A skeleton is the structural frame that supports the body of an
animal
Animals are multicellular, eukaryotic organisms in the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic material, Cellular respiration#Aerobic respiration, breathe oxygen, are Motilit ...
. There are several types of skeletons, including the
exoskeleton
An exoskeleton (from Greek ''éxō'' "outer" and ''skeletós'' "skeleton") is an external skeleton that supports and protects an animal's body, in contrast to an internal skeleton (endoskeleton) in for example, a human. In usage, some of the ...
, which is the stable outer shell of an organism, the
endoskeleton
An endoskeleton (From Greek ἔνδον, éndon = "within", "inner" + σκελετός, skeletos = "skeleton") is an internal support structure of an animal, composed of mineralized tissue.
Overview
An endoskeleton is a skeleton that is on the i ...
, which forms the support structure inside the body, and the
hydroskeleton
A hydrostatic skeleton, or hydroskeleton, is a flexible skeleton supported by fluid pressure. Hydrostatic skeletons are common among simple invertebrate organisms. While more advanced organisms can be considered hydrostatic, they are sometimes refe ...
, a flexible internal skeleton supported by fluid pressure.
Vertebrate
Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, ...
s are animals with a
vertebral column
The vertebral column, also known as the backbone or spine, is part of the axial skeleton. The vertebral column is the defining characteristic of a vertebrate in which the notochord (a flexible rod of uniform composition) found in all chordata, ...
, and their skeletons are typically composed of bone and cartilage.
Invertebrate
Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chordate ...
s are animals that lack a vertebral column. The skeletons of invertebrates vary, including hard exoskeleton shells, plated endoskeletons, or
spicules
Spicules are any of various small needle-like anatomical structures occurring in organisms
Spicule may also refer to:
*Spicule (sponge), small skeletal elements of sea sponges
*Spicule (nematode), reproductive structures found in male nematodes ( ...
. Cartilage is a rigid
connective tissue
Connective tissue is one of the four primary types of animal tissue, along with epithelial tissue, muscle tissue, and nervous tissue. It develops from the mesenchyme derived from the mesoderm the middle embryonic germ layer. Connective tiss ...
that is found in the skeletal systems of vertebrates and invertebrates.
Etymology
The term ''skeleton'' comes . ''Sceleton'' is an archaic form of the word.
Classification
Skeletons can be defined by several attributes. Solid skeletons consist of hard substances, such as
bone
A bone is a Stiffness, rigid Organ (biology), organ that constitutes part of the skeleton in most vertebrate animals. Bones protect the various other organs of the body, produce red blood cell, red and white blood cells, store minerals, provid ...
,
cartilage
Cartilage is a resilient and smooth type of connective tissue. In tetrapods, it covers and protects the ends of long bones at the joints as articular cartilage, and is a structural component of many body parts including the rib cage, the neck an ...
, or
cuticle
A cuticle (), or cuticula, is any of a variety of tough but flexible, non-mineral outer coverings of an organism, or parts of an organism, that provide protection. Various types of "cuticle" are non- homologous, differing in their origin, structu ...
. These can be further divided by location; internal skeletons are endoskeletons, and external skeletons are exoskeletons. Skeletons may also be defined by rigidity, where pliant skeletons are more elastic than rigid skeletons.
[.] Fluid or
hydrostatic skeleton
A hydrostatic skeleton, or hydroskeleton, is a flexible skeleton supported by fluid pressure. Hydrostatic skeletons are common among simple invertebrate organisms. While more advanced organisms can be considered hydrostatic, they are sometimes refe ...
s do not have hard structure like solid skeletons, instead functioning via pressurized fluids. Hydrostatic skeletons are always internal.
Exoskeletons
Exoskeletons are external skeletons that cover the body of an animal, serving as armor to protect an animal from predators. Arthropods have exoskeletons that encase their bodies and have to undergo periodic
moult
In biology, moulting (British English), or molting (American English), also known as sloughing, shedding, or in many invertebrates, ecdysis, is the manner in which an animal routinely casts off a part of its body (often, but not always, an outer ...
ing or
ecdysis
Ecdysis is the moulting of the cuticle in many invertebrates of the clade Ecdysozoa. Since the cuticle of these animals typically forms a largely inelastic exoskeleton, it is shed during growth and a new, larger covering is formed. The remna ...
as the animal grows. The
shells of
molluscs
Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant taxon, extant species of molluscs are recognized. The number of fossil sp ...
are another form of exoskeleton.
