mesoderm

mesoderm in biology, middle layer of tissue formed in the gastrula stage of the developing embryo. At the end of the blastula stage, cells of the embryo are arranged in the form of a hollow ball. Continued cell movement results in an invagination of the bottom region of the embryo, producing a form that resembles a double-layered cup. A third layer, the mesoderm, is formed between the other two by growth of cells derived from a marginal zone. The mesoderm is the germ layer that forms many muscles, the circulatory and excretory systems, and the dermis, skeleton, and other supportive and connective tissue. It also gives rise to the notochord , a supporting structure between the neural canal and the primitive gut. In many animals, including vertebrates, the mesoderm surrounds a cavity known as the coelom , the space that contains the viscera. See embryo .

mesoderm In a triploblastic embryo, the middle layer of cells, which form a hollow ring surrounding the endoderm, giving rise to muscle, blood vessel, some nervous tissue, and the organs of the body within the mesoderm cavity. On development of the notochord, the mesoderm develops bilaterally paired blocks on either side of the notochord, known as somites, in which voluntary muscles, vertebrae, and deep portions of the skin develop. Somites are connected by undifferentiated mesoderm, known as lateral plate mesoderm. The ectodermal layer of the mesoderm is called the somatic mesoderm and the endodermal layer is called the splanchnic mesoderm layer. The mesoderm region between the somite and lateral plate mesoderm eventually separates and forms a nephric ridge which will develop into the kidney. A genital ridge later forms from the medial border of the kidneys which gives rise to the gonads. After differentiation of the mesoderm, organogenesis occurs.

mesoderm (mes-oh-derm) n. the middle germ layer of the early embryo. It gives rise to cartilage, muscle, bone, blood, kidneys, gonads and their ducts, and connective tissue.
—mesodermal adj.