Evolution of the eye - wikipedia Evolution of the eye Jump to : navigation , search Major stages in the evolution of the eye in vertebrates . The evolution of the eye is attractive to study , because the eye distinctively exemplifies an analogous organ found in many animal forms . Complex , image - forming eyes have evolved independently several times . The evolutionary geneticist Walter Gering holds a different opinion . According his theory , the genes engaged with photoreceptor cells ( so called Pax 6 genes ) appeared probably at once in the life history , at a very early stage of the evolution . Probably , the complex , image forming kinds of eyes are at least five : Chidaria - lens eye , complex eye ; Molluscs - lens eye , everse retina ; Annelids - lens eye , everse retina ; Arthropods - facet eye ; Vertebrate - lens eye , inverse retina Complex eyes appeared first within the few million years of the Cambrian explosion . From before the Cambrian , no evidence of eyes has survived , but diverse eyes are known from the Burgess shale of the Middle Cambrian , and from the slightly older Emu Bay Shale . Eyes are adapted to the various requirements of their owners . They vary in their visual acuity , the range of wavelengths they can detect , their sensitivity in low light , their ability to detect motion or to resolve objects , and whether they can discriminate colours . Contents ( hide ) 1 History of research 2 Rate of evolution 3 One origin or many ? 4 Stages of eye evolution 4.1 Early eyes 4.2 Lens formation and diversification 4.3 Other developments 4.3. 1 Color vision 4.3. 2 Polarization vision 4.3. 3 Focusing mechanism 4.3. 4 Location 5 Evolutionary baggage 6 Footnotes 7 See also 8 References 9 Further reading 10 External links History of research ( edit ) The human eye , showing the iris In 1802 , philosopher William Paley called it a miracle of `` design '' . Charles Darwin himself wrote in his Origin of Species , that the evolution of the eye by natural selection seemed at first glance `` absurd in the highest possible degree '' . However , he went on that despite the difficulty in imagining it , its evolution was perfectly feasible : ... if numerous gradations from a simple and imperfect eye to one complex and perfect can be shown to exist , each grade being useful to its possessor , as is certainly the case ; if further , the eye ever varies and the variations be inherited , as is likewise certainly the case and if such variations should be useful to any animal under changing conditions of life , then the difficulty of believing that a perfect and complex eye could be formed by natural selection , though insuperable by our imagination , should not be considered as subversive of the theory . He suggested a stepwise evolution from `` an optic nerve merely coated with pigment , and without any other mechanism '' to `` a moderately high stage of perfection '' , and gave examples of existing intermediate steps . Darwin 's suggestions were soon shown to be correct , and current research is investigating the genetic mechanisms underlying eye development and evolution . Biologist D.E. Nilsson has independently theorized about four general stages in the evolution of a vertebrate eye from a patch of photoreceptors . Nilsson and S. Pelger estimated in a classical paper how many generations are needed to evolve a complex eye in vertebrates . Another researcher , G.C. Young , has used the fossil record to infer evolutionary conclusions , based on the structure of eye orbits and openings in fossilized skulls for blood vessels and nerves to go through . All this adds to the growing amount of evidence that supports Darwin 's theory . Rate of evolution ( edit ) The first fossils of eyes found to date are from the lower Cambrian period ( about 540 million years ago ) . The lower Cambrian had a burst of apparently rapid evolution , called the `` Cambrian explosion '' . One of the many hypotheses for `` causes '' of the Cambrian explosion is the `` Light Switch '' theory of Andrew Parker : It holds that the evolution of eyes started an arms race that accelerated evolution . Before the Cambrian explosion , animals may have sensed light , but did not use it for fast locomotion or navigation by vision . The rate of eye evolution is difficult to estimate , because the fossil record , particularly of the lower Cambrian , is poor . How fast a circular patch of photoreceptor cells evolve into a fully functional vertebrate eye has been estimated based on rates of mutation , relative advantage to the organism , and natural selection . However , the time needed for each state was consistently overestimated and the generation time was set to one year , which is common in small animals . Even with these pessimistic values , the vertebrate eye would still evolve from a patch of photoreceptor cells in less than 364,000 years . One origin or many ? ( edit ) Whether the eye evolved once or many times depends on the definition of an eye . All eyed animals share much of the genetic machinery for eye development . This suggests that the ancestor of eyed animals had some form of light - sensitive machinery -- even if it was not a dedicated optical organ . However , even photoreceptor cells may have evolved more than once from molecularly similar chemoreceptor cells . Probably , photoreceptor cells existed long before the Cambrian explosion . Higher - level similarities -- such as the use of the protein crystallin in the independently derived cephalopod and vertebrate lenses -- reflect the co-option of a more fundamental protein to a new function within the eye . A shared trait common to all light - sensitive organs are opsins . Opsins belong to a family of photo - sensitive proteins and fall into nine groups , which already existed in the urbilaterian , the last common ancestor of all bilateral symmetrical animals . Additionally , the genetic toolkit for positioning eyes is shared by all animals : The PAX6 gene controls where eyes develop in animals ranging from octopuses to mice and fruit flies . Such high - level genes are , by implication , much older than many of the structures that they control today ; they must originally have served a different purpose , before they were co-opted for eye development . Eyes and other sensory organs probably evolved before the brain : There is no need for an information - processing organ ( brain ) before there is information to process . Stages of eye evolution ( edit ) The stigma ( 2 ) of the euglena hides a light - sensitive spot . The earliest predecessors of the eye were photoreceptor proteins that sense light , found even in unicellular organisms , called `` eyespots '' . Eyespots can only sense ambient brightness : they can distinguish light from dark , sufficient for photoperiodism and daily synchronization of circadian rhythms . They are insufficient for vision , as they can not distinguish shapes or determine the direction light is coming from . Eyespots are found in nearly all major animal groups , and are common among unicellular organisms , including euglena . The euglena 's eyespot , called a stigma , is located at its anterior end . It is a small splotch