Exoskeletons provide a surface for the attachment of muscles, and specialized appendanges of the exoskeleton can assist with movement and defense. In arthropods, the exoskeleton also assists with
sensory perception
Perception () is the organization, identification, and interpretation of sensory information in order to represent and understand the presented information or environment. All perception involves signals that go through the nervous system ...
.
An external skeleton can be quite heavy in relation to the overall mass of an animal, so on land, organisms that have an exoskeleton are mostly relatively small. Somewhat larger aquatic animals can support an exoskeleton because weight is less of a consideration underwater. The
southern giant clam
''Tridacna derasa'', the southern giant clam or smooth giant clam, is a species of extremely large marine clam in the family Cardiidae.
Description
The southern giant clam is one of the largest of the "giant clams", reaching up to 60 cm in ...
, a species of extremely large saltwater clam in the Pacific Ocean, has a shell that is massive in both size and weight. ''
Syrinx aruanus
''Syrinx aruanus'', common name the Australian trumpet or false trumpet, is a species of extremely large sea snail measuring up to 91 cm long and weighing up to 18 kg. It is a marine gastropod mollusk in the family Turbinellidae, and ...
'' is a species of sea snail with a very large shell.
Endoskeletons
Endoskeletons are the internal support structure of an animal, composed of
mineralized tissues
Mineralized tissues are biological tissues that incorporate minerals into soft matrices. Typically these tissues form a protective shield or structural support. Bone, mollusc shells, deep sea sponge ''Euplectella'' species, radiolarians, diato ...
, such as the bone skeletons found in most vertebrates.
Endoskeletons are highly specialized and vary significantly between animals.
They vary in complexity from functioning purely for support (as in the case of
sponges
Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the diploblasts. They are multicellular organisms that have bodies full of pores and channels allowing water to circulate through ...
), to serving as an attachment site for muscles and a mechanism for transmitting muscular forces. A true endoskeleton is derived from
mesodermal
The mesoderm is the middle layer of the three germ layers that develops during gastrulation in the very early development of the embryo of most animals. The outer layer is the ectoderm, and the inner layer is the endoderm.Langman's Medical Embry ...
tissue. Endoskeletons occur in
chordate
A chordate () is an animal of the phylum Chordata (). All chordates possess, at some point during their larval or adult stages, five synapomorphies, or primary physical characteristics, that distinguish them from all the other taxa. These fiv ...
s, echinoderms, and sponges.
Rigidity
Pliant skeletons are capable of movement; thus, when
stress
Stress may refer to:
Science and medicine
* Stress (biology), an organism's response to a stressor such as an environmental condition
* Stress (linguistics), relative emphasis or prominence given to a syllable in a word, or to a word in a phrase ...
is applied to the skeletal structure, it deforms and then regains its original shape. This skeletal structure is used in some invertebrates, for instance in the hinge of
bivalve shell
A bivalve shell is part of the body, the exoskeleton or shell, of a bivalve mollusk. In life, the shell of this class of mollusks is composed of two hinged parts or ''valves''. Bivalves are very common in essentially all aquatic locales, including ...
s or the
mesoglea
Mesoglea refers to the extracellular matrix found in cnidarians like coral or jellyfish that functions as a hydrostatic skeleton. It is related to but distinct from mesohyl, which generally refers to extracellular material found in sponges.
Desc ...
of
cnidarians
Cnidaria () is a phylum under kingdom Animalia containing over 11,000 species of aquatic animals found both in freshwater and marine environments, predominantly the latter.
Their distinguishing feature is cnidocytes, specialized cells that th ...
such as
jellyfish
Jellyfish and sea jellies are the informal common names given to the medusa-phase of certain gelatinous members of the subphylum Medusozoa, a major part of the phylum Cnidaria. Jellyfish are mainly free-swimming marine animals with umbrella- ...
. Pliant skeletons are beneficial because only
muscle
Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of muscl ...
contractions are needed to bend the skeleton; upon muscle relaxation, the skeleton will return to its original shape.
Cartilage
Cartilage is a resilient and smooth type of connective tissue. In tetrapods, it covers and protects the ends of long bones at the joints as articular cartilage, and is a structural component of many body parts including the rib cage, the neck an ...
is one material that a pliant skeleton may be composed of, but most pliant skeletons are formed from a mixture of
proteins
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...
,
polysaccharides
Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrates found in food. They are long chain polymeric carbohydrates composed of monosaccharide units bound together by glycosidic linkages. This carbohydrate can react with w ...
