Editing Endoskeleton

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

An '''endoskeleton''' (From [[Ancient Greek]] ἔνδον, éndon = "within", "inner" + σκελετός, skeletos = "skeleton") is a [[structural frame]] ([[skeleton]]) — usually composed of [[mineralized tissues|mineralized tissue]] — on the inside of an [[animal]], overlaid by [[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]] against [[gravity]] and [[mechanical load]]s, and provide anchoring attachment sites for [[skeletal muscle]]s to transmit force and allow movements and [[animal locomotion|locomotion]].
 
[[Vertebrate]]s and the closely related [[cephalochordate]]s are the predominant animal [[clade]] with endoskeletons (made of mostly [[bone]] and sometimes [[cartilage]], as well as [[notochord]]al [[glycoprotein]] and [[collagen fiber]]s), although [[invertebrate]]s such as [[sponge]]s also have evolved a form of "[[rebar]]" endoskeletons made of diffuse meshworks of [[calcite]]/[[silica]] [[structural element]]s called [[sponge spicule|spicule]]s, and [[echinoderm]]s have a [[dermal]] calcite endoskeleton known as [[ossicle (echinoderm)|ossicle]]s. Some [[coleoid]] [[cephalopod]]s ([[squid]]s and [[cuttlefish]]) have an internalized [[vestigial]] [[aragonite]]/calcite-[[chitin]] [[mollusc shell|shell]] known as [[gladius (cephalopod)|gladius]] or [[cuttlebone]], which can serve as muscle attachments but the main function is often to maintain [[buoyancy]] rather than to give structural support, and their body shape is largely maintained by [[hydroskeleton]].

Compared to the [[exoskeleton]]s of many invertebrates, endoskeletons allow much larger overall body sizes for the same skeletal [[mass]], as most soft tissues and [[organ (biology)|organ]]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]], making it easier for the [[circulatory system]] to [[perfusion|perfuse]] and [[oxygenation (medicine)|oxygenate]], as well as higher [[tissue (biology)|tissue]] [[density]] against [[stress (mechanics)|stress]]. The external nature of muscle attachments also allows [[physiological cross-sectional area|thicker]] and more diverse [[muscle architecture]]s, as well as more versatile [[range of motion]]s.

==Overview==
A true endoskeleton is derived from [[mesoderm]]al tissue. In three [[phylum|phyla]] of animals, [[Chordata]] (chordates), [[Echinoderm]]ata (echinoderms) and [[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]]s and a mechanism for transmitting muscular forces as in chordates and echinoderms, which provides a means of [[animal locomotion|locomotion]].

Compared to the [[exoskeleton]] structure in many [[invertebrate]]s (particularly [[panarthropod]]s), the endoskeleton has several advantages:
* The capacity for larger body sizes under the same skeletal [[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]] is not restricted by the endoskeleton itself, but by the [[weight]] of soft tissues that can be attached and supported by it, while the capacity of an exoskeleton's internal [[body cavity|cavity]] restricts how much [[organ (biology)|organ]]s and tissues can be supported. Because of skeletal rigidity, many invertebrates have to repeatedly [[moulting|moult]] ([[ecdysis]]) during the [[juvenile (organism)|juvenile]] [[biological life cycle|stages of life]] 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]]s. In contrast, exoskeletons are more "spread thin" over the exterior, meaning that when [[mechanical stress|stress]] is applied to one area of the body, most of the remaining exoskeleton often just plays "dead weight". Increasing the skeletal [[mechanical strength|strength]] of a local area often means having to increase the [[cuticle]] thickness and [[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 [[perfusion|perfuse]]d and maintained from both inside (via [[nutrient arteries]] of the [[bone marrow|marrow]]) and outside (via [[periosteal]] [[arteriole]]s). The tissue catchment volume that the [[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 [[Injury|trauma]] 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 [[muscle architecture|muscular layout]]s as well as bigger [[physiological cross-sectional area]], which translates to greater [[muscle contraction|contractile]] [[physical strength|strength]] and adaptability. Having external muscles also means the potential for greater [[lever]]age as the muscle can attach further down from a [[joint]] (comparatively, exoskeletal muscles cannot attach farther than the internal diameter of the corresponding joint cavity), although the muscles (especially [[flexor]]s) themselves can sometimes physically hinder the joint's [[range of motion]].