of red pigment which shades a collection of light sensitive crystals . Together with the leading flagellum , the eyespot allows the organism to move in response to light , often toward the light to assist in photosynthesis , and to predict day and night , the primary function of circadian rhythms . Visual pigments are located in the brains of more complex organisms , and are thought to have a role in synchronising spawning with lunar cycles . By detecting the subtle changes in night - time illumination , organisms could synchronise the release of sperm and eggs to maximise the probability of fertilisation . Vision itself relies on a basic biochemistry which is common to all eyes . However , how this biochemical toolkit is used to interpret an organism 's environment varies widely : eyes have a wide range of structures and forms , all of which have evolved quite late relative to the underlying proteins and molecules . At a cellular level , there appear to be two main `` designs '' of eyes , one possessed by the protostomes ( molluscs , annelid worms and arthropods ) , the other by the deuterostomes ( chordates and echinoderms ) . The functional unit of the eye is the photoreceptor cell , which contains the opsin proteins and responds to light by initiating a nerve impulse . The light sensitive opsins are borne on a hairy layer , to maximise the surface area . The nature of these `` hairs '' differs , with two basic forms underlying photoreceptor structure : microvilli and cilia . In the eyes of protostomes , they are microvilli : extensions or protrusions of the cellular membrane . But in the eyes of deuterostomes , they are derived from cilia , which are separate structures . However , outside the eyes an organism may use the other type of photoreceptor cells , for instance the clamworm Platynereis dumerilii uses microvilliar cells in the eyes but has additionally deep brain ciliary photoreceptor cells . The actual derivation may be more complicated , as some microvilli contain traces of cilia -- but other observations appear to support a fundamental difference between protostomes and deuterostomes . These considerations centre on the response of the cells to light -- some use sodium to cause the electric signal that will form a nerve impulse , and others use potassium ; further , protostomes on the whole construct a signal by allowing more sodium to pass through their cell walls , whereas deuterostomes allow less through . This suggests that when the two lineages diverged in the Precambrian , they had only very primitive light receptors , which developed into more complex eyes independently . Early eyes ( edit ) The basic light - processing unit of eyes is the photoreceptor cell , a specialized cell containing two types of molecules in a membrane : the opsin , a light - sensitive protein , surrounding the chromophore , a pigment that distinguishes colors . Groups of such cells are termed `` eyespots '' , and have evolved independently somewhere between 40 and 65 times . These eyespots permit animals to gain only a very basic sense of the direction and intensity of light , but not enough to discriminate an object from its surroundings . Developing an optical system that can discriminate the direction of light to within a few degrees is apparently much more difficult , and only six of the thirty - some phyla possess such a system . However , these phyla account for 96 % of living species . The planarian has `` cup '' eyespots that can slightly distinguish light direction . These complex optical systems started out as the multicellular eyepatch gradually depressed into a cup , which first granted the ability to discriminate brightness in directions , then in finer and finer directions as the pit deepened . While flat eyepatches were ineffective at determining the direction of light , as a beam of light would activate exactly the same patch of photo - sensitive cells regardless of its direction , the `` cup '' shape of the pit eyes allowed limited directional differentiation by changing which cells the lights would hit depending upon the light 's angle . Pit eyes , which had arisen by the Cambrian period , were seen in ancient snails , and are found in some snails and other invertebrates living today , such as planaria . Planaria can slightly differentiate the direction and intensity of light because of their cup - shaped , heavily pigmented retina cells , which shield the light - sensitive cells from exposure in all directions except for the single opening for the light . However , this proto - eye is still much more useful for detecting the absence or presence of light than its direction ; this gradually changes as the eye 's pit deepens and the number of photoreceptive cells grows , allowing for increasingly precise visual information . When a photon is absorbed by the chromophore , a chemical reaction causes the photon 's energy to be transduced into electrical energy and relayed , in higher animals , to the nervous system . These photoreceptor cells form part of the retina , a thin layer of cells that relays visual information , including the light and day - length information needed by the circadian rhythm system , to the brain . However , some jellyfish , such as Cladonema , have elaborate eyes but no brain . Their eyes transmit a message directly to the muscles without the intermediate processing provided by a brain . During the Cambrian explosion , the development of the eye accelerated rapidly , with radical improvements in image - processing and detection of light direction . The primitive nautilus eye functions similarly to a pinhole camera . After the photosensitive cell region invaginated , there came a point when reducing the width of the light opening became more efficient at increasing visual resolution than continued deepening of the cup . By reducing the size of the opening , organisms achieved true imaging , allowing for fine directional sensing and even some shape - sensing . Eyes of this nature are currently found in the nautilus . Lacking a cornea or lens , they provide poor resolution and dim imaging , but are still , for the purpose of vision , a major improvement over the early eyepatches . Overgrowths of transparent cells prevented contamination and parasitic infestation . The chamber contents , now segregated , could slowly specialize into a transparent humour , for optimizations such as colour filtering , higher refractive index , blocking of ultraviolet radiation , or the ability to operate in and out of water . The layer may , in certain classes , be related to the moulting of the organism 's shell or skin . An example of this can be observed in Onychophorans where the cuticula of the shell continues to the cornea . The cornea is composed of either one or two cuticular layers depending on how recently the animal has moulted . Along with the lens and two humors , the cornea is responsible for converging light and aiding the focusing of it on the back of the retina . The cornea protects the eyeball while at the same time accounting for approximately 2 / 3 of the eye 's total refractive power . It is likely that a key reason eyes specialize in detecting