# Endoskeleton

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{{Short description|Internal support structure of an animal}}[[File:Swordfish skeleton.jpg|thumb|upright=1.2|Endoskeleton of a [swordfish](/source/swordfish)]]
{{biomineralization sidebar|endoskeletons}}

An '''endoskeleton''' (from [Ancient Greek](/source/Ancient_Greek) ἔνδον (''éndon''), meaning "inside", and σκελετός (''skeletós''), meaning "skeleton") is a [structural frame](/source/structural_frame) ([skeleton](/source/skeleton)) — usually composed of [mineralized tissue](/source/mineralized_tissues) — on the inside of an [animal](/source/animal), overlaid by [soft tissue](/source/soft_tissue)s.<ref>{{Cite book |last=Hyman |first=Libbie Henrietta |url=https://books.google.com/books?id=VKlWjdOkiMwC |title=Hyman's Comparative Vertebrate Anatomy |date=1992-09-15 |publisher=University of Chicago Press |isbn=978-0-226-87013-7 |pages=192–236 |language=en}}</ref><ref>{{Citation |last=Gillis |first=J. Andrew |title=The Development and Evolution of Cartilage |date=2019 |url=https://linkinghub.elsevier.com/retrieve/pii/B9780128096338907702 |work=Reference Module in Life Sciences |access-date=2023-10-03 |publisher=Elsevier |language=en |doi=10.1016/b978-0-12-809633-8.90770-2 |isbn=978-0-12-809633-8|url-access=subscription }}</ref> Endoskeletons serve as [structural support](/source/structural_support) against [gravity](/source/gravity) and [mechanical load](/source/mechanical_load)s, and provide anchoring attachment sites for [skeletal muscle](/source/skeletal_muscle)s to transmit force and allow movements and [locomotion](/source/animal_locomotion).
 
[Vertebrate](/source/Vertebrate)s and the closely related [cephalochordate](/source/cephalochordate)s are the predominant animal [clade](/source/clade) with endoskeletons (made of mostly [bone](/source/bone) and sometimes [cartilage](/source/cartilage), as well as [notochord](/source/notochord)al [glycoprotein](/source/glycoprotein) and [collagen fiber](/source/collagen_fiber)s), although [invertebrate](/source/invertebrate)s such as [sponge](/source/sponge)s also have evolved a form of "[rebar](/source/rebar)" endoskeletons made of diffuse meshworks of [calcite](/source/calcite)/[silica](/source/silica) [structural element](/source/structural_element)s called [spicule](/source/sponge_spicule)s, and [echinoderm](/source/echinoderm)s have a [dermal](/source/dermal) calcite endoskeleton known as [ossicle](/source/ossicle_(echinoderm))s. Some [coleoid](/source/coleoid) [cephalopod](/source/cephalopod)s ([squid](/source/squid)s and [cuttlefish](/source/cuttlefish)) have an internalized [vestigial](/source/vestigial) [aragonite](/source/aragonite)/calcite-[chitin](/source/chitin) [shell](/source/mollusc_shell) known as [gladius](/source/gladius_(cephalopod)) or [cuttlebone](/source/cuttlebone); these can serve as muscle attachments, but their main function is often to maintain [buoyancy](/source/buoyancy) rather than to give structural support, and their body shape is largely maintained by their [hydroskeleton](/source/hydroskeleton).

Compared to the [exoskeleton](/source/exoskeleton)s of many invertebrates, endoskeletons allow much larger overall body sizes for the same skeletal [mass](/source/mass), as most soft tissues and [organ](/source/organ_(biology))s are positioned ''outside'' the skeleton rather than within it, thus unrestricted by the volume and internal capacity of the skeleton itself. Being more centralized in structure also means more compact [volume](/source/volume), making it easier for the [circulatory system](/source/circulatory_system) to [perfuse](/source/perfusion) and [oxygenate](/source/oxygenation_(medicine)), as well as higher [tissue](/source/tissue_(biology)) [density](/source/density) against [stress](/source/stress_(mechanics)). The external nature of muscle attachments also allows [thicker](/source/physiological_cross-sectional_area) and more diverse [muscle architecture](/source/muscle_architecture)s, as well as more versatile [range of motion](/source/range_of_motion)s.