, and water.
For additional structure or protection, pliant skeletons may be supported by rigid skeletons. Organisms that have pliant skeletons typically live in water, which supports body structure in the absence of a rigid skeleton.
Rigid skeletons are not capable of movement when stressed, creating a strong support system most common in
terrestrial animals
Terrestrial animals are animals that live predominantly or entirely on land (e.g. cats, dogs, ants, spiders), as compared with aquatic animals, which live predominantly or entirely in the water (e.g. fish, lobsters, octopuses), and amphibians, ...
. Such a skeleton type used by animals that live in water are more for protection (such as
barnacle and
snail
A snail is, in loose terms, a shelled gastropod. The name is most often applied to land snails, terrestrial pulmonate gastropod molluscs. However, the common name ''snail'' is also used for most of the members of the molluscan class Gastro ...
shells) or for fast-moving animals that require additional support of musculature needed for swimming through water. Rigid skeletons are formed from materials including
chitin
Chitin ( C8 H13 O5 N)n ( ) is a long-chain polymer of ''N''-acetylglucosamine, an amide derivative of glucose. Chitin is probably the second most abundant polysaccharide in nature (behind only cellulose); an estimated 1 billion tons of chit ...
(in arthropods),
calcium
Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to ...
compounds such as
calcium carbonate (in
stony coral
Scleractinia, also called stony corals or hard corals, are marine animals in the phylum Cnidaria that build themselves a hard skeleton. The individual animals are known as polyps and have a cylindrical body crowned by an oral disc in which a ...
s and
mollusk
Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is e ...
s) and
silicate
In chemistry, a silicate is any member of a family of polyatomic anions consisting of silicon and oxygen, usually with the general formula , where . The family includes orthosilicate (), metasilicate (), and pyrosilicate (, ). The name is al ...
(for
diatom
A diatom (Neo-Latin ''diatoma''), "a cutting through, a severance", from el, διάτομος, diátomos, "cut in half, divided equally" from el, διατέμνω, diatémno, "to cut in twain". is any member of a large group comprising sev ...
s and
radiolarian
The Radiolaria, also called Radiozoa, are protozoa of diameter 0.1–0.2 mm that produce intricate mineral skeletons, typically with a central capsule dividing the cell into the inner and outer portions of endoplasm and ectoplasm. The elab ...
s).
Hydrostatic skeletons
Hydrostatic skeletons are flexible cavities within an animal that provide structure through fluid pressure, occurring in some types of
soft-bodied organisms, including jellyfish, flatworms, nematodes, and earthworms. The walls of these cavities are made of muscle and connective tissue.
In addition to providing structure for an animal's body, hydrostatic skeletons transmit the forces of muscle contraction, allowing an animal to move by alternating contractions and expansions of muscles along the animal's length.
Cytoskeleton
The cytoskeleton (''cyto-'' meaning cell) is used to stabilize and preserve the form of the cells. It is a dynamic structure that maintains cell shape, protects the cell, enables cellular motion using structures such as
flagella
A flagellum (; ) is a hairlike appendage that protrudes from certain plant and animal sperm cells, and from a wide range of microorganisms to provide motility. Many protists with flagella are termed as flagellates.
A microorganism may have f ...
,
cilia
The cilium, plural cilia (), is a membrane-bound organelle found on most types of eukaryotic cell, and certain microorganisms known as ciliates. Cilia are absent in bacteria and archaea. The cilium has the shape of a slender threadlike projecti ...
and
lamellipodia
The lamellipodium (plural lamellipodia) (from Latin ''lamella'', related to ', "thin sheet", and the Greek radical ''pod-'', "foot") is a cytoskeletal protein actin projection on the leading edge of the cell. It contains a quasi-two-dimensional ...
, and transport within cells such as the movement of
vesicles
Vesicle may refer to:
; In cellular biology or chemistry
* Vesicle (biology and chemistry), a supramolecular assembly of lipid molecules, like a cell membrane
* Synaptic vesicle
; In human embryology
* Vesicle (embryology), bulge-like features o ...
and
organelle
In cell biology, an organelle is a specialized subunit, usually within a cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts of cells, as organs are to the body, hence ''organelle,'' the ...
s, and plays a role in cellular division. The cytoskeleton is not a skeleton in the sense that it provides the structural system for the body of an animal; rather, it serves a similar function at the cellular level.