a specific , narrow range of wavelengths on the electromagnetic spectrum -- the visible spectrum -- is because the earliest species to develop photosensitivity were aquatic , and only two specific wavelength ranges of electromagnetic radiation , blue and green visible light , can travel through water . This same light - filtering property of water also influenced the photosensitivity of plants . Lens formation and diversification ( edit ) Light from a distant object and a near object being focused by changing the curvature of the lens In a lensless eye , the light emanating from a distant point hits the back of the eye with about the same size as the eye 's aperture . With the addition of a lens this incoming light is concentrated on a smaller surface area , without reducing the overall intensity of the stimulus . The focal length of an early lobopod with lens - containing simple eyes focused the image behind the retina , so while no part of the image could be brought into focus , the intensity of light allowed the organism to see in deeper ( and therefore darker ) waters . A subsequent increase of the lens 's refractive index probably resulted in an in - focus image being formed . The development of the lens in camera - type eyes probably followed a different trajectory . The transparent cells over a pinhole eye 's aperture split into two layers , with liquid in between . The liquid originally served as a circulatory fluid for oxygen , nutrients , wastes , and immune functions , allowing greater total thickness and higher mechanical protection . In addition , multiple interfaces between solids and liquids increase optical power , allowing wider viewing angles and greater imaging resolution . Again , the division of layers may have originated with the shedding of skin ; intracellular fluid may infill naturally depending on layer depth . Note that this optical layout has not been found , nor is it expected to be found . Fossilization rarely preserves soft tissues , and even if it did , the new humour would almost certainly close as the remains desiccated , or as sediment overburden forced the layers together , making the fossilized eye resemble the previous layout . Compound eye of Antarctic krill Vertebrate lenses are composed of adapted epithelial cells which have high concentrations of the protein crystallin . These crystallins belong to two major families , the α - crystallins and the βγ - crystallins . Both were categories of proteins originally used for other functions in organisms , but eventually were adapted for the sole purpose of vision in animal eyes . In the embryo , the lens is living tissue , but the cellular machinery is not transparent so must be removed before the organism can see . Removing the machinery means the lens is composed of dead cells , packed with crystallins . These crystallins are special because they have the unique characteristics required for transparency and function in the lens such as tight packing , resistance to crystallization , and extreme longevity , as they must survive for the entirety of the organism 's life . The refractive index gradient which makes the lens useful is caused by the radial shift in crystallin concentration in different parts of the lens , rather than by the specific type of protein : it is not the presence of crystallin , but the relative distribution of it , that renders the lens useful . It is biologically difficult to maintain a transparent layer of cells . Deposition of transparent , nonliving , material eased the need for nutrient supply and waste removal . Trilobites used calcite , a mineral which today is known to be used for vision only in a single species of brittle star . In other compound eyes and camera eyes , the material is crystallin . A gap between tissue layers naturally forms a biconvex shape , which is optically and mechanically ideal for substances of normal refractive index . A biconvex lens confers not only optical resolution , but aperture and low - light ability , as resolution is now decoupled from hole size -- which slowly increases again , free from the circulatory constraints . Independently , a transparent layer and a nontransparent layer may split forward from the lens : a separate cornea and iris . ( These may happen before or after crystal deposition , or not at all . ) Separation of the forward layer again forms a humour , the aqueous humour . This increases refractive power and again eases circulatory problems . Formation of a nontransparent ring allows more blood vessels , more circulation , and larger eye sizes . This flap around the perimeter of the lens also masks optical imperfections , which are more common at lens edges . The need to mask lens imperfections gradually increases with lens curvature and power , overall lens and eye size , and the resolution and aperture needs of the organism , driven by hunting or survival requirements . This type is now functionally identical to the eye of most vertebrates , including humans . Indeed , `` the basic pattern of all vertebrate eyes is similar . '' Other developments ( edit ) Color vision ( edit ) This section needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . ( October 2016 ) ( Learn how and when to remove this template message ) Five classes of visual opsins are found in vertebrates . All but one of these developed prior to the divergence of Cyclostomata and fish . The five opsin classes are variously adapted depending on the light spectrum encountered . As light travels through water , longer wavelengths , such as reds and yellows , are absorbed more quickly than the shorter wavelengths of the greens and blues . This creates a gradient of light as the depth of water increases . The visual opsins in fish are more sensitive to the range of light in their habitat and depth . However , land environments do not vary in wavelength composition , so that the opsin sensitivities among land vertebrates does not vary much . This directly contributes to the significant presence of communication colors . Color vision gives distinct selective advantages , such as better recognition of predators , food , and mates . Indeed , it is thought that simple sensory - neural mechanisms may selectively control general behavior patterns , such as escape , foraging , and hiding . Many examples of wavelength - specific behaviors have been identified , in two primary groups : Below 450 nm , associated with direct light , and above 450 nm , associated with reflected light . As opsin molecules were tuned to detect different wavelengths of light , at some point color vision developed when the photoreceptor cells used differently tuned opsins . This may have happened at any of the early stages of the eye 's evolution , and may have disappeared and reevolved as organisms became predator or prey . Similarly , night and day vision emerged when photoreceptor cells differentiated into rods and cones , respectively . Further information : Evolution of color vision Polarization vision ( edit ) As discussed earlier , the properties of light under water differ from those in air . One example of this is the polarization of light . Polarization is the organization