The mesoderm is a germ layer present in animal embryos that will give rise to specialized tissue types. The mesoderm is one of three germ layers found in triploblastic organisms; it is found between the ectoderm and endoderm. All bilaterally-symmetrical animals are triploblasts, whereas some simpler animals such as cnidaria and ctenophores (jellyfish and comb jellies) have only two germ layers and are diploblasts. The mesoderm plays an important role in animal development. It goes on to form many central structures including the skeletal system, muscular system, and the notochord.
The tissue layers, or germ layers, form during gastrulation. Early in development the egg consists of a darker animal region and a yolky vegetal region. Future mesoderm cells form from animal region cells at the boundary of these two regions as an equatorial band. Unlike the other two germ layers whose fate is determined by maternal factors in the egg, future mesodermal cells form in response to signals from the future endoderm cells in the vegetal region.
In the early blastula, the future mesoderm cells are found on the surface. During gastrulation the cells will rearrange until the mesoderm (and endoderm) are on the inside of the embryo, and the ectoderm is on the outside surface. This occurs by invagination of the mesoderm and endoderm cells; they migrate to the interior while the ectoderm spreads to cover the exterior.
The mesoderm is responsible for the formation of a number of critical structures and organs within the developing embryo including the skeletal system, the muscular system, the excretory system, the circulatory system, the lymphatic system, and the reproductive system. It also gives rise to connective tissues, the dermis of the skin, the lining of the coelom, the adrenal cortex, and many of the internal organs. The mesoderm is generally separated into a number of regions between the dorsal and ventral sides. The dorsal mesoderm will form the notochord, the central region will form the heart and muscles, and the most ventral region will form the blood and associated organs (e.g., the kidney).
Towards the end of gastrulation, the dorsal mesoderm forms a rod-like notochord that runs along the embryo from head to tail below where the nervous system will form, just above the archenteron (the primitive gut). The notochord then sends a signal to the dorsal ectoderm to form the neural tube that will go on to form the central nervous system. The notochord acts as an anchor around which the vertebrae will form. The notochord itself is transient and will ultimately be incorporated as the inner portion of the vertebral disks.
On either side of the notochord lie paired segments of mesoderm that are arranged successively. These somites are formed from lateral-ventral mesoderm. They will ultimately give rise to muscles, cartilage, the vertebral column, and the dermis of the skin. What structures are formed from each of the somite pairs is specified by Hox genes and signals from adjacent cells.
Some of the mesoderm found along the notochord will form mesenchyme cells that will then relocate to new locations. This lateral plate mesoderm will form internal organs including the kidney and the heart. The kidney is formed from mesenchymal cells that have formed tubules. These tubules combine and each develops a glomerulus which acts in filtration. The heart is formed as a single tubule made up of two epithelial layers of mesenchyme: the endocardium and the myocardium. The tube develops into the two heart chambers, the atrium and the ventricle, which then divide further in mammals and birds to form the four-chamber heart.
In many cases, the mesoderm does not develop properly in the embryo. This can result in a number of conditions including heart defects, skeletal abnormalities, or death of the embryo. An example of a mesoderm abnormality is the mutation of the mammalian T gene that gives rise to the brachyury protein. The homozygous form of this mutation is lethal as the mesoderm does not form properly and the skeleton does not develop in the posterior. Heterozygotes for the T gene mutation generally lack some of the sacral vertebrae and have a short tail.