Vertebrate skeletons
![Huxley - Mans Place in Nature](https://upload.wikimedia.org/wikipedia/commons/9/91/Huxley_-_Mans_Place_in_Nature.jpg)
In most vertebrates, the main skeletal component is bone.
Bones compose a unique skeletal system for each type of animal. Another important component is cartilage which in
mammal
Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...
s is found mainly in the joint areas. In other animals, such as the
cartilaginous fishes, which include the
shark
Sharks are a group of elasmobranch fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachimo ...
s, the skeleton is composed entirely of
cartilage
Cartilage is a resilient and smooth type of connective tissue. In tetrapods, it covers and protects the ends of long bones at the joints as articular cartilage, and is a structural component of many body parts including the rib cage, the neck an ...
. The
segmental pattern of the skeleton is present in all vertebrates, with basic units being repeated, such as in the vertebral column and the ribcage.
Bones are rigid
organs
In biology, an organ is a collection of tissues joined in a structural unit to serve a common function. In the hierarchy of life, an organ lies between tissue and an organ system. Tissues are formed from same type cells to act together in a f ...
that form part of the endoskeleton of vertebrates. They provide structural support for the body, assist in movement by opposing muscular contraction, and create a protective wall around internal organs. Bones are primarily made of inorganic minerals, such as
hydroxyapatite
Hydroxyapatite, also called hydroxylapatite (HA), is a naturally occurring mineral form of calcium apatite with the formula Ca5(PO4)3(OH), but it is usually written Ca10(PO4)6(OH)2 to denote that the crystal unit cell comprises two entities. ...
, while the remainder is made of an organic matrix and water. The hollow tubular structure of bones provide considerable resistance against compression while staying lightweight. Most cells in bones are either
osteoblast
Osteoblasts (from the Greek language, Greek combining forms for "bone", ὀστέο-, ''osteo-'' and βλαστάνω, ''blastanō'' "germinate") are cell (biology), cells with a single Cell nucleus, nucleus that synthesize bone. However, in the p ...
s,
osteoclast
An osteoclast () is a type of bone cell that breaks down bone tissue. This function is critical in the maintenance, repair, and remodeling of bones of the vertebral skeleton. The osteoclast disassembles and digests the composite of hydrated pro ...
s, or
osteocyte
An osteocyte, an oblate shaped type of bone cell with dendritic processes, is the most commonly found cell in mature bone. It can live as long as the organism itself. The adult human body has about 42 billion of them. Osteocytes do not divide and ...
s.
Bone tissue is a type of dense
connective tissue
Connective tissue is one of the four primary types of animal tissue, along with epithelial tissue, muscle tissue, and nervous tissue. It develops from the mesenchyme derived from the mesoderm the middle embryonic germ layer. Connective tiss ...
. One of the types of tissue that makes up bone tissue is
mineralized tissue
Mineralized tissues are biological tissues that incorporate minerals into soft matrices. Typically these tissues form a protective shield or structural support. Bone, mollusc shells, deep sea sponge ''Euplectella'' species, radiolarians, diato ...
and this gives it rigidity and a
honeycomb-like three-dimensional internal structure. Bones also produce
red
Red is the color at the long wavelength end of the visible spectrum of light, next to orange and opposite violet. It has a dominant wavelength of approximately 625–740 nanometres. It is a primary color in the RGB color model and a secondar ...
and
white blood cell
White blood cells, also called leukocytes or leucocytes, are the cell (biology), cells of the immune system that are involved in protecting the body against both infectious disease and foreign invaders. All white blood cells are produced and de ...
s and serve as calcium and phosphate storage at the cellular level. Other types of tissue found in bones include
marrow,
endosteum
The endosteum (plural endostea) is a thin vascular membrane of connective tissue that lines the inner surface of the bony tissue that forms the medullary cavity of long bones.
This endosteal surface is usually resorbed during long periods of ma ...
and
periosteum
The periosteum is a membrane that covers the outer surface of all bones, except at the articular surfaces (i.e. the parts within a joint space) of long bones. Endosteum lines the inner surface of the medullary cavity of all long bones.
Structure ...
,
nerve
A nerve is an enclosed, cable-like bundle of nerve fibers (called axons) in the peripheral nervous system.
A nerve transmits electrical impulses. It is the basic unit of the peripheral nervous system. A nerve provides a common pathway for the e ...
s,
blood vessel
The blood vessels are the components of the circulatory system that transport blood throughout the human body. These vessels transport blood cells, nutrients, and oxygen to the tissues of the body. They also take waste and carbon dioxide away ...
s and cartilage.