==Overview==
A true endoskeleton is derived from [mesoderm](/source/mesoderm)al tissue. In three [phyla](/source/phylum) of animals, [Chordata](/source/Chordata) (chordates), [Echinoderm](/source/Echinoderm)ata (echinoderms) and [Porifera](/source/Porifera) (sponges), endoskeletons of various complexity are found. An endoskeleton may function purely for structural support (as in the case of Porifera), but often also serves as an attachment site for [muscle](/source/muscle)s and a mechanism for transmitting muscular forces as in chordates and echinoderms, which provides a means of [locomotion](/source/animal_locomotion).

Compared to the [exoskeleton](/source/exoskeleton) structure in many [invertebrate](/source/invertebrate)s (particularly [panarthropod](/source/panarthropod)s), the endoskeleton has several advantages:
* The capacity for larger body sizes under the same skeletal [mass](/source/mass), as the endoskeleton has a "flesh-over-bone" construct rather than a "flesh-in-bone" one as in exoskeletons. This means that the body's overall [volume](/source/volume) is not restricted by the endoskeleton itself, but by the [weight](/source/weight) of soft tissues that can be attached and supported by it, while the capacity of an exoskeleton's internal [cavity](/source/body_cavity) restricts how much [organ](/source/organ_(biology))s and tissues can be supported. Because of skeletal rigidity, many invertebrates have to repeatedly [moult](/source/moulting) ([ecdysis](/source/ecdysis)) during the [juvenile](/source/juvenile_(organism)) [stages of life](/source/biological_life_cycle) to grow bigger.
* Endoskeletons have a more concentrated layout due to its internalized nature, so a greater proportion of skeletal tissue can be recruited to handle [mechanical load](/source/mechanical_load)s. In contrast, exoskeletons are more "spread thin" over the exterior, meaning that when [stress](/source/mechanical_stress) is applied to one area of the body, most of the remaining exoskeleton often just plays "dead weight". Increasing the skeletal [strength](/source/mechanical_strength) of a local area often means having to increase the [cuticle](/source/cuticle) thickness and [density](/source/density) of an entire part of the body, which increase the overall weight significantly, especially with larger body sizes.
* Being internal means the skeletal tissue can be [perfuse](/source/perfusion)d and maintained from both inside (via [nutrient arteries](/source/nutrient_arteries) of the [marrow](/source/bone_marrow)) and outside (via [periosteal](/source/periosteal) [arteriole](/source/arteriole)s). The tissue catchment volume that the [circulatory system](/source/circulatory_system) is required to cover is also smaller than that of exoskeletons, making it easier to maintain skeletal health.
* Endoskeletons are typically cushioned from [trauma](/source/Injury) by the overlying soft tissues, while exoskeletons are directly exposed to external insults.
* Having other tissues attached outside the skeleton means that endoskeletons can have a more diverse [muscular layout](/source/muscle_architecture)s as well as bigger [physiological cross-sectional area](/source/physiological_cross-sectional_area), which translates to greater [contractile](/source/muscle_contraction) [strength](/source/physical_strength) and adaptability. Having external muscles also means the potential for greater [lever](/source/lever)age as the muscle can attach further down from a [joint](/source/joint) (comparatively, exoskeletal muscles cannot attach farther than the internal diameter of the corresponding joint cavity), although the muscles (especially [flexor](/source/flexor)s) themselves can sometimes physically hinder the joint's [range of motion](/source/range_of_motion).

=== Chordates ===
{{Skeleton}}
All [chordate](/source/chordate)s have a [notochord](/source/notochord), a flexible [glycoprotein](/source/glycoprotein) rod cross-wrapped by two [collagen](/source/collagen)-[elastin](/source/elastin) helices, which their [body plan](/source/body_plan)s develop around as [embryo](/source/embryo)s. With the exception of the [subphylum](/source/subphylum) [Tunicata](/source/Tunicata) (whose members only retain the notochord during [larva](/source/larva)l [stage](/source/biological_life_cycle)s and as [adult](/source/adult)s are either [soft-bodied](/source/soft-bodied_organism) or, in the case of [sea squirt](/source/sea_squirt)s, supported by a [cellulose](/source/cellulose) exoskeleton known as a [test](/source/test_(biology))), chordate bodies are developed along an [axial](/source/axial_skeleton) endoskeleton derived from the notochord. Like many macroscopically [motile](/source/motile) [bilaterian](/source/bilaterian) animals that need to be capable of sufficient [locomotive](/source/animal_locomotion) [propulsion](/source/propulsion), chordates evolved specialized [striated muscle](/source/striated_muscle)s over their endoskeletons, which have serialized [sarcomere](/source/sarcomere)s and parallel [myofibril](/source/myofibril)s bundled in [fascicle](/source/muscle_fascicle)s to both generate greater [force](/source/force) and optimize [contractile](/source/muscle_contraction) speed.

