 | This articleneeds additional citations forverification. Please helpimprove this article byadding citations to reliable sources. Unsourced material may be challenged and removed. Find sources: "Germ layer" – news ·newspapers ·books ·scholar ·JSTOR(September 2022) (Learn how and when to remove this message) |
Agerm layer is a primary layer ofcells that forms duringembryonic development.[1] The three germ layers invertebrates are particularly pronounced; however, alleumetazoans (animals that are sister taxa to thesponges) produce two or three primary germ layers. Some animals, likecnidarians, produce two germ layers (theectoderm andendoderm) making themdiploblastic. Other animals such asbilaterians produce a third layer (themesoderm) between these two layers, making themtriploblastic. Germ layers eventually give rise to all of an animal'stissues andorgans through the process oforganogenesis.
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 (Keimblatt) concept, stating that the external, internal and middle layers form respectively the epidermis, the gut, and the intervening musculature and vasculature.[2][3][4] The term "mesoderm" was introduced into English byHuxley in 1871, and "ectoderm" and "endoderm" byLankester in 1873.
Amonganimals,sponges show the least amount of compartmentalization, having a single germ layer. Although they havedifferentiated cells (e.g.collar cells), they lack true tissue coordination.Diploblastic animals,Cnidaria andCtenophora, show an increase in compartmentalization, having two germ layers, theendoderm andectoderm. Diploblastic animals are organized into recognisable tissues. Allbilaterian animals (from flatworms to humans) aretriploblastic, possessing amesoderm in addition to the germ layers found in Diploblasts. Triploblastic animals develop recognizable organs.
Fertilization leads to the formation of azygote. During the next stage,cleavage,mitotic cell divisions transform the zygote into a hollow ball of cells, ablastula. This early embryonic form undergoesgastrulation, forming agastrula with either two or three layers (the germ layers). In allvertebrates, these progenitor cells differentiate into all adult tissues and organs.[5]
In thehuman embryo, after about three days, the zygote forms a solid mass of cells by mitotic division, called amorula. This then changes to ablastocyst, consisting of an outer layer called atrophoblast, and an inner cell mass called theembryoblast. Filled with uterine fluid, the blastocyst breaks out of thezona pellucida and undergoesimplantation. The inner cell mass initially has two layers: thehypoblast andepiblast. At the end of the second week, aprimitive streak appears. The epiblast in this region moves towards the primitive streak, dives down into it, and forms a new layer, called theendoderm, pushing the hypoblast out of the way (this goes on to form theamnion.) The epiblast keeps moving and forms a second layer, themesoderm. The top layer is now called theectoderm.[6]
Gastrulation occurs in reference to the primarybody axis. Germ layer formation is linked to the primary body axis as well, however it is less reliant on it than gastrulation is.Hydractinia shows that germ layer formation that transpires as a mixed delamination.[7]
In mice, germ layer differentiation is controlled by twotranscription factors:Sox2 andOct4 proteins. These transcription factors cause thepluripotent mouseembryonic stem cells to select a germ layer fate. Sox2 promotes ectodermal differentiation, while Oct4 promotes mesendodermal differentiation. Each gene inhibits what the other promotes. Amounts of each protein are different throughout the genome, causing the embryonic stem cells to select their fate.[8]
Theendoderm is one of the germ layers formed during animalembryonic development. Cells migrating inward along thearchenteron form the inner layer of thegastrula, which develops into theendoderm.
The endoderm consists at first of flattened cells, which subsequently become columnar. It forms the epithelial lining of the whole of thedigestive tract except 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 tract, including those of the liver and pancreas; the epithelium of the auditory tube and tympanic cavity; the trachea, bronchi, and alveoli of the lungs; the bladder and part of the urethra; and the follicle lining of the thyroid gland and thymus.
The endoderm forms: thepharynx, theesophagus, thestomach, thesmall intestine, thecolon, theliver, thepancreas, thebladder, theepithelial parts of thetrachea andbronchi, thelungs, thethyroid, and theparathyroid.
Themesoderm germ layer forms in theembryos oftriploblasticanimals. Duringgastrulation, some of the cells migrating inward contribute to the mesoderm, an additional layer between the endoderm and theectoderm.[9] The formation of a mesoderm leads to the development of acoelom. Organs formed inside a coelom can freely move, grow, and develop independently of the body wall while fluid cushions protects them from shocks.[10]
The mesoderm has several components which develop into tissues:intermediate mesoderm,paraxial mesoderm,lateral plate mesoderm, andchorda-mesoderm. The chorda-mesoderm develops into the notochord. The intermediate mesoderm develops into kidneys and gonads. The paraxial mesoderm develops into cartilage, skeletal muscle, and dermis. The lateral plate mesoderm develops into the circulatory system (including the heart and spleen), the wall of the gut, and wall of the human body.[11]
Through cell signaling cascades and interactions with the ectodermal and endodermal cells, the mesodermal cells begin the process ofdifferentiation.[12]
The mesoderm forms: muscle (smooth andstriated),bone,cartilage,connective tissue,adipose tissue,circulatory system,lymphatic system,dermis,dentine of teeth,genitourinary system,serous membranes,spleen andnotochord.
Theectoderm generates the outer layer of the embryo, and it forms from the embryo'sepiblast.[13] Theectoderm develops into thesurface ectoderm,neural crest, and theneural tube.[14]
The surface ectoderm develops into:epidermis,hair,nails,lens of the eye,sebaceous glands,cornea,tooth enamel, the epithelium of themouth andnose.
The neural crest of the ectoderm develops into:peripheral nervous system,adrenal medulla,melanocytes, facial cartilage.
The neural tube of the ectoderm develops into:brain,spinal cord,posterior pituitary,motor neurons,retina.
Note: The anterior pituitary develops from the ectodermal tissue ofRathke's pouch.
Because of its great importance, the neural crest is sometimes considered a fourth germ layer.[15] It is, however, derived from the ectoderm.
| International | |
|---|---|
| National | |
| Other | |