During
embryonic development
An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sperm ...
, bones are developed individually from skeletogenic cells in the ectoderm and mesoderm. Most of these cells develop into separate bone, cartilage, and joint cells, and they are then articulated with one another. Specialized skeletal tissues are unique to vertebrates. Cartilage grows more quickly than bone, causing it to be more prominent earlier in an animal's life before it is overtaken by bone. Cartilage is also used in vertebrates to resist stress at points of articulation in the skeleton. Cartilage in vertebrates is usually encased in
perichondrium
The perichondrium (from Greek el, περί, peri, around, label=none and el, χόνδρος, chondros, cartilage, label=none) is a layer of dense irregular connective tissue that surrounds the cartilage of developing bone. It consists of two sep ...
tissue.
Ligament
A ligament is the fibrous connective tissue that connects bones to other bones. It is also known as ''articular ligament'', ''articular larua'', ''fibrous ligament'', or ''true ligament''. Other ligaments in the body include the:
* Peritoneal li ...
s are elastic tissues that connect bones to other bones, and
tendon
A tendon or sinew is a tough, high-tensile-strength band of dense fibrous connective tissue that connects muscle to bone. It is able to transmit the mechanical forces of muscle contraction to the skeletal system without sacrificing its ability ...
s are elastic tissues that connect muscles to bones.
Amphibians and reptiles
The skeletons of turtles have evolved to develop a
shell
Shell may refer to:
Architecture and design
* Shell (structure), a thin structure
** Concrete shell, a thin shell of concrete, usually with no interior columns or exterior buttresses
** Thin-shell structure
Science Biology
* Seashell, a hard o ...
from the ribcage, forming an exoskeleton. The skeletons of
snake
Snakes are elongated, Limbless vertebrate, limbless, carnivore, carnivorous reptiles of the suborder Serpentes . Like all other Squamata, squamates, snakes are ectothermic, amniote vertebrates covered in overlapping Scale (zoology), scales. Ma ...
s and
caecilian
Caecilians (; ) are a group of limbless, vermiform or serpentine amphibians. They mostly live hidden in the ground and in stream substrates, making them the least familiar order of amphibians. Caecilians are mostly distributed in the tropics of ...
s have significantly more vertebrae than other animals. Snakes often have over 300, compared to the 65 that is typical in lizards.
Birds
The skeletons of birds are adapted for
flight
Flight or flying is the process by which an object moves through a space without contacting any planetary surface, either within an atmosphere (i.e. air flight or aviation) or through the vacuum of outer space (i.e. spaceflight). This can be a ...
. The bones in bird skeletons are hollow and lightweight to reduce the metabolic cost of flight. Several attributes of the shape and structure of the bones are optimized to endure the physical stress associated with flight, including a round and thin
humeral shaft and the fusion of skeletal elements into single
ossification
Ossification (also called osteogenesis or bone mineralization) in bone remodeling is the process of laying down new bone material by cells named osteoblasts. It is synonymous with bone tissue formation. There are two processes resulting in t ...
s. Because of this, birds usually have a smaller number of bones than other terrestrial vertebrates. Birds also lack teeth or even a true
jaw
The jaw is any opposable articulated structure at the entrance of the mouth, typically used for grasping and manipulating food. The term ''jaws'' is also broadly applied to the whole of the structures constituting the vault of the mouth and serv ...
, instead having evolved a
beak
The beak, bill, or rostrum is an external anatomical structure found mostly in birds, but also in turtles, non-avian dinosaurs and a few mammals. A beak is used for eating, preening, manipulating objects, killing prey, fighting, probing for food ...
, which is far more lightweight. The beaks of many baby birds have a projection called an
egg tooth
An egg tooth is a temporary, sharp projection present on the bill or snout of an oviparous animal at hatching. It allows the hatchling to penetrate the eggshell from inside and break free. Birds, reptiles, and monotremes possess egg teeth as ha ...
, which facilitates their exit from the amniotic egg.
Fish
The skeleton, which forms the support structure inside the fish is either made of cartilage as in the
Chondrichthyes
Chondrichthyes (; ) is a class that contains the cartilaginous fishes that have skeletons primarily composed of cartilage. They can be contrasted with the Osteichthyes or ''bony fishes'', which have skeletons primarily composed of bone tissue. ...
, or bones as in the
Osteichthyes
Osteichthyes (), popularly referred to as the bony fish, is a diverse superclass of fish that have skeletons primarily composed of bone tissue. They can be contrasted with the Chondrichthyes, which have skeletons primarily composed of cartilage ...