of originally disordered light , from the sun , into linear arrangements . This occurs when light passes through slit like filters , as well as when passing into a new medium . Sensitivity to polarized light is especially useful for organisms whose habitats are located more than a few meters under water . In this environment , color vision is less dependable , and therefore a weaker selective factor . While most photoreceptors have the ability to distinguish partially polarized light , terrestrial vertebrates ' membranes are orientated perpendicularly , such that they are insensitive to polarized light . However , some fish can discern polarized light , demonstrating that they possess some linear photoreceptors . Additionally , cuttlefish are capable of perceiving the polarization of light with high visual fidelity , although they appear to lack any significant capacity for color differentiation . Like color vision , sensitivity to polarization can aid in an organism 's ability to differentiate surrounding objects and individuals . Because of the marginal reflective interference of polarized light , it is often used for orientation and navigation , as well as distinguishing concealed objects , such as disguised prey . Focusing mechanism ( edit ) By utilizing the iris sphincter muscle , some species move the lens back and forth , some stretch the lens flatter . Another mechanism regulates focusing chemically and independently of these two , by controlling growth of the eye and maintaining focal length . In addition , the pupil shape can be used to predict the focal system being utilized . A slit pupil can indicate the common multifocal system , while a circular pupil usually specifies a monofocal system . When using a circular form , the pupil will constrict under bright light , increasing the focal length , and will dilate when dark in order to decrease the depth of focus . Note that a focusing method is not a requirement . As photographers know , focal errors increase as aperture increases . Thus , countless organisms with small eyes are active in direct sunlight and survive with no focus mechanism at all . As a species grows larger , or transitions to dimmer environments , a means of focusing need only appear gradually . Location ( edit ) Prey generally have eyes on the sides of their head so to have a larger field of view , from which to avoid predators . Predators , however , have eyes in front of their head in order to have better depth perception . Flatfish are predators which lie on their side on the bottom , and have eyes placed asymmetrically on the same side of the head . A transitional fossil from the common symmetric position is Amphistium . Evolutionary baggage ( edit ) Vertebrate Octopus Vertebrates and octopodes developed the camera eye independently . In the vertebrate version the nerve fibers pass in front of the retina , and there is a blind spot where the nerves pass through the retina . In the vertebrate example , 4 represents the blind spot , which is notably absent from the octopus eye . In both vertebrates and octopodes , 1 represents the retina , 2 represents the nerve fibers , and 3 represents the optic nerve . Main article : Evolutionary baggage The eyes of many animals record their evolutionary history in their contemporary anatomy . The vertebrate eye , for instance , is built `` backwards and upside down '' , requiring `` photons of light to travel through the cornea , lens , aqueous fluid , blood vessels , ganglion cells , amacrine cells , horizontal cells , and bipolar cells before they reach the light - sensitive rods and cones that transduce the light signal into neural impulses , which are then sent to the visual cortex at the back of the brain for processing into meaningful patterns . '' While such a construct has some drawbacks , it also allows the outer retina of the vertebrates to sustain higher metabolic activities as compared to the non-inverted design . It also allowed for the evolution of the choroid layer , including the retinal pigment epithelial ( RPE ) cells , which play an important role in protecting the photoreceptive cells from photo - oxidative damage . The camera eyes of cephalopods , in contrast , are constructed the `` right way out '' , with the nerves attached to the rear of the retina . This means that they do not have a blind spot . This difference may be accounted for by the origins of eyes ; in cephalopods they develop as an invagination of the head surface whereas in vertebrates they originate as an extension of the brain . Footnotes ( edit ) Jump up ^ David Berlinski , an intelligent design proponent , questioned the basis of the calculations , and the author of the original paper refuted Berlinski 's criticism . Berlinski , David ( April 2001 ) . `` Commentary magazine '' . Nilsson , Dan - E. `` Beware of Pseudo-science : a response to David Berlinski 's attack on my calculation of how long it takes for an eye to evolve '' . `` Evolution of the Eye '' on PBS Jump up ^ The precise number varies from author to author . See also ( edit ) Ocelloid References ( edit ) Jump up ^ `` '' An important cue for understanding eye evolution is the distinction between different types of photoreceptor cells . Salvini - Plawen and Mayr ( 1977 ) noted a remarkable diversity of photoreceptor cell morphology across the animal kingdom , and suggested that photoreceptors evolved independently numerous times . '' - Land , M.F. and Nilsson , D. - E. , Animal Eyes ( 2 - nd ed . ) , Oxford University Press , Oxford ( 2012 ) , p. 7 . 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Jump up ^ Malstrom , T ; Kroger , R ( 2006 ) . `` pupil shape and lens optics in the eyes of terrestrial vertebrates '' . The Journal of Experimental Biology . Jump up ^ `` Carnivores '' . U.S. Department of the Interior , Bureau of Land Management . 2009 - 12 - 14 . Retrieved 2011 - 03 - 28 . Jump up ^ Boroditsky , Lera ( 1999 - 06 - 24 ) . `` Light & Eyes : Lecture Notes '' . Lecture Notes . Stanford . Retrieved 11 May 2010 . Jump up ^ Dr. Michael Shermer , as quoted by Christopher Hitchens in his book `` God is Not Great '' ( pg. 82 ) Jump up ^ Reichenbach A , Bringmann A. ( 2010 ) . Müller cells in the healthy and diseased retina . New York : Springer . pp 15 - 20 . Jump up ^ http://www.photobiology.info/Rozanowska.html Jump up ^ Diagrammatic representation of disc shedding and phagosome retrieval into the pigment epithelial cell. ( 1 ) Jump up ^ Serb , Jeanne M. ; Eernisse , Douglas J. ( 2008 - 09 - 25 ) . `` Charting Evolution 's Trajectory : Using Molluscan Eye Diversity to Understand Parallel and Convergent Evolution '' . Evolution : Education and Outreach. 1 ( 4 ) : 439 -- 447 . doi : 10.1007 / s12052 - 008 - 0084 - 1 . ISSN 1936 - 6426 . Further reading ( edit ) Lamb TD , Collin SP , Pugh EN ( December 2007 ) . `` Evolution of the vertebrate eye : opsins , photoreceptors , retina and eye cup '' . Nat. Rev. Neurosci. 8 ( 12 ) : 960 -- 76 . doi : 10.1038 / nrn2283 . PMC 3143066 . PMID 18026166 . Illustration . Review Lamb , TD ( 2011 ) . `` Evolution of the Eye '' ( PDF ) . Scientific American . 