==== Cephalochordates ====
In the more [basal](/source/basal_(phylogenetics)) subphylum [Cephalochordata](/source/Cephalochordata) ([lancelet](/source/lancelet)s), the endoskeleton solely consists of a single notochord. Alternating muscle contractions bend the notochord from side to side, which stores and releases [elastic energy](/source/elastic_energy) like a [spring](/source/spring_(device)), resulting in a [body-caudal fin locomotion](/source/body-caudal_fin_locomotion) with better energy efficiency, although [extant](/source/extant_taxon) cephalochordates (only three [genera](/source/genera) with 32 [species](/source/species) from the family [Branchiostomatidae](/source/Branchiostomatidae)) are [burrowing](/source/burrowing) [filter feeder](/source/filter_feeder)s who mostly remain immobile in the [substrate](/source/substrate_(aquatic_environment)).

==== Vertebrates ====
Chordates in the [crown group](/source/crown_group) subphylum [Vertebrata](/source/Vertebrata) (i.e. ''vertebrates'', such as [fish](/source/fish), [amphibian](/source/amphibian)s, [reptile](/source/reptile)s, [bird](/source/bird)s and [mammal](/source/mammal)s), the endoskeleton is greatly expanded. During [embryonic development](/source/embryonic_development), the notochord becomes [segment](/source/body_segment)ally replaced by a much tougher [vertebral column](/source/vertebral_column) (i.e. the ''spine'') composed of stiffer [structural element](/source/structural_element)s called [vertebra](/source/vertebra)e. Notochord [remnant](/source/vestigiality)s are transformed into [intervertebral disc](/source/intervertebral_disc)s, which give some [range of motion](/source/range_of_motion) between the adjacent vertebrae, allowing the overall spinal column to flex and rotate. The vertebrate endoskeleton is made up of two types of [mineralized tissues](/source/mineralized_tissues), i.e. [bone](/source/bone) and [cartilage](/source/cartilage), with the [joint](/source/joint)s reinforced by [ligament](/source/ligament)s made of [Type I collagen](/source/Type_I_collagen). Unlike the singular axial skeleton of cephalochordates, the vertebrate skeletal elements expand axially, ventrally and laterally to form the [cranium](/source/cranium), [rib cage](/source/rib_cage) and [appendicular skeleton](/source/appendicular_skeleton), giving vertebrates a much more widened endoskeleton.

Vertebrates also have bulkier, more complexly organized striated muscles called [skeletal muscle](/source/skeletal_muscle)s inserted over both the axial and appendicular skeletons, which can transmit significant forces via [dense connective tissue](/source/dense_connective_tissue) cords/bands called [tendon](/source/tendon)s and [aponeuroses](/source/aponeuroses). In [terrestrial](/source/terrestrial_animal) vertebrates ([tetrapod](/source/tetrapod)s), both the axial and ''especially'' the appendicular endoskeleton (the latter of which [evolve](/source/evolution)d into [limb](/source/limb_(anatomy)) skeletons) have become significantly strengthened to adapt for the added burden of [gravity](/source/gravity) and [locomotion on dry land](/source/terrestrial_locomotion), as their bodies' weight is not offset by [buoyancy](/source/buoyancy) as in aquatic environments. In some vertebrate species, parts of the endoskeleton become specialized for [flight](/source/animal_flight) (as [wing](/source/wing)s), [balance](/source/balance_(ability)) (in [arboreal](/source/arboreal) species), [communication](/source/animal_communication) (as [vocalization](/source/animal_language)s or [fin](/source/fish_fin)/[sail](/source/neural_spinal_sail)/[crest](/source/crest_(anatomy)) [display](/source/display_(zoology))), [hearing](/source/hearing) ([mammalian](/source/mammalian) [ossicle](/source/ossicle)s), [digestion](/source/digestion) (particularly [mastication](/source/mastication)) and [prehensility](/source/prehensility) ([grasping](/source/grasping), [object manipulation](/source/object_manipulation) and [fine motor activities](/source/fine_motor_skill)).

