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A germ layer is a collection of cells, formed during animal embryogenesis. Germ layers are only really pronounced in the vertebrates. However, all animals more complex than sponges (eumetazoans and agnotozoans) produce two or three primary tissue layers (sometimes called primary germ layers). Animals with radial symmetry, like cnidarians, produce two called ectoderm and endoderm. Animals with bilateral symmetry produce a third layer in-between called mesoderm. Germ layers will eventually give rise to all of an animal’s tissues and organs through a process called organogenesis.

Germ layerEdit

Caspar Friedrich Wolff observed organization of the early embryo in leaf-like layers. Later, Heinz Christian Pander discovered germ layers while studying chick embryos. According to the number of layers produced, animals are classified as diploblastic (two layers) or triploblastic (three layers). All animals, except for those in the branch radiata, are triploblastic.


DevelopmentEdit

Fertilization leads to the formation of a zygote. During the next stage, cleavage, mitotic cell divisions transform the zygote into a tiny ball of cells called a blastula. This early embryonic form undergoes a massive reorganization called gastrulation forming a gastrula with either two or three layers (the germ layers). In all vertebrates, these are the forerunners of all adult tissues and organs. The appearance of the archenteron marks the onset of gastrulation.

Gastrulation

Gastrulation of a diploblast: The formation of germ layers from a (1) blastula to a (2) gastrula. Some of the ectoderm cells (orange) move inward forming the endoderm (red).

EctodermEdit

The ectoderm is the start of a tissue that covers the body surfaces. It emerges first and forms from the outermost of the germ layers.

What forms from it (general)?Edit

Cell differentiation

Organs derived from each germ layer. Image from NCBI.

What forms from it (vertebrates)?Edit

In vertebrates, the ectoderm has three parts: external ectoderm, the neural crest, and neural tube.

EndodermEdit

Cells migrating inward along the archenteron form the inner layer of the gastrula, which develops into the endoderm.

What forms from it?Edit

MesodermEdit

Mesoderm forms in the embryos of animals more complex than cnidarians. Some of the cells migrating inward to form the endoderm form an additional layer between the other two.

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 can freely move, grow, and develop independently of the body wall while fluid cushions and protects them from shocks.

Categorizing AnimalsEdit

Not all triploblastic animals have a coelom, like flatworms, the simplest animals with organs that form from three tissue layers. 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

What forms from it (general)?Edit

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

What forms from it (vertebrates)?Edit

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

  • Chordamesoderm
    • lies along the central axis, under the neural tube
    • gives rise to the notochordal process which later becomes the notochord
  • Paraxial Mesoderm
    • at the sides of the neural tube
    • gives rise to the somites and head mesoderm.
      • somites form the vertebral column dermis and skeletal muscle
      • head mesoderm will develop into facial muscle and cartilage
  • Lateral Plate Mesoderm
    • found at the periphery of the embryo
    • will split into two layers, the somatic layer/mesoderm and the splanchnic layer/mesoderm
      • somatic layer forms the future body wall
      • splanchnic layer forms the circulatory system and future gut wall.
      • between the two is the coelom

ReferencesEdit

See alsoEdit

eo:Ektodermo fr:Ectoderme he:אקטודרםpt:Ectoderme (embrião)

de:Mesoderm eo:Mezodermo fr:Mésodermept:Mesoderme

de:Entoderm es:Endodermo eo:Endodermo fr:Endoderme

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