Germ layer - Wikipedia, the free encyclopedia 11/17/11 2:24 PM 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 have differentiated cells (e.g. collar cells), they lack true Gastrulation of a diploblast: The formation of tissue coordination. Diploblastic animals, Cnidaria and germ layers from a (1) blastula to a (2) gastrula. Ctenophora, show an increase in complexity, having two Some of the ectoderm cells (orange) move inward germ layers, the endoderm and ectoderm. Diploblastic forming the endoderm (red). 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.

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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 [citation needed] gastrulation. Micrograph of a teratoma, a tumour that characteristically has tissue from In humans, after about three days, the zygote forms a solid mass of all three germ layers. The image cells by mitotic division, called a morula. This then changes to a shows tissue derived from the blastocyst, consisting of an outer layer called a trophoblast, and an mesoderm (immature cartilage - left- inner cell mass called the embryoblast. Filled with uterine fluid, the upper corner of image), endoderm blastocyst breaks out of the zona pellucida and undergoes (gastrointestinal glands - center- implantation. The inner cell mass initially has two layers: the bottom of image) and ectoderm hypoblast and epiblast. At the end of the second week, a primitive (epidermis - right of image). H&E streak appears. The epiblast in this region moves towards the stain. 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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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 The mesoderm aids in the production of cardiac dermis of skin, connective tissue, the urogenital system, the muscle, skeletal muscle, smooth muscle, tissues heart, blood (lymph cells), the kidney, and the spleen. 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. The ectoderm produces tissues within the Neural crest epidermis, aids in the formation of neurons within the brain, and constructs Because of its great importance, the neural crest is sometimes melanocytes. considered a fourth germ layer.[2] It is, however, derived from the ectoderm. 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" http://en.wikipedia.org/wiki/Germ_layer Page 3 of 4 Germ layer - Wikipedia, the free encyclopedia 11/17/11 2:24 PM

Categories: Embryology Developmental biology Gastrulation

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