Definitions

mesoderm

mesoderm

[mez-uh-durm, mes-, mee-zuh-, -suh-]
mesoderm, in biology, middle layer of tissue formed in the gastrula stage of the developing embryo. At the end of the blastula stage, cells of the embryo are arranged in the form of a hollow ball. Continued cell movement results in an invagination of the bottom region of the embryo, producing a form that resembles a double-layered cup. A third layer, the mesoderm, is formed between the other two by growth of cells derived from a marginal zone. The mesoderm is the germ layer that forms many muscles, the circulatory and excretory systems, and the dermis, skeleton, and other supportive and connective tissue. It also gives rise to the notochord, a supporting structure between the neural canal and the primitive gut. In many animals, including vertebrates, the mesoderm surrounds a cavity known as the coelom, the space that contains the viscera. See embryo.
One of the three germ layers found in the embryos of animals more complex than cnidarians, making them triploblastic. Mesoderm forms 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.

Mesoderm is found in all large, complex animals, and allows the formation of a coelom, which allows more room for independent growth of the body organs and the coelomic fluid may also act to cushion and protect them from shocks.

Categorizing animals

Not all triploblastic animals have a coelom, like the simplest animals with organs that form from three tissue layers: flatworms. There are three different ways in which the mesoderm is found aligned with the ectoderm and these form a method of categorizing animals into three broad groupings.

  • Acoelomates
    • no coelom
    • tissues and organs packed between gut and body wall
  • Pseudocoelomates (blastocoelomat)
    • false coelom
    • unlined or partially lined body cavity between gut and body wall
  • Coelomates
    • proper coelom
    • lined cavity between gut and body wall

Derivatives

General

The body organs, tissues and systems derived from the mesoderm in most triploblastic animals can be listed as follows:

Vertebrates

In addition to the general list, the mesoderm of a developing vertebrate differentiates into the following:

See also

Additional images

References

  • Evers, Christine A., Lisa Starr. Biology:Concepts and Applications. 6th ed. United States:Thomson, 2006. ISBN 0-534-46224-3.

External links

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