. The main skeletal element is the vertebral column, composed of articulating vertebrae which are lightweight yet strong. The ribs attach to the spine and there are no limbs or limb girdles. They are supported only by the muscles. The main external features of the fish, the
fins
A fin is a thin component or appendage attached to a larger body or structure. Fins typically function as foils that produce lift or thrust, or provide the ability to steer or stabilize motion while traveling in water, air, or other fluids. Fin ...
, are composed of either bony or soft spines called rays which, with the exception of the caudal fin (tail fin), have no direct connection with the spine. They are supported by the muscles which compose the main part of the trunk.
Cartilaginous fish, such as sharks, rays, skates, and chimeras, have skeletons made entirely of cartilage. The lighter weight of cartilage allows these fish to expend less energy when swimming.
Mammals
Marine mammals
![Zalophus californianus 01](https://upload.wikimedia.org/wikipedia/commons/d/d5/Zalophus_californianus_01.JPG)
To facilitate the movement of
marine mammal
Marine mammals are aquatic mammals that rely on the ocean and other marine ecosystems for their existence. They include animals such as seals, whales, manatees, sea otters and polar bears. They are an informal group, unified only by their reli ...
s in water, the hind legs were either lost altogether, as in the whales and
manatee
Manatees (family Trichechidae, genus ''Trichechus'') are large, fully aquatic, mostly herbivorous marine mammals sometimes known as sea cows. There are three accepted living species of Trichechidae, representing three of the four living species ...
s, or united in a single
tail fin as in the
pinnipeds
Pinnipeds (pronounced ), commonly known as seals, are a widely range (biology), distributed and diverse clade of carnivorous, fin-footed, semiaquatic, mostly marine mammal, marine mammals. They comprise the extant taxon, extant family (biology ...
(seals). In the whale, the
cervical vertebrae
In tetrapods, cervical vertebrae (singular: vertebra) are the vertebrae of the neck, immediately below the skull. Truncal vertebrae (divided into thoracic and lumbar vertebrae in mammals) lie caudal (toward the tail) of cervical vertebrae. In ...
are typically fused, an adaptation trading flexibility for stability during swimming.
Humans
![Leonardo Skeleton 1511](https://upload.wikimedia.org/wikipedia/commons/0/02/Leonardo_Skeleton_1511.jpg)
The skeleton consists of both fused and individual bones supported and supplemented by ligaments, tendons,
muscle
Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of muscl ...
s and cartilage. It serves as a scaffold which supports organs, anchors muscles, and protects organs such as the brain,
lungs
The lungs are the primary organs of the respiratory system in humans and most other animals, including some snails and a small number of fish. In mammals and most other vertebrates, two lungs are located near the backbone on either side of th ...
,
heart
The heart is a muscular organ in most animals. This organ pumps blood through the blood vessels of the circulatory system. The pumped blood carries oxygen and nutrients to the body, while carrying metabolic waste such as carbon dioxide t ...
and
spinal cord
The spinal cord is a long, thin, tubular structure made up of nervous tissue, which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column (backbone). The backbone encloses the central canal of the spi ...
. The biggest bone in the body is the
femur
The femur (; ), or thigh bone, is the proximal bone of the hindlimb in tetrapod vertebrates. The head of the femur articulates with the acetabulum in the pelvic bone forming the hip joint, while the distal part of the femur articulates with ...
in the upper leg, and the smallest is the
stapes
The ''stapes'' or stirrup is a bone in the middle ear of humans and other animals which is involved in the conduction of sound vibrations to the inner ear. This bone is connected to the oval window by its annular ligament, which allows the foo ...
bone in the
middle ear
The middle ear is the portion of the ear medial to the eardrum, and distal to the oval window of the cochlea (of the inner ear).
The mammalian middle ear contains three ossicles, which transfer the vibrations of the eardrum into waves in the ...
. In an adult, the skeleton comprises around 13.1% of the total body weight, and half of this weight is water.
Fused bones include those of the
pelvis
The pelvis (plural pelves or pelvises) is the lower part of the trunk, between the abdomen and the thighs (sometimes also called pelvic region), together with its embedded skeleton (sometimes also called bony pelvis, or pelvic skeleton).
The ...
and the
cranium
The skull is a bone protective cavity for the brain. The skull is composed of four types of bone i.e., cranial bones, facial bones, ear ossicles and hyoid bone. However two parts are more prominent: the cranium and the mandible. In humans, the ...
