Histogenesis

Histogenesis is the formation of different tissues from undifferentiated cells.[1] These cells are constituents of three primary germ layers, the endoderm, mesoderm, and ectoderm. The science of the microscopic structures of the tissues formed within histogenesis is termed histology.

Germ layers
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).
Main article: Germ layer

A germ layer is a collection of cells, formed during animal and mammalian embryogenesis. Germ layers are typically pronounced within vertebrate organisms; however, animals or mammals more complex than sponges (eumetazoans and agnotozoans) produce two or three primary tissue layers. Animals with radial symmetry, such as cnidarians, produce two layers, called the ectoderm and endoderm. They are diploblastic. Animals with bilateral symmetry produce a third layer in-between called mesoderm, making them triploblastic. Germ layers will eventually give rise to all of an animal's or mammal's tissues and organs through a process called organogenesis.

Endoderm
Main article: Endoderm

The endoderm is one of the germ layers formed during animal embryogenesis. Cells migrating inward along the archenteron form the inner layer of the gastrula, which develops into the endoderm. Initially, the endoderm consists of flattened cells, which subsequently become columnar.

Mesoderm
Main article: Mesoderm

The mesoderm germ layer forms in the embryos of animals and mammals more complex than cnidarians, making them triploblastic. During gastrulation, some of the cells migrating inward to form the endoderm form an additional layer between the endoderm and the ectoderm. A theory suggests that 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.

Ectoderm
Main article: Ectoderm

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.

Production
The endoderm produces tissue within the lungs, thyroid, and pancreas. The mesoderm aids in the production of cardiac muscle, skeletal muscle, smooth muscle, tissues within the kidneys, and red blood cells. The ectoderm produces tissues within the epidermis and aids in the formation of neurons within the brain, and melanocytes.

The proceeding graph represents the products produced by the three germ layers.

Germ LayerCategoryProduct
EndodermGeneral[2]Gastrointestinal tract
EndodernGeneralRespiratory tract
EndodermGeneralEndocrine glands and organs (liver and pancreas)
MesodermVertebrate[3]Bones and most cartilage
MesodermGeneralMost of the Circulatory system
MesodermGeneralConnective tissues of the gut and integuments
MesodermGeneralExcretory Tract
MesodermGeneralMesenchyme
MesodermGeneralMesothelium
MesodermGeneralMuscles
MesodermGeneralPeritoneum
MesodermGeneralReproductive System
MesodermGeneralUrinary System
MesodermVertebrateLateral plate mesoderm
EctodermGeneralNervous system
EctodermGeneralOuter part of integument
EctodermVertebrateSkin (along with glands, hair, nails)
EctodermVertebrateEpithelium of the mouth and nasal cavity
EctodermVertebrateLens and cornea of the eye
EctodermVertebrateMelanocytes
EctodermVertebratePeripheral nervous system
EctodermVertebrateFacial cartilage
EctodermVertebrateDentin (in teeth)
EctodermVertebrateBrain (rhombencephalon, mesencephalon & prosencephalon)
EctodermVertebrateSpinal cord and motor neurons
EctodermVertebrateRetina
EctodermVertebratePosterior pituitary

See also

References
  1. histogenesis definition - Dictionary - MSN Encarta. Archived from the original on 2009-03-28.
  2. The General category denotes that all or most of the animals containing this layer produce the adjacent product.
  3. The Vertebrate category denotes that all or most of the vertebrates containing this layer produce the adjacent product.

Further reading

  • Microsoft Corporation. Histogenesis Archived 2009-03-28 at the Wayback Machine (Archived 2009-10-31). Encarta World English Dictionary (North American Edition). URL accessed on 14 May 2005.
  • Evers, Christine A., Lisa Starr. Biology:Concepts and Applications. 6th ed. United States:Thomson, 2006. ISBN 978-0-534-46224-6.
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