mesoderm

The mesoderm is the middle of the three primary germ layers formed during gastrulation in triploblastic animals. It originates when cells of the embryonic epiblast migrate between the ectoderm and endoderm through the primitive streak, becoming mesodermal tissue that lies between the other two layers.

The mesoderm differentiates into regional domains that give rise to a wide array of structures. The axial (or chordamesoderm) contributes to the notochord, a key signaling center that patterns surrounding tissues and supports proper development of the vertebral column. The paraxial mesoderm forms somites, which subdivide into the sclerotome (vertebrae and ribs), myotome (skeletal muscles), and dermatome (dermis of the dorsal skin). The intermediate mesoderm gives rise to much of the urogenital system, including kidneys, ureters, and gonads, along with associated ducts. The lateral plate mesoderm splits into a somatic (parietal) layer and a splanchnic (visceral) layer, with the coelomic cavity forming between them. The somatic mesoderm contributes to the body wall and limb connective tissues, while the splanchnic mesoderm contributes to the circulatory system, the heart, and the mesenteries and visceral organs; the mesothelium lining body cavities derives from it.

Overall, mesoderm forms muscles (skeletal, smooth, and cardiac), most of the skeleton (bone and cartilage), connective tissues, the cardiovascular system, blood cells and vessels, kidneys and gonads, and various supportive and lining tissues throughout the body. It plays a central role in organizing the body plan via signaling centers like the notochord.