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Germ layer
From Wikipedia, the free encyclopedia
A germ layer, occasionally[1] referred to as a germinal epithelium, is a group of cells, formed during
animal embryogenesis. Germ layers are particularly 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 germ
layers (the ectoderm and endoderm) making them diploblastic. Animals with bilateral symmetry produce a
third layer between these two layers (appropriately called the mesoderm) making them triploblastic. Germ
layers eventually give rise to all of an animal’s tissues and organs through the process of organogenesis.
Contents
1 Germ layers
1.1 Development
1.2 Endoderm
1.3 Mesoderm
1.4 Ectoderm
1.5 Neural crest
2 See also
3 References
Germ layers
Caspar Friedrich Wolff observed organization of the
early embryo in leaf-like layers. In 1817, Heinz Christian
Pander discovered three primordial germ layers while
studying chick embryos. Between 1850 and 1855,
Robert Remak had further refined the germ cell layer
concept, and introduced into English were the terms
"mesoderm" by Huxley in 1871 and "ectoderm" and
"endoderm" by Lankester in 1873.
Among animals, sponges show the simplest
organization, having a single germ layer. Although they
Gastrulation of a diploblast: The formation of
have differentiated cells (e.g. collar cells), they lack true
germ layers from a (1) blastula to a (2) gastrula.
tissue coordination. Diploblastic animals, Cnidaria and
Some of the ectoderm cells (orange) move inward
Ctenophora, show an increase in complexity, having two
forming the endoderm (red).
germ layers, the endoderm and ectoderm. Diploblastic
animals are organized into recognisable tissues. All
higher animals (from flatworms to humans) are triploblastic, possessing a mesoderm in addition to the germ
layers found in Diploblasts. Triploblastic animals develop recognisable organs.
Development
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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, a blastula. This early embryonic form undergoes
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.[citation needed]
The appearance of the archenteron marks the onset of
gastrulation.[citation needed]
Micrograph of a teratoma, a tumour
that characteristically has tissue from
all three germ layers. The image
shows tissue derived from the
mesoderm (immature cartilage - leftupper corner of image), endoderm
(gastrointestinal glands - centerbottom of image) and ectoderm
(epidermis - right of image). H&E
stain.
In humans, after about three days, the zygote forms a solid mass of
cells by mitotic division, called a morula. This then changes to a
blastocyst, consisting of an outer layer called a trophoblast, and an
inner cell mass called the embryoblast. Filled with uterine fluid, the
blastocyst breaks out of the zona pellucida and undergoes
implantation. The inner cell mass initially has two layers: the
hypoblast and epiblast. At the end of the second week, a primitive
streak appears. The epiblast in this region moves towards the
primitive streak, dives down into it, and forms a new layer, called the
endoderm, pushing the hypoblast out of the way (this goes on to form
the amnion.) The epiblast keeps moving and forms a second layer, the mesoderm. The top layer is now
called the ectoderm.[citation needed]
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.
The endoderm consists at first of flattened cells, which subsequently
become columnar. It forms the epithelial lining of the whole of the
digestive tube except part of the mouth and pharynx and the terminal
part of the rectum (which are lined by involutions of the ectoderm). It
The endoderm produces tissue
also forms the lining cells of all the glands which open into the digestive
within the lungs, thyroid, and
tube, including those of the liver and pancreas; the epithelium of the
pancreas.
auditory tube and tympanic cavity; the trachea, bronchi, and air cells of
the lungs; the urinary bladder and part of the urethra; and the follicle lining of the thyroid gland and thymus.
The endoderm forms: the stomach, the colon, the liver, the pancreas, the urinary bladder, the lining of the
urethra, the epithelial parts of trachea, the lungs, the pharynx, the thyroid, the parathyroid, and the intestines.
Mesoderm
Main article: Mesoderm
The mesoderm germ layer forms in the embryos of triploblastic animals. During gastrulation, some of the
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cells migrating inward contribute to the mesoderm, an
additional layer between the endoderm and the ectoderm.
The formation of a mesoderm led to the development 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.
The mesoderm forms: skeletal muscle, the skeleton, the
dermis of skin, connective tissue, the urogenital system, the
heart, blood (lymph cells), the kidney, and the spleen.
The mesoderm aids in the production of cardiac
muscle, skeletal muscle, smooth muscle, tissues
within the kidneys, and red blood cells.
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.
The ectoderm forms: the central nervous system, the lens of the
eye, cranial and sensory, the ganglia and nerves, pigment cells,
head connective tissues, the epidermis, hair, and mammary
glands.
Neural crest
Because of its great importance, the neural crest is sometimes
considered a fourth germ layer.[2] It is, however, derived from
the ectoderm.
The ectoderm produces tissues within the
epidermis, aids in the formation of neurons
within the brain, and constructs
melanocytes.
See also
Cellular differentiation
Embryogenesis
Histogenesis
Neurulation
Organogenesis
References
1. ^ Gilbert, Scott F (2003). "The Epidermis and the Origin of Cutaneous Structures"
(http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=dbio&part=A2929) . Developmental Biology. Sinauer
Associates. http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=dbio&part=A2929.
2. ^ Hall BK (2000) The neural crest as a fourth germ layer and vertebrates as quadroblastic not triploblastic. Evolution
& Development 2, 3-5 pmid=11256415 url=http://onlinelibrary.wiley.com/resolve/openurl?
genre=article&sid=nlm:pubmed&issn=1520-541X&date=2000&volume=2&issue=1&spage=3
Retrieved from "http://en.wikipedia.org/w/index.php?title=Germ_layer&oldid=457507952"
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Categories:
Embryology
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Developmental biology
Gastrulation
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