Germ layer 1065882 222054036 2008-06-27T09:59:02Z BotKung 3479822 robot Adding: [[th:Germ layer]] [[Image:cell differentiation.gif|right|thumb|300px|Organs derived from each germ layer. Image from [http://www.ncbi.nih.gov/About/primer/genetics_cell.html NCBI].]] A '''germ layer''' is a collection of [[cell (biology)|cell]]s, formed during animal [[embryogenesis]]. Germ layers are particularly pronounced in the [[vertebrate]]s; however, all [[animal]]s more complex than [[sea sponge|sponges]] ([[eumetazoa]]ns and [[wikispecies:agnotozoa|agnotozoans]]) produce two or three '''primary tissue layers''' (sometimes called primary germ layers). Animals with [[symmetry (biology)#Radial symmetry|radial symmetry]], like [[cnidaria]]ns, produce two germ layers (the [[ectoderm]] and [[endoderm]]) making them [[diploblastic]]. Animals with [[symmetry (biology)#Bilateral symmetry|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 [[biological tissue|tissues]] and [[organ (anatomy)|organ]]s through the process of [[organogenesis]]. ==Germ layers== [[Image:Gastrulation.png|thumb|300px|'''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).]] [[Caspar Friedrich Wolff]] observed organization of the early embryo in leaf-like layers. Later, [[Heinz Christian Pander]] discovered germ layers while studying chick embryos. Among [[animal]]s, [[sea sponge|sponges]] show the simplest organization, having a single germ layer. Although they have differentiated cells (e.g. [[Choanocyte|collar cell]]s), they lack true tissue coordination. [[Diploblastic]] animals, [[Cnidaria]] and [[ctenophores]], show an increase in complexity, having two germ layers, the [[endoderm]] and [[ectoderm]]. Diploblastic animals are organized into recognisable tissues. All higher animals (from flatworms to man) are [[triploblastic]], possessing a [[mesoderm]] in additition to the germ layers found in Diploblasts. Triploblastic animals develop recognisable organs. ===Development=== [[fertilisation|Fertilization]] leads to the formation of a [[zygote]]. During the next stage, [[cleavage (embryo)|cleavage]], [[mitosis|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 [[vertebrate]]s, these are the forerunners of all adult tissues and organs. The appearance of the [[archenteron]] marks the onset of gastrulation. 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]]. ===Endoderm===<!-- This section is linked from [[Liver]] --> [[Image:Endoderm2.png|thumb|200px|The [[endoderm]] produces tissue within the [[lung]]s, [[thyroid]], and [[pancreas]].]] {{main|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 excepting part of the mouth and pharynx and the terminal part of the rectum (which are lined by involutions of the ectoderm). It also forms the lining cells of all the glands which open into the digestive tube, including those of the liver and pancreas; the epithelium of the 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===<!-- This section is linked from [[Echinoderm]] --> [[Image:Mesoderm.png|thumb|275px|The [[mesoderm]] aids in the production of [[cardiac muscle]], [[skeletal muscle]], [[smooth muscle]], tissues within the [[kidney]]s, and [[red blood cell]]s.]] {{main|Mesoderm}} The '''mesoderm''' germ layer forms in the [[embryo]]s of [[triploblastic]] [[animal]]s. During [[gastrulation]], some of the cells migrating inward contribute to the [[mesoderm]], an additional layer between the endoderm and the [[ectoderm]]. 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 development of a [[body cavity|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, the crystal lens of the eye, connective tissue, the urogenital system, the [[heart]], blood ([[lymph]] cells), and the spleen. ===Ectoderm=== [[Image:Ectoderm.png|thumb|250px|The [[ectoderm]] produces tissues within the [[Epidermis (skin)|epidermis]], aids in the formation of [[neuron]]s within the brain, and constructs [[melanocytes]].]] {{main|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. It is, however, derived from the ectoderm. ==References== *Evers, Christine A., Lisa Starr. ''Biology:Concepts and Applications.'' 6th ed. United States:Thomson, 2006. ISBN 0-534-46224-3. ==See also== *[[Cellular differentiation]] *[[Embryogenesis]] *[[Histogenesis]] *[[Neurulation]] *[[Organogenesis]] {{Embryology}} {{Germ layer}} [[Category:Embryology]] [[Category:Developmental biology]] [[Category:Gastrulation]] [[ar:طبقة التبرعم]] [[cs:Zárodečný list]] [[de:Keimblatt]] [[es:Capa germinativa]] [[it:Foglietto embrionale]] [[nl:Kiemblad]] [[ja:胚葉]] [[pl:Listki zarodkowe]] [[pt:Camada germinativa]] [[sr:Клицин лист]] [[th:Germ layer]] [[uk:Зародкові листки]]