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:

• bones
• most of the circulatory system, including the heart and major blood vessels
• connective tissues of the gut and integuments
• mesenchyme
• mesothelium
• muscles
• peritoneum (lining of the coelom)
• reproductive system
• urinary system, including the kidneys

Vertebrates

Transverse section of a chick embryo of forty-five hours' incubation.

• Chordamesoderm: yellow, at the notochord.
  
• Paraxial mesoderm: red, comprising the somites.
  
• Intermediate mesoderm: purple, which includes the Wolffian duct.
  
• Lateral plate mesoderm: purple, comprising somatic and splanchic mesoderm.

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

• Chordamesoderm
• Paraxial mesoderm
• Intermediate mesoderm
• Lateral plate mesoderm

See also

• Embryo
• Embryogenesis
• Embryology
• Ectoderm
• Endoderm
• Germ layer
• Histogenesis
• Organogenesis

Additional images

<gallery> Image:Gray32.png|Section through ovum imbedded in the uterine decidua Image:Gray35.png|Transverse section of a chorionic villus. Image:Gray36.png|Primary chorionic villi. Diagrammatic. Image:Gray37.png|Secondary chorionic villi. Diagrammatic. </gallery>

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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