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.

mes·o·derm / ˈmezəˌdərm; ˈmē-/ • n. Embryology the middle layer of an embryo in early development, between the endoderm and ectoderm.DERIVATIVES: mes·o·der·mal / ˌmezəˈdəməl; mē-/ adj.mes·o·der·mic / -ˈdərmik/ adj.

mesoderm The layer of cells in the gastrula that lies between the ectoderm and endoderm. It develops into the muscles, circulatory system, and sex organs and in vertebrates also into the excretory system and skeleton. See also germ layers.

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.