The combination of a more [robust](/source/robust) endoskeleton and a stronger, more versatile [muscular system](/source/muscular_system), supported by a [heart](/source/heart)-pumped [closed circulatory system](/source/closed_circulatory_system), a [myelin](/source/myelin)ated [nervous system](/source/nervous_system) with faster [saltatory conduction](/source/saltatory_conduction)s (in all [jawed vertebrate](/source/jawed_vertebrate)s) and [centralized](/source/centralized) neural control by an highly functional [brain](/source/brain), have allowed the vertebrates to achieve much larger body sizes than [invertebrate](/source/invertebrate)s while still maintaining responsive [sensory perception](/source/sensory_perception) and [motor control](/source/motor_control). As a result, vertebrates have gradually dominated all [high-level](/source/trophic_level) [niche](/source/ecological_niche)s in both [aquatic](/source/aquatic_ecosystem) and [terrestrial ecosystem](/source/terrestrial_ecosystem)s since the [Devonian](/source/Devonian) (circa. 420-359&nbsp;[Mya](/source/million_years_ago)).

=== Echinoderms ===
Echinoderms have a [mesoderm](/source/mesoderm)al skeleton in the [dermis](/source/dermis), composed of [calcite](/source/calcite)-based plates known as [ossicle](/source/ossicle_(echinoderm))s, which form a porous structure known as [stereom](/source/stereom).<ref>{{cite book |last1=Behrens |first1=Peter |last2=Bäuerlein |first2=Edmund |title=Handbook of Biomineralization: Biomimetic and bioinspired chemistry' |year=2007 |publisher=Wiley-VCH |isbn=978-3-527-31805-6 |page=393 }}</ref><ref>{{cite book | last1=Brusca | first1=Richard C. | last2=Moore | first2=Wendy | last3=Shuster | first3=Stephen M. | title=Invertebrates |edition=3rd | publisher = Sinauer Associates | publication-place=Sunderland, Massachusetts | date=2016 | isbn=978-1-60535-375-3 | oclc=928750550 |pages=979–980}}</ref> In [sea urchin](/source/sea_urchin)s, the ossicles are fused together into a [test](/source/test_(biology)), while in the arms of [sea star](/source/sea_star)s, [brittle star](/source/brittle_star)s and [crinoid](/source/crinoid)s (sea lilies) they articulate to form flexible joints. The ossicles may bear external projections in the form of [spine](/source/spine_(zoology))s, granules or warts that are supported by a tough [epidermis](/source/epidermis_(zoology)). Echinoderm skeletal elements are sometimes deployed in specialized ways such as the [chewing](/source/chewing) organ in sea urchins called "[Aristotle's lantern](/source/Aristotle's_lantern)", the supportive stalks of crinoids, and the structural "lime ring" of [sea cucumber](/source/sea_cucumber)s.<ref>{{cite book |last1=Ruppert |first1=Edward E. |last2=Fox |first2=Richard S. |last3=Barnes |first3=Robert D. |title=Invertebrate Zoology |edition=7th |year=2004 |publisher=Cengage Learning |isbn=81-315-0104-3|page=873 }}</ref>