305 : 64 -- 69 . doi : 10.1038 / scientificamerican0711 - 64 . Land , Michael F. ; Nilsson , Dan - Eric ( 2012 ) . `` The origin of vision '' . Animal Eyes ( 2 ed . ) . Oxford : Oxford University Press . pp. 1 -- 22 . ISBN 978 - 0199581146 . Journal Evolution : Education and Outreach Volume 1 , Number 4 / October 2008 . Special Issue : The Evolution of Eyes . 26 articles , free access . Ivan R. Schwab ( 2012 ) . Evolution 's Witness : How Eyes Evolved . New York : Oxford University Press . ISBN 9780195369748 . Hayakawa S , Takaku Y , Hwang JS , Horiguchi T , Suga H , Gehring W , et al. ( 2015 ) . `` Function and evolutionary origin of unicellular camera - type eye structure '' . PLoS ONE . 10 ( 3 ) : e0118415 . Bibcode : 2015PLoSO ... 1018415H . doi : 10.1371 / journal. pone. 0118415 . PMC 4348419 . PMID 25734540 . Greuet , C ( 1968 ) . `` Organisation ultrastructurale de l'ocelle de deux Peridiniens Warnowiidae , Erythropsis pavillardi Kofoid et Swezy et Warnowia pulchra Schiller '' . Protistologica. 4 : 209 -- 230 . Gregory S. Gavelis , Shiho Hayakawa , Richard A. White III , Takashi Gojobori , Curtis A. Suttle , Patrick J. Keeling , Brian S. Leander ( 2015 ) . `` Eye - like ocelloids are built from different endosymbiotically acquired components '' . Nature . 523 : 204 -- 7 . Bibcode : 2015Natur. 523 ... 204G . doi : 10.1038 / nature14593 . PMID 26131935 . CS1 maint : Uses authors parameter ( link ) Oakley , Todd H. ; Speiser , Daniel I. ( 2015 ) . `` How Complexity Originates : The Evolution of Animal Eyes '' . Annual Review of Ecology , Evolution , and Systematics. 46 : 237 -- 260 . doi : 10.1146 / annurev - ecolsys - 110512 - 135907 . Ed Young ; photographs by David Liittschwager ( February 2016 ) . `` Inside the Eye : Nature 's Most Exquisite Creation '' . National Geographic . 229 ( 2 ) : 30 -- 57 . External links ( edit ) `` Evolution of the Eye '' . WGBH Educational Foundation and Clear Blue Sky Productions . PBS . 2001 . Creationism Disproved ? Video from the National Center for Science Education on the evolution of the eye Evolution : Education and Outreach Special Issue : Evolution and Eyes volume 1 , number 4 , October 2008 , pages 351 - 559 . ISSN 1936 - 6426 ( Print ) 1936 - 6434 ( Online ) Evolutionary biology Evolutionary history of life Index of evolutionary biology articles Introduction Outline of evolution Timeline of evolution Evolution Abiogenesis Adaptation Adaptive radiation Cladistics Coevolution Common descent Convergence Divergence Earliest known life forms Evidence of common descent Extinction Event Gene - centered view Homology Last universal common ancestor Macroevolution Microevolution Origin of life Panspermia Parallel evolution Prehistoric Autopsy Speciation Taxonomy Population genetics Biodiversity Gene flow Genetic drift Mutation Natural selection Variation Development Canalisation Evolutionary developmental biology Inversion Modularity Phenotypic plasticity Of taxa Birds origin Brachiopods Molluscs Cephalopods Dinosaurs Fish Fungi Insects butterflies Life Mammals cats canids wolves dogs hyenas dolphins and whales horses primates humans lemurs sea cows Plants Reptiles Spiders Tetrapods Viruses influenza Of organs Cell DNA Flagella Eukaryotes symbiogenesis chromosome endomembrane system mitochondria nucleus plastids In animals eye hair auditory ossicle nervous system brain Of processes Aging Death Programmed cell death Avian flight Biological complexity Cooperation Color vision in primates Emotion Empathy Ethics Eusociality Immune system Metabolism Monogamy Morality Mosaic evolution Multicellularity Sexual reproduction Gamete differentiation / sexes Life cycles / nuclear phases Mating types Sex - determination Snake venom Tempo and modes Gradualism / Punctuated equilibrium / Saltationism Micromutation / Macromutation Uniformitarianism / Catastrophism Speciation Allopatric Anagenesis Catagenesis Cladogenesis Cospeciation Ecological Hybrid Parapatric Peripatric Reinforcement Sympatric History Renaissance and Enlightenment Transmutation of species Charles Darwin On the Origin of Species History of paleontology Transitional fossil Blending inheritance Mendelian inheritance The eclipse of Darwinism Modern synthesis History of molecular evolution Extended evolutionary synthesis Philosophy Darwinism Alternatives Catastrophism Lamarckism Orthogenesis Mutationism Saltationism Structuralism Spandrel Theistic Vitalism Teleology in biology Related Biogeography Ecological genetics Molecular evolution Phylogenetics Tree Polymorphism Protocell Systematics Commons Portal WikiProject Vision in animals Vision Birds Cats Chameleons Dinosaurs Dogs Fishes Horses Toads Eyes Arthropod eye Compound eye Eagle eye Eye shine Simple eye in invertebrates Mammalian eye Human eye Mollusc eye Cephalopod eye Gastropod eye Holochroal eye Parietal eye Schizochroal eye Evolution Evolution of the eye Evolution of color vision Evolution of color vision in primates Coloration Albinism in biology Animal coloration Aposematism Camouflage Chromatophore Counter-illumination Countershading Crypsis Deimatic behaviour Disruptive coloration Eyespot ( mimicry ) Mimicry Structural coloration Underwater camouflage Related topics Animal senses Blindness in animals Eyespot apparatus Feature detection Infrared sensing in snakes Monocular deprivation Ommatidium Palpebral ( bone ) Pseudopupil Rhopalium Underwater vision Visual perception Retrieved from `` https://en.wikipedia.org/w/index.php?title=Evolution_of_the_eye&oldid=838517245 '' Categories : Evolution by phenotype Eye Hidden categories : Webarchive template wayback links All articles with unsourced statements Articles with unsourced statements from May 2011 Wikipedia articles needing clarification from December 2008 Articles with unsourced statements from April 2009 All pages needing factual verification Wikipedia articles needing factual verification from February 2009 Wikipedia articles needing clarification from March 2017 Articles needing additional references from October 2016 All articles needing additional references Articles with specifically marked weasel - worded phrases from March 2016 Articles with unsourced statements from March 2016 All articles with dead external links Articles with dead external links from November 2017 CS1 maint : Uses authors parameter Use dmy dates from May 2011 Talk Contents About Wikipedia বাংলা Deutsch Ελληνικά Español فارسی Français مصرى 日本 語 Simple English Suomi ไทย Türkçe 4 more Edit links This page was last edited on 27 April 2018 , at 14 : 37 . 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Gallbladder - Wikipedia Gallbladder This is the latest accepted revision , reviewed on 18 January 2018 . Jump to : navigation , search Gallbladder The gallbladder sits beneath the liver . Details Precursor Foregut System Digestive system Artery Cystic artery Vein Cystic vein Nerve Celiac ganglia , Vagus nerve Identifiers Latin Vesica biliaris , vesica fellea MeSH A03. 159.439 Dorlands / Elsevier 12383343 TA A05. 8.02. 