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).

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. Therefore, 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

The endoderm is one of the germ to penes 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

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

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 Layer	Category	Product
Endoderm	General[2]	Gastrointestinal tract
Endodern	General	Respiratory tract
Endoderm	General	Endocrine glands and organs (liver and pancreas)
Mesoderm	General	Bones
Mesoderm	General	Most of the Circulatory system
Mesoderm	General	Connective tissues of the gut and integuments
Mesoderm	General	Excretory Tract
Mesoderm	General	Mesenchyme
Mesoderm	General	Mesothelium
Mesoderm	General	Muscles
Mesoderm	General	Peritoneum
Mesoderm	General	Reproductive System
Mesoderm	General	Urinary System
Mesoderm	Vertebrate[3]	Chordamesoderm
Mesoderm	Vertebrate	Paraxial mesoderm
Mesoderm	Vertebrate	Intermediate mesoderm
Mesoderm	Vertebrate	Lateral plate mesoderm
Ectoderm	General	Nervous system
Ectoderm	General	Outer part of integument
Ectoderm	Vertebrate	Skin (along with glands, hair, nails)
Ectoderm	Vertebrate	Epithelium of the mouth and nasal cavity
Ectoderm	Vertebrate	Lens and cornea of the eye
Ectoderm	Vertebrate	Melanocytes
Ectoderm	Vertebrate	Peripheral nervous system
Ectoderm	Vertebrate	Facial cartilage
Ectoderm	Vertebrate	Dentin (in teeth)
Ectoderm	Vertebrate	Brain (rhombencephalon, mesencephalon and prosencephalon)
Ectoderm	Vertebrate	Spinal cord and motor neurons
Ectoderm	Vertebrate	Retina
Ectoderm	Vertebrate	Posterior pituitary

See also

• Histology

References

1. histogenesis definition - Dictionary - MSN Encarta. Archived from the original on 2009-10-31.
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.

• Microsoft Corporation. Histogenesis (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.

External links

• (English) Derivatives of the Ectoderm
• (English) Derivatives of the Endoderm and Mesoderm