=== Sponges===
The poriferan "skeleton" consists of mesh-like network of microscopic [spicule](/source/sponge_spicule)s. The soft [connective tissue](/source/connective_tissue)s of sponges are composed of gelatinous [mesohyl](/source/mesohyl) reinforced by fibrous [spongin](/source/spongin), forming a [composite](/source/composite_material) [matrix](/source/matrix_(biology)) that has decent [tensile strength](/source/tensile_strength) but severely lacks the [rigidity](/source/stiffness) needed to resist [deformation](/source/deformation_(engineering)) from [ocean current](/source/ocean_current)s. The spicules act as [structural element](/source/structural_element)s that add much needed [compressive](/source/compressive_strength) and [shear strength](/source/shear_strength)s that help maintain the sponge's shape (which is needed to ensure optimal [filter feeding](/source/filter_feeding)), much like the [aggregate](/source/aggregate_(composite))s and [rebar stirrup](/source/rebar)s within [reinforced concrete](/source/reinforced_concrete). Sponges can have spicules made of [calcium carbonate](/source/calcium_carbonate) ([calcite](/source/calcite) or [aragonite](/source/aragonite)) or more commonly [silica](/source/silica), which separate sponges into two main [clade](/source/clade)s, [calcareous sponge](/source/calcareous_sponge)s ([class](/source/class_(biology)) [Calcarea](/source/Calcarea)) and [siliceous sponge](/source/siliceous_sponge)s, the latter being the dominant extant clade with two classes [Demospongiae](/source/Demospongiae) ([common sponge](/source/common_sponge)s) and [Hexactinellida](/source/Hexactinellida) ([glass sponge](/source/glass_sponge)s). There are however species (such as [bath sponge](/source/Spongia_officinalis) and [lake sponge](/source/Spongilla_lacustris)) that have no or severely reduced spicules, which gives them an overall soft "spongy" structure.

Deep-sea demosponges from the family [Cladorhizidae](/source/Cladorhizidae) have evolved a unique [carnivorous](/source/carnivorous) survival strategy, by having tiny [grappling hook](/source/grappling_hook)-like spicules ([microsclere](/source/microsclere)s) that extends outwards like [bur](/source/bur)s to snag and trap passing-by aquatic animals such as small fish and [crustacean](/source/crustacean)s. As sponges don't have dedicated [digestive system](/source/digestive_system)s, these predatory sponges rely on [symbiotic](/source/symbiotic) organisms such as [scale worm](/source/scale_worm)s and [microbe](/source/microbe)s to help digest the seized prey and release [nutrient](/source/nutrient)s that can then be absorbed by the sponges' cells.

=== Coleoids ===
The [Coleoidea](/source/Coleoidea), a [subclass](/source/subclass_(taxonomy)) of [cephalopod](/source/cephalopod) [mollusc](/source/mollusc)s who [evolve](/source/evolution)d an internalized [shell](/source/mollusc_shell), do not have a true endoskeleton in the physiological sense. The internal shell has evolved into a [buoyancy](/source/buoyancy) [organ](/source/organ_(biology)) called the [gladius](/source/gladius_(cephalopod)) or [cuttlebone](/source/cuttlebone), which may provide muscle attachment but does ''not'' support the cephalopod's body shape (which is maintained solely by a [hydroskeleton](/source/hydroskeleton)). Coleoids from the [order](/source/order_(biology)) [Octopoda](/source/Octopus) (octopuses) even have lost that internalized shell completely.

== Gallery ==
<gallery>
File:Human skeleton -Booth Museum, Brighton and Hove, East Sussex, England-20Oct2011.jpg|A [human skeleton](/source/human_skeleton) on display at [Booth Museum of Natural History](/source/Booth_Museum_of_Natural_History)
File:Orionides.jpg|[Fossil](/source/Fossil)ized skeleton of various [dinosaur](/source/dinosaur)s
File:Kitefin Shark.jpg|The skeleton of a [kitefin shark](/source/kitefin_shark), a [cartilaginous fish](/source/cartilaginous_fish)
File:Branchiostoma (I1342) (29014085923).jpg|The [notochord](/source/notochord) endoskeleton of ''[Branchiostoma](/source/Branchiostoma)'', a [cephalochordate](/source/cephalochordate) ([lancelet](/source/lancelet)s)
File:Starfish 9-legged skeleton ThE.jpg|The dermal ossicles of a [starfish](/source/starfish), an echinoderm
File:Expn4384 (27840605922).jpg|The silica spicule skeleton of a [Venus' flower basket](/source/Venus'_flower_basket), a [glass sponge](/source/glass_sponge)
</gallery>

==See also==
*[Exoskeleton](/source/Exoskeleton)
*[Hydrostatic skeleton](/source/Hydrostatic_skeleton)

==References==
{{Reflist}}
{{Authority control}}

Category:Animal anatomy
Category:Biomechanics
Category:Skeletal system
Category:Zoology

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Adapted from the Wikipedia article [Endoskeleton](https://en.wikipedia.org/wiki/Endoskeleton) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Endoskeleton?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.