001 FMA 7202 Anatomical terminology ( edit on Wikidata ) In vertebrates , the gallbladder is a small hollow organ where bile is stored and concentrated before it is released into the small intestine . In humans , the pear - shaped gallbladder lies beneath the liver , although the structure and position of the gallbladder can vary significantly among animal species . It receives and stores bile , produced by the liver , via the common hepatic duct , and releases it via the common bile duct into the duodenum , where the bile helps in the digestion of fats . The gallbladder can be affected by gallstones , formed by material that can not be dissolved -- usually cholesterol or bilirubin , a product of haemoglobin breakdown . These may cause significant pain , particularly in the right upper corner of the abdomen , and are often treated with removal of the gallbladder called a cholecystectomy . Cholecystitis , inflammation of the gallbladder , has a wide range of causes , including result from the impaction of gallstones , infection , and autoimmune disease . Contents ( hide ) 1 Structure 1.1 Microanatomy 1.2 Variation 1.3 Development 2 Function 3 Clinical significance 3.1 Gallstones 3.2 Inflammation 3.3 Gallbladder removal 3.4 Cancer 3.5 Tests 4 Society and culture 5 Other animals 6 History 7 See also 8 References 9 External links Structure ( edit ) The gallbladder is a hollow organ that sits in a shallow depression below the right lobe of the liver , that is grey - blue in life . In adults , the gallbladder measures approximately 7 to 10 centimetres ( 2.8 to 3.9 inches ) in length and 4 centimetres ( 1.6 in ) in diameter when fully distended . The gallbladder has a capacity of about 50 millilitres ( 1.8 imperial fluid ounces ) . The gallbladder is shaped like a pear , with its tip opening into the cystic duct . The gallbladder is divided into three sections : the fundus , body , and neck . The fundus is the rounded base , angled so that it faces the abdominal wall . The body lies in a depression in the surface of the lower liver . The neck tapers and is continuous with the cystic duct , part of the biliary tree . The gallbladder fossa , against which the fundus and body of the gallbladder lie , is found beneath the junction of hepatic segments IVB and V . The cystic duct unites with the common hepatic duct to become the common bile duct . At the junction of the neck of the gallbladder and the cystic duct , there is an out - pouching of the gallbladder wall forming a mucosal fold known as `` Hartmann 's pouch '' . Microanatomy ( edit ) Micrograph of a normal gallbladder wall . H&E stain . The gallbladder wall is composed of a number of layers . The gallbladder wall 's innermost surface is lined by a single layer of columnar cells with a brush border of microvilli , very similar to intestinal absorptive cells . Underneath the epithelium is an underlying lamina propria , a muscular layer , an outer perimuscular layer and serosa . Unlike elsewhere in the intestinal tract , the gallbladder does not have a muscularis mucosae , and the muscular fibres are not arranged in distinct layers . The mucosa , the inner portion of the gallbladder wall , consists of a lining of a single layer of columnar cells , with cells possessing small hair - like attachments called microvilli . This sits on a thin layer of connective tissue , the lamina propria . The mucosa is curved and collected into tiny outpouchings called rugae . A muscular layer sits beneath the mucosa . This is formed by smooth muscle , with fibres that lie in longitudinal , oblique and transverse directions , and are not arranged in separate layers . The muscle fibres here contract to expel bile from the gallbladder . A distinctive feature of the gallbladder is the presence of Rokitansky -- Aschoff sinuses , deep outpouchings of the mucosa that can extend through the muscular layer . The muscular layer is surrounded by a layer of connective and fat tissue . The outer layer of the fundus of gallbladder , and the surfaces not in contact with the liver , are covered by a thick serosa , which is exposed to the peritoneum . The serosa contains blood vessels and lymphatics . The surfaces in contact with the liver are covered in connective tissue . Variation ( edit ) The gallbladder varies in size , shape , and position between different people . Rarely , two or even three gallbladders may coexist , either as separate bladders draining into the cystic duct , or sharing a common branch that drains into the cystic duct . Additionally , the gallbladder may fail to form at all . Gallbladders with two lobes separated by a septum may also exist . These abnormalities are not likely to affect function and are generally asymptomatic . The location of the gallbladder in relation to the liver may also vary , with documented variants including gallbladders found within , above , on the left side of , behind , and detached or suspended from the liver . Such variants are very rare : from 1886 to 1998 , only 110 cases of left - lying liver , or less than one per year , were reported in scientific literature . An anatomical variation can occur , known as a Phrygian cap , which is an innocuous fold in the fundus , named after its resemblance to the Phrygian cap . Development ( edit ) The gallbladder develops from an endodermal outpouching of the embryonic gut tube . Early in development , the human embryo has three germ layers and abuts an embryonic yolk sac . During the second week of embryogenesis , as the embryo grows , it begins to surround and envelop portions of this sac . The enveloped portions form the basis for the adult gastrointestinal tract . Sections of this foregut begin to differentiate into the organs of the gastrointestinal tract , such as the esophagus , stomach , and intestines . During the fourth week of embryological development , the stomach rotates . The stomach , originally lying in the midline of the embryo , rotates so that its body is on the left . This rotation also affects the part of the gastrointestinal tube immediately below the stomach , which will go on to become the duodenum . By the end of the fourth week , the developing duodenum begins to spout a small outpouching on its right side , the hepatic diverticulum , which will go on to become the biliary tree . Just below this is a second outpouching , known as the cystic diverticulum , that will eventually develop into the gallbladder . Function ( edit ) 1 . Bile ducts : 2 . Intrahepatic bile ducts , 3 . Left and right hepatic ducts , 4 . Common hepatic duct , 5 . Cystic duct , 6 . Common bile duct , 7 . Ampulla of Vater , 8 . Major duodenal papilla 9 . Gallbladder , 10 -- 11 . Right and left lobes of liver . 12 . Spleen . 13 . Esophagus. 14 . Stomach . 15 . Pancreas : 16 . Accessory pancreatic duct , 17 . Pancreatic duct . 