization of endodermal and ectodermal cells. E.g the dorsal mesoderm induces the ectoderm to become neural ectoderm.Mesodermal Derivatives Dorsal: notochord and somites (muscle and cartilage) Lateral: Limb(muscle and cartilage), Heart (cardiac muscle) and Gut (smooth muscle) Ventral: Kidney and Blood islands (erthrocytes)

Spemann's OrganizerThe ORGANIZER or SPEMANN'S ORGANIZER contains the deep vegetal cells of the dorsal blastopore lip that controls gastrulation, The organizer is induced by the Nieuwkoop Centre. The ORGANIZER directly patterns the mesoderm from dorsal to ventral Identifying the OrganizerIf you transfer the dorsal lip from one embryo to a different posotion in another embryo. The transplanted endoderm and mesoderm will recruit the host epithelium to also involute and a secondary embryo with clear anterior-posterior and dorsal-ventral axes will form

Formation and patterning of MesodermIf you culture the equatorial region of an early blastula in isolation it will form ectoderm. If you culture the equatorial region of a late blastula in isolation, it will form mesoderm.Therefore the vegetal cells must have INDUCED the equatorial cells to become mesoderm by the late blastula stage.Induction:The switching of cells from one differentiation pathway into another by the influence of adjacent cells.You can stimulate the inductive process that occurs before the late blastula stage by co-culturing the animal cap with the vegetal cells. The normal fate of cells of the animal cap is to form ectodermal cells. However if it the animal cap cells contact cells of the vegetal pole they can be enduced to form mesodermal cells. The dorsal vegetal cells induce dorsal mesoderm (notochord and muslce). The ventral cells induce blood.Dorsalizing the mesodermVentral mesoderm will normally become blood but if you place it adjacent to the Organizer it will become derivatives of lateral mesodermal cells.Mesoderm induction is mediated by morphogensMorphogen:An inducing factor that can evoke more than one positive response from the responding tissue. In a gradient, cells respond according to the concentration of morphogen.

Patterning the Animal to Vegetal Axis: Vegetal cells secrete a TGF beta family member. Xnr: xenopus Nodal Related. High concentrations specify endoderm and lower concentrations specift mesoderm. Vegetal cells also secrete a TGF beta family member that antagonises NODAL, it is called LEFTY whixh diffuses more rapidly than Nodal. There is a gradient in Nodal activity from the vegetal to the animal pole which specifies endoderm, mesoderm and ectoderm. Patterning Dorsal-ventral axis BMPs (bone morphogenic proteins) are secreted throughout the embryo. The BMP antagonists CHORDIN and NOGGIN are secreted from the dorsal side by the organizer. Ectodermal cells which see high BMP activity become epidermal. Ectodermal cells which see low BMP activity remain neural. If you treat animal poles cells with intermediate concentrations of nodal and BMP they will become blood (ventral mesoderm)