Mesoderm

The germ layer mesoderm forms 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 form an additional layer between the endoderm and the ectoderm.

This key innovation evolved hundreds of millions of years ago and led to the evolution of nearly all large, complex animals. The formation of a mesoderm led to the formation of a coelom. Organs formed inside a coelom (body cavity) can freely move, grow, and develop independently of the body wall while fluid cushions and protects them from shocks.

Not all triploblastic animals have a coelom, like the simplest animals with organs that form from three tissue layers: flatworms. Three different configurations of mesoderm in relation to ectoderm form a method of categorizing animals.

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

Note: Not all triploblasts produce all of the items listed.

• 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

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

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

• Ectoderm
• Endoderm
• Germ layer
• Histogenesis
• Organogenesis

• 
  Transverse section of a chorionic villus.

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

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