Question

Explain the following observationNoggin secreting cells placed in lateral plate mesoderm causes somites to form there.

Answer 1

ans:-) One of the major tasks of gastrulation is to create a mesodermal layer between the endoderm and the ectoderm. the formation of mesodermal and endodermal organs is not subsequent to neural tube formation, but occurs synchronously. The notochord extends beneath the neural tube from the base of the head into the tail. On either side of the neural tube lie thick bands of mesodermal cells. These bands of paraxial mesoderm are referred to as the segmental plate (in birds) and the unsegmented mesoderm (in mammals). As the primitive streak regresses and the neural folds begin to gather at the center of the embryo, the paraxial mesoderm separates into blocks of cells called somites. and Noggin was also first identified as a dorsalizing factor promoting neural tissue formation secreted by the Spemann organizer. It is a 67-kD homodimeric glycoprotein, which was the first BMP antagonist to have its crystallographic structure resolved at high resolution  Noggin exists as a dimer of identical subunits and resembles a flat “butterfly-like” shape with the helical regions of the two subunits in close contact with each other as the body of the butterfly, and the β-strands of each subunit projecting outward as wings in parallel with each other, and at some 45° away from the axis of symmetry of the dimer. In the Noggin–BMP-7 complex, the two proteins face toward each other and make contact through the tips of their β-strand loops. the BMP antagonist Noggin is expressed within the dorsomedial lip of the dermomyotome, where Pax3-expressing cells first initiate the expression of MyoD and Myf5 to give rise to myotomal cells in the medial somite. Consistent with the expression of Noggin in dorsomedial dermomyotomal cells that lie adjacent to the dorsal neural tube, we have found that coculture of somites with fibroblasts programmed to secrete Wnt1, which is expressed in dorsal neural tube, can induce somitic Noggin expression. Ectopic expression of Noggin lateral to the somite dramatically expands MyoD expression into the lateral regions of the somite, represses Pax3 expression in this tissue, and induces formation of a lateral myotome. Together, our findings indicate that the timing and location of myogenesis within the somite is controlled by relative levels of BMP activity and localized expression of a BMP antagonist. which is Noggin.