18 . Small intestine : 19 . Duodenum , 20 . Jejunum 21 -- 22 . Right and left kidneys . The front border of the liver has been lifted up ( brown arrow ) . The main purpose of the gallbladder is to store bile , also called gall , needed for the digestion of fats in food . Produced by the liver , bile flows through small vessels into the larger hepatic ducts and ultimately though the cystic duct ( parts of the biliary tree ) into the gallbladder , where it is stored . At any one time , 30 to 60 millilitres ( 1.0 to 2.0 US fl oz ) of bile is stored within the gallbladder . When food containing fat enters the digestive tract , it stimulates the secretion of cholecystokinin ( CCK ) from I cells of the duodenum and jejunum . In response to cholecystokinin , the gallbladder rhythmically contracts and releases its contents into the common bile duct , eventually draining into the duodenum . The bile emulsifies fats in partly digested food , thereby assisting their absorption . Bile consists primarily of water and bile salts , and also acts as a means of eliminating bilirubin , a product of hemoglobin metabolism , from the body . The bile that is secreted by the liver and stored in the gallbladder is not the same as the bile that is secreted by the gallbladder . During gallbladder storage of bile , it is concentrated by removal of some water and electrolytes . This is through the active transport of sodium ions across the epithelium of the gallbladder , which creates an osmotic pressure that also causes water and other electrolytes such as chloride to be reabsorbed . Clinical significance ( edit ) Main article : Gallbladder disease Gallstones ( edit ) Main article : Gallstones Gallstones form when the bile is saturated , usually with either cholesterol or bilirubin . Most gallstones do not cause symptoms , with stones either remaining in the gallbladder or passed along the biliary system . When symptoms occur , severe `` colicky '' pain in the upper right part of the abdomen is often felt . If the stone blocks the gallbladder , inflammation known as cholecystitis may result . If the stone lodges in the biliary system , jaundice may occur ; and if the stone blocks the pancreatic duct , then pancreatitis may occur . Gallstones are diagnosed using ultrasound . When a symptomatic gallstone occurs , it is often managed by waiting it to be passed naturally . Given the likelihood of recurrent gallstones , surgery to remove the gallbladder is often considered . Some medication , such as ursodeoxycholic acid , may be used ; and lithotripsy , a procedure used to break down the stones , may also be used . Inflammation ( edit ) Main article : Cholecystitis Known as cholecystitis , inflammation of the gallbladder is commonly caused by obstruction of the duct with gallstones , which is known as cholelithiasis . Blocked bile accumulates , and pressure on the gallbladder wall may lead to the release of substances that cause inflammation , such as phospholipase . There is also the risk of bacterial infection . An inflamed gallbladder is likely to cause pain and fever , and tenderness in the upper , right corner of the abdomen , and may have a positive Murphy 's sign . Cholecystitis is often managed with rest and antibiotics , particularly cephalosporins and , in severe cases , metronidazole . Gallbladder removal ( edit ) Main article : Cholecystectomy A cholecystectomy is a procedure in which the gallbladder is removed . It may be removed because of recurrent gallstones , and is considered an elective procedure . A cholecystectomy may be an open procedure , or one conducted by laparoscopy . In the surgery , the gallbladder is removed from the neck to the fundus , and so bile will drain directly from the liver into the biliary tree . About 30 percent of patients may experience some degree of indigestion following the procedure , although severe complications are much rarer . About 10 percent of surgeries lead to a chronic condition of postcholecystectomy syndrome . Cancer ( edit ) Main article : Gallbladder carcinoma Cancer of the gallbladder is uncommon and mostly occurs in later life . When cancer occurs , it is mostly of the glands lining the surface of the gallbladder ( adenocarcinoma ) . Gallstones are thought to be linked to the formation of cancer , and other risk factors include large ( > 1 cm ) gallbladder polyps and having a highly calcified `` porcelain '' gallbladder . Cancer of the gallbladder can cause attacks of biliary pain , yellowing of the skin ( jaundice ) , and weight loss . A large gallbladder may be able to felt in the abdomen . Liver function tests may be elevated , particularly involving GGT and ALP , with ultrasound and CT scans being considered medical imaging investigations of choice . Cancer of the gallbladder is managed by removing the gallbladder , however As of 2010 the prognosis remains poor . Cancer of the gallbladder may also be found incidentally after surgical removal of the gallbladder , with 1 - 3 % of cancers identified in this way . Gallbladder polyps are mostly benign growths or lesions resembling growths that form in the gallbladder wall , and are only associated with cancer when they are larger in size ( > 1 cm ) . Cholesterol polyps , often associated with cholesterolosis ( `` strawberry gallbladder '' , a change in the gallbladder wall due to excess cholesterol. ) Such polyps often cause no symptoms and so are often detected in this way . Tests ( edit ) An ultrasound showing the gallbladder ( central dark area ) with a large gallstone ( white circular area found near the middle ) . Tests used to investigate for gallbladder disease include blood tests and medical imaging . A full blood count may reveal an increased white cell count suggestive of inflammation or infection . Tests such as bilirubin and liver function tests may reveal if there is inflammation linked to the biliary tree or gallbladder , and whether this is associated with inflammation of the liver , and a lipase or amylase may be elvated if there is pancreatitis . Bilirubin may rise when there is obstruction of the flow of bile . A CA 19 - 9 level may be taken to investigate for cholangiocarcinoma . Ultrasound is often the first medical imaging test performed when gallbladder disease such as gallstones are suspected . An abdominal X-ray or CT scan is another form of imaging that may be used to examine the gallbladder and surrounding organs . Other imaging options include MRCP ( magnetic resonance cholangiopancreatography ) , ERCP and percutaneous or intraoperative cholangiography . A cholescintigraphy scan is a nuclear imaging procedure used to assess the condition of the gallbladder . Society and Culture ( edit ) To have ' gall ' is associated with bold behaviour , whereas to have ' bile ' is associated with bitterness . In the Chinese language , the gallbladder ( Chinese : 膽 ) is associated with courage and a myriad of related idioms , including using terms such as `` a body completely ( of ) gall '' ( Chinese : 渾身 是 膽 ) to describe a brave person , and `` single gallbladder hero '' ( Chinese : 孤膽 英雄 ) to describe a lone hero . In the Zangfu theory of Chinese medicine , the gallbladder not only has a digestive role , but is seen as the seat of decision - making . Other animals ( edit ) Most vertebrates have gallbladders , but the form and arrangement of the bile ducts may vary considerably . In many species , for example , there are several separate ducts running to the intestine , rather than the single common bile duct found in humans . Several species of mammals ( including horses , deer , rats , and laminoids ) , several species of birds , lampreys and all invertebrates lack a gallbladder altogether . The bile from several species of bears is used in Traditional Chinese medicine ; bile bears are kept alive in captivity while their bile is painfully extracted , in an industry characterized by animal cruelty . History ( edit ) Depictions of the gallbladder and biliary tree are found in Babylonian models found from 2000 BCE , and in ancient Etruscan model from 200 BCE , with models associated with divine worship . Diseases of the gallbladder have affected humans since antiquity , with gallstones found in the mummy of Princess Amenen of Thebes dating to 1500 BCE . Some historians believe the death of Alexander the Great may have been associated with an acute episode of cholecystitis . The existence of the gallbladder has been noted since the 5th century , but it is only relatively recently that the function and the diseases of the gallbladder has been documented , particularly in the last two centuries . The first descriptions of gallstones appear to have been in the Renaissance , perhaps because of the low incidence of gallstones in earlier times owing to a diet with more cereals and vegetables , and less meat . Anthonius Benevinius in 1506 was the first to draw a connection between symptoms and the presence of gallstones . Courvoisier after examining a number of cases in 1890 that gave rise to the eponymous Courvoisier 's law stating that in an enlarged , nontender gallbladder , the cause of jaundice is unlikely to be gallstones . The first surgical removal of a gallstone ( cholecystolithotomy ) was in 1676 by physician Joenisius , who removed the stones from a spontaneously occurring biliary fistula . Stough Hobbs in 1867 performed the first recorded cholecystotomy , although such an operation was in fact described earlier by French surgeon Jean Louis Petit in the mid eighteenth century . German surgeon Carl Langenbuch performed the first cholecystectomy in 1882 for a sufferer of cholelithiasis . Before this , surgery had focused on creating a fistula for drainage of gallstones . Langenbuch reasoned that given several other species of mammal has no gallbladder , humans could survive without one . The debate whether surgical removal of the gallbladder or simply gallstones was preferred was settled in the 1920s , with consensus that removal of the gallbladder was preferred . It was only in the mid and late parts of the twentieth century that medical imaging techniques such as use of contrast medium and CT scans were used to view the gallbladder . The first laparoscopic cholecystectomy performed by Erich Mühe of Germany in 1985 , although French surgeons Phillipe Mouret and Francois Dubois are often credited for their operations in 1987 and 1988 respectively . See also ( edit ) Medicine portal Enterohepatic circulation References ( edit ) Jump up ^ Ginsburg , Ph. D. , J.N. ( August 22 , 2005 ) . `` Control of Gastrointestinal Function '' . In Thomas M. Nosek , Ph. D. Gastrointestinal Physiology . Essentials of Human Physiology . Augusta , Georgia , United State : Medical College of Georgia . pp. p. 30 . Archived from the original on April 1 , 2008 . Retrieved June 29 , 2007 . ^ Jump up to : Gray 's Anatomy 2008 , p. 1187 - 81 . Jump up ^ Jon W. Meilstrup ( 1994 ) . Imaging Atlas of the Normal Gallbladder and Its Variants . Boca Raton : CRC Press . p. 4 . ISBN 0 - 8493 - 4788 - 2 . Jump up ^ Nagral , Sanjay ( 2005 ) . `` Anatomy relevant to cholecystectomy '' . 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Jump up ^ Segura - Sampedro , JJ ; Navarro - Sánchez , A ; Ashrafian , H ; Martínez - Isla , A ( February 2015 ) . `` Laparoscopic approach to the intrahepatic gallbladder . A case report '' . Revista espanola de enfermedades digestivas : organo oficial de la Sociedad Espanola de Patologia Digestiva. 107 ( 2 ) : 122 -- 3 . PMID 25659400 . Archived from the original on March 4 , 2016 . Jump up ^ Dhulkotia , A ; Kumar , S ; Kabra , V ; Shukla , HS ( March 1 , 2002 ) . `` Aberrant gallbladder situated beneath the left lobe of liver '' . HPB. 4 ( 1 ) : 39 -- 42 . doi : 10.1080 / 136518202753598726 . Jump up ^ Naganuma , S. ; Ishida , H. ; Konno , K. ; Hamashima , Y. ; Hoshino , T. ; Naganuma , H. ; Komatsuda , T. ; Ohyama , Y. ; Yamada , N. ; Ishida , J. ; Masamune , O. ( March 6 , 2014 ) . `` Sonographic findings of anomalous position of the gallbladder '' . Abdominal Imaging. 23 ( 1 ) : 67 -- 72 . doi : 10.1007 / s002619900287 . PMID 9437066 . Jump up ^ Meilstrup JW ; Hopper KD ; Thieme GA ( December 1991 ) . `` Imaging of gallbladder variants '' . AJR Am J Roentgenol. 157 ( 6 ) : 1205 -- 8 . doi : 10.2214 / ajr. 157.6. 1950867 . PMID 1950867 . ^ Jump up to : Gary C. Schoenwolf ; et al. ( 2009 ) . Larsen 's human embryology ( Thoroughly rev. and updated 4th ed . ) . Philadelphia : Churchill Livingstone / Elsevier . pp. `` Development of the Gastrointestinal Tract '' . ISBN 978 - 0 - 443 - 06811 - 9 . Jump up ^ editor - in - chief , Susan Standring ; section editors , Neil R. Borley ; et al. ( 2008 ) . Gray 's anatomy : the anatomical basis of clinical practice ( 40th ed . ) . London : Churchill Livingstone . pp. 1163 , 1177 , 1185 -- 6 . ISBN 978 - 0 - 8089 - 2371 - 8 . CS1 maint : Explicit use of et al. ( link ) CS1 maint : Multiple names : authors list ( link ) CS1 maint : Extra text : authors list ( link ) ^ Jump up to : Hall , Arthur C. Guyton , John E. ( 2005 ) . Textbook of medical physiology ( 11th ed . ) . 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Jump up ^ Reynolds , Walker ( January -- March 2001 ) . `` The First Laparoscopic Cholecystectomy '' . Journal of the Society of Laparoendoscopic surgeons. 5 ( 1 ) : 89 -- 94 . Books editor - in - chief , Susan Standring ; section editors , Neil R. Borley ; et al. ( 2008 ) . Gray 's anatomy : the anatomical basis of clinical practice ( 40th ed . ) . London : Churchill Livingstone . ISBN 978 - 0 - 8089 - 2371 - 8 . CS1 maint : Explicit use of et al. ( link ) CS1 maint : Multiple names : authors list ( link ) CS1 maint : Extra text : authors list ( link ) External links ( edit ) Wikimedia Commons has media related to Gallbladder . Diagram of Human Stomach and Gallbladder -- Human Anatomy Online dd , MyHealthScore.com . ( hide ) Anatomy of the liver , pancreas and biliary tree Liver Bare area Cantlie line Ligamentum venosum Porta hepatis Round ligament Lobes of liver Caudate Quadrate Fibrous capsule of Glisson Perisinusoidal space Liver sinusoid Periportal space Portal triad Lobules of liver Liver segment Microanatomy Hepatocyte Hepatic stellate cell Kupffer cell Biliary tract Bile ducts Intrahepatic bile ducts Bile canaliculus Canals of Hering Interlobular Left hepatic duct Right hepatic duct Common hepatic duct Gallbladder Cystic duct Common bile duct Ampulla of Vater Sphincter of Oddi Cells Cholecystocyte Pancreas Gross Tail Body Neck Head ( Uncinate process ) Ducts main accessory Microanatomy Pancreatic islets Centroacinar cell Stellate cell GND : 4071544 - 9 NDL : 00572670 Retrieved from `` https://en.wikipedia.org/w/index.php?title=Gallbladder&oldid=821194740 '' Categories : Abdomen Animal anatomy Gallbladder Organs ( anatomy ) Hidden categories : CS1 maint : Explicit use of et al . 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Question: where is gall bladder situated in human body