In humans, the mesoderm is one of the three primary germ cell layers - the other two are the ectoderm and endoderm - in the very early embryo. The mesoderm is the middle layer. It differentiates to give rise to a number of tissues and structures including bone, cartilage, muscle, connective tissue (including that of the dermis), blood vascular, reproductive, excretory and urinogenital systems and contributes to some glands. Some cells in mesodermal tissues retain the capacity to differentiate in diverse directions. For example, some cells in the bone marrow (mesoderm) can become liver (endoderm).
More generally, the mesoderm is one of the three germ layers found in the embryos of Bilaterian animals (all animals except sponges, Cnidarians, and Placozoans), making them triploblastic. Mesoderm forms in the embryo during gastrulation when some of the cells migrating inward to form the endoderm, produce an additional layer that lies between the endoderm and the ectoderm. The formation of the mesoderm may be mediated by the signalling factor NODAL.
Mesoderm is found in most large, complex animals, and allows the formation of a coelom, which allows more room for independent growth of the body organs.
The body organs, tissues and systems derived from the mesoderm in most triploblastic animals can be listed as follows:
Before formation of the items in the above list, the mesoderm of a developing vertebrate transitionally differentiates into the following sub-types:
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