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

==== Cephalochordates ====
In the more [[basal (phylogenetics)|basal]] subphylum [[Cephalochordata]] ([[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]] like a [[spring (device)|spring]], resulting in a [[body-caudal fin locomotion]] with better energy efficiency, although [[extant taxon|extant]] cephalochordates (only three [[genera]] with 32 [[species]] from the family [[Branchiostomatidae]]) are [[burrowing]] [[filter feeder]]s who mostly remain immobile in the [[substrate (aquatic environment)|substrate]].

==== Vertebrates ====
Chordates in the [[crown group]] subphylum [[Vertebrata]] (i.e. ''vertebrates'', such as [[fish]], [[amphibian]]s, [[reptile]]s, [[bird]]s and [[mammal]]s), the endoskeleton is greatly expanded. During [[embryonic development]], the notochord becomes [[body segment|segment]]ally replaced by a much tougher [[vertebral column]] (i.e. the ''spine'') composed of stiffer [[structural element]]s called [[vertebra]]e. Notochord [[vestigiality|remnant]]s are transformed into [[intervertebral disc]]s, which give some [[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]], i.e. [[bone]] and [[cartilage]], with the [[joint]]s reinforced by [[ligament]]s made of [[Type I collagen]]. Unlike the singular axial skeleton of cephalochordates, the vertebrate skeletal elements expand axially, ventrally and laterally to form the [[cranium]], [[rib cage]] and [[appendicular skeleton]], giving vertebrates a much more widened endoskeleton.

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

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

=== Echinoderms ===
Echinoderms have a [[mesoderm]]al skeleton in the [[dermis]], composed of [[calcite]]-based plates known as [[ossicle (echinoderm)|ossicle]]s, which form a porous structure known as [[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 |pp=979–980}}</ref> In [[sea urchin]]s, the ossicles are fused together into a [[test (biology)|test]], while in the arms of [[sea star]]s, [[brittle star]]s and [[crinoid]]s (sea lilies) they articulate to form flexible joints. The ossicles may bear external projections in the form of [[spine (zoology)|spine]]s, granules or warts that are supported by a tough [[epidermis (zoology)|epidermis]]. Echinoderm skeletal elements are sometimes deployed in specialized ways such as the [[chewing]] organ in sea urchins called "[[Aristotle's lantern]]", the supportive stalks of crinoids, and the structural "lime ring" of [[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|p=873 }}</ref>

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

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

=== Coleoids ===
The [[Coleoidea]], a [[subclass (taxonomy)|subclass]] of [[cephalopod]] [[mollusc]]s who [[evolution|evolve]]d an internalized [[mollusc shell|shell]], do not have a true endoskeleton in the physiological sense. The internal shell has evolved into a [[buoyancy]] [[organ (biology)|organ]] called the [[gladius (cephalopod)|gladius]] or [[cuttlebone]], which may provide muscle attachment but does ''not'' support the cephalopod's body shape (which is maintained solely by a [[hydroskeleton]]). Coleoids from the [[order (biology)|order]] [[Octopus|Octopoda]] (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]] on display at [[Booth Museum of Natural History]]
File:Orionides.jpg|[[Fossil]]ized skeleton of various [[dinosaur]]s
File:Kitefin Shark.jpg|The skeleton of a [[kitefin shark]], a [[cartilaginous fish]]
File:Branchiostoma (I1342) (29014085923).jpg|The [[notochord]] endoskeleton of ''[[Branchiostoma]]'', a [[cephalochordate]] ([[lancelet]]s)
File:Starfish 9-legged skeleton ThE.jpg|The dermal ossicles of a [[starfish]], an echinoderm
File:Expn4384 (27840605922).jpg|The silica spicule skeleton of a [[Venus' flower basket]], a [[glass sponge]]
</gallery>

==See also==
*[[Exoskeleton]]
*[[Hydrostatic skeleton]]

==References==
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[[Category:Animal anatomy]]
[[Category:Biomechanics]]
[[Category:Skeletal system]]
[[Category:Zoology]]