. Not all bones are interconnected directly: There are three bones in each
middle ear
The middle ear is the portion of the ear medial to the eardrum, and distal to the oval window of the cochlea (of the inner ear).
The mammalian middle ear contains three ossicles, which transfer the vibrations of the eardrum into waves in the ...
called the
ossicles
The ossicles (also called auditory ossicles) are three bones in either middle ear that are among the smallest bones in the human body. They serve to transmit sounds from the air to the fluid-filled labyrinth (cochlea). The absence of the auditory ...
that articulate only with each other. The
hyoid bone, which is located in the neck and serves as the point of attachment for the
tongue
The tongue is a muscular organ (anatomy), organ in the mouth of a typical tetrapod. It manipulates food for mastication and swallowing as part of the digestive system, digestive process, and is the primary organ of taste. The tongue's upper surfa ...
, does not articulate with any other bones in the body, being supported by muscles and ligaments.
There are 206 bones in the adult human skeleton, although this number depends on whether the pelvic bones (the
hip bone
The hip bone (os coxae, innominate bone, pelvic bone or coxal bone) is a large flat bone, constricted in the center and expanded above and below. In some vertebrates (including humans before puberty) it is composed of three parts: the Ilium (bone), ...
s on each side) are counted as one or three bones on each side (ilium, ischium, and pubis), whether the coccyx or tail bone is counted as one or four separate bones, and does not count the variable
wormian bone
Wormian bones, also known as intrasutural bones or sutural bones, are extra bone pieces that can occur within a suture (joint) in the skull. These are irregular isolated bones that can appear in addition to the usual centres of ossification of the ...
s between skull sutures. Similarly, the sacrum is usually counted as a single bone, rather than five fused vertebrae. There is also a variable number of small sesamoid bones, commonly found in tendons. The patella or kneecap on each side is an example of a larger sesamoid bone. The patellae are counted in the total, as they are constant. The number of bones varies between individuals and with age – newborn babies have over 270 bones some of which fuse together. These bones are organized into a longitudinal axis, the
axial skeleton
The axial skeleton is the part of the skeleton that consists of the bones of the head and trunk (anatomy), trunk of a vertebrate. In the human skeleton, it consists of 80 bones and is composed of six parts; the human skull, skull (22 bones), als ...
, to which the
appendicular skeleton
The appendicular skeleton is the portion of the skeleton of vertebrates consisting of the bones that support the appendages. There are 126 bones. The appendicular skeleton includes the skeletal elements within the limbs, as well as supporting shou ...
is attached.
[.]
The human skeleton takes 20 years before it is fully developed, and the bones contain
marrow, which produces blood cells.
There exist several general differences between the male and female skeletons. The male skeleton, for example, is generally larger and heavier than the female skeleton. In the female skeleton, the bones of the skull are generally less angular. The female skeleton also has wider and shorter breastbone and slimmer wrists. There exist significant differences between the male and female pelvis which are related to the female's pregnancy and childbirth capabilities. The female pelvis is wider and shallower than the male pelvis. Female pelvises also have an enlarged pelvic outlet and a wider and more circular pelvic inlet. The angle between the pubic bones is known to be sharper in males, which results in a more circular, narrower, and near heart-shaped pelvis.
[.]
Invertebrate skeletons
Invertebrates are defined by a lack of vertebral column, and they do not have bone skeletons. Arthropods have exoskeletons and echinoderms have endoskeletons. Some soft-bodied organisms, such as
jellyfish
Jellyfish and sea jellies are the informal common names given to the medusa-phase of certain gelatinous members of the subphylum Medusozoa, a major part of the phylum Cnidaria. Jellyfish are mainly free-swimming marine animals with umbrella- ...
and
earthworm
An earthworm is a terrestrial invertebrate that belongs to the phylum Annelida. They exhibit a tube-within-a-tube body plan; they are externally segmented with corresponding internal segmentation; and they usually have setae on all segments. Th ...
s, have hydrostatic skeletons.
Arthropods
The skeletons of
arthropod
Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and Arth ...
s, including
insect
Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body ( head, thorax and abdomen), three pairs ...
s,
crustacean
Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, amphipods and mantis shrimp. The crustacean group ...
s, and
arachnid
Arachnida () is a class of joint-legged invertebrate animals (arthropods), in the subphylum Chelicerata. Arachnida includes, among others, spiders, scorpions, ticks, mites, pseudoscorpions, harvestmen, camel spiders, whip spiders and vinegaroon ...
s, are cuticle exoskeletons. They are composed of
chitin
Chitin ( C8 H13 O5 N)n ( ) is a long-chain polymer of ''N''-acetylglucosamine, an amide derivative of glucose. Chitin is probably the second most abundant polysaccharide in nature (behind only cellulose); an estimated 1 billion tons of chit ...
secreted by the
epidermis
The epidermis is the outermost of the three layers that comprise the skin, the inner layers being the dermis and hypodermis. The epidermis layer provides a barrier to infection from environmental pathogens and regulates the amount of water rele ...
. The cuticle covers the animal's body and lines several internal organs, including parts of the digestive system. Arthropods molt as they grow through a process of
ecdysis
Ecdysis is the moulting of the cuticle in many invertebrates of the clade Ecdysozoa. Since the cuticle of these animals typically forms a largely inelastic exoskeleton, it is shed during growth and a new, larger covering is formed. The remna ...
, developing a new exoskeleton, digesting part of the previous skeleton, and leaving the
remainder
In mathematics, the remainder is the amount "left over" after performing some computation. In arithmetic, the remainder is the integer "left over" after dividing one integer by another to produce an integer quotient (integer division). In algebr ...
behind. An arthropod's skeleton serves many functions, working as an
integument
In biology, an integument is the tissue surrounding an organism's body or an organ within, such as skin, a husk, shell, germ or rind.
Etymology
The term is derived from ''integumentum'', which is Latin for "a covering". In a transferred, or ...
to provide a barrier and support the body, providing appendages for movement and defense, and assisting in sensory perception. Some arthropods, such as crustaceans, absorb biominerals like calcium carbonate from the environment to strengthen the cuticle.
Echinoderms
The skeletons of
echinoderm
An echinoderm () is any member of the phylum Echinodermata (). The adults are recognisable by their (usually five-point) radial symmetry, and include starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers, as well as the sea ...
s, such as
starfish
Starfish or sea stars are star-shaped echinoderms belonging to the class Asteroidea (). Common usage frequently finds these names being also applied to ophiuroids, which are correctly referred to as brittle stars or basket stars. Starfish ...
and
sea urchin
Sea urchins () are spiny, globular echinoderms in the class Echinoidea. About 950 species of sea urchin live on the seabed of every ocean and inhabit every depth zone from the intertidal seashore down to . The spherical, hard shells (tests) of ...
s, are endoskeletons that consist of large, well-developed
sclerite
A sclerite (Greek , ', meaning "hard") is a hardened body part. In various branches of biology the term is applied to various structures, but not as a rule to vertebrate anatomical features such as bones and teeth. Instead it refers most commonly ...
plates that adjoin or overlap to cover the animal's body. The skeletons of
sea cucumber
Sea cucumbers are echinoderms from the class Holothuroidea (). They are marine animals with a leathery skin and an elongated body containing a single, branched gonad. Sea cucumbers are found on the sea floor worldwide. The number of holothuria ...
s are an exception, having a reduced size to assist in feeding and movement. Echinoderm skeletons are composed of
stereom, made up of
calcite
Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...
with a
monocrystal
In materials science, a single crystal (or single-crystal solid or monocrystalline solid) is a material in which the crystal lattice of the entire sample is continuous and unbroken to the edges of the sample, with no grain boundaries.RIWD. "Re ...
structure. They also have a significant
magnesium
Magnesium is a chemical element with the symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 of the periodic ta ...
content, forming up to 15% of the skeleton's composition. The stereome structure is porous, and the pores fill with connective
stromal tissue as the animal ages. Sea urchins have as many as ten variants of stereome structure. Among extant animals, such skeletons are unique to echinoderms, though similar skeletons were used by some
Paleozoic
The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon.
The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838
by combining the Greek words ''palaiós'' (, "old") and ' ...
animals. The skeletons of echinoderms are
mesoderm
The mesoderm is the middle layer of the three germ layers that develops during gastrulation in the very early development of the embryo of most animals. The outer layer is the ectoderm, and the inner layer is the endoderm.Langman's Medical E ...
al, as they are mostly encased by soft tissue. Plates of the skeleton may be interlocked or connected through muscles and ligaments. Skeletal elements in echinoderms are highly specialized and take many forms, though they usually retain some form of symmetry. The spines of sea urchins are the largest type of echinoderm skeletal structure.
Molluscs
Some molluscs, such as conchs, scallops, and snails, have shells that serve as exoskeