Mesoderm
Mesoderm is one of the three primary germ layers of a vertebrate embryo. A zygote is formed after fertilization which develops by cell division into a blastula. This is followed by gastrulation which is the process of cell movements and translocations that results in the formation of a three layered embryo. The superficial layer is known as ectoderm, the interior layer is endoderm, and the middle germ layer is known as mesoderm. Through the process of differentiation, mesoderm gives rise to all the organs between the ectoderm and endoderm. These include striated (voluntary) skeletal muscle, heart muscle, blood, spleen, adrenal cortex, kidneys, and the various kinds of connective tissue, e.g., cartilage, bone, smooth muscle (this includes the involuntary muscle of the uterus, the muscular layers of the gut, and urinary bladder), the connective tissue layers of the skin, and endothelial lining of blood vessels.
A fish is quite different from a frog, a lizard, a mouse or a human.
Yet, the mesoderm gives rise to the same organs in each of these diverse forms. The kidneys of a frog (mesonephros) and the kidneys of a human (metanephros) are both derived from mesoderm as is the kidney of all vertebrates. The germ layer origin of differentiated tissues is strictly conserved which is why students of embryology still learn about the primary germ layers.
Clearly, the most of the mass of the adult body is of mesodermal origin. Malignancies of mesodermal origin are referred to as sarcomas. Prefixes to that term give a more precise definition of that which is malignant. For example, chondrosarcoma is a malignancy of cartilage and fibrosarcoma is a malignancy of fibrous connective tissue. While mesoderm is the dominant tissue of the body, it is vulnerable to fewer malignancies than epithelial cells.
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