Germ layer

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 layer


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.

Development
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.

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.

What forms from it (general)?

 * Nervous system
 * Outer part of integument

What forms from it (vertebrates)?
In vertebrates, the ectoderm has three parts: external ectoderm, the neural crest, and neural tube.


 * External Ectoderm
 * skin (along with glands, hair, nails)
 * epithelium of the mouth and nasal cavity
 * lens and cornea of the eye


 * Neural Crest
 * melanocytes
 * peripheral nervous system
 * facial cartilage
 * dentin (in teeth)


 * Neural Tube
 * brain (rhombencephalon, mesencephalon and prosencephalon)
 * spinal cord and motor neurons
 * retina
 * pituitary

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

What forms from it?

 * gastrointestinal tract
 * respiratory tract
 * endocrine glands (including liver and pancreas)

Mesoderm
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 Animals
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)?
Note: Not all triploblasts produce all of the items listed.
 * Bones
 * most of the Circulatory system
 * connective tissues of the gut and integuments
 * Excretory Tract
 * Mesenchyme
 * Mesothelium
 * Muscles
 * Peritoneum (lining of the coelom)
 * Reproductive System
 * Urinary System

What forms from it (vertebrates)?
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


 * Intermediate Mesoderm
 * located between the paraxial mesoderm and the lateral plate
 * develops into the part of the urogenital system (kidneys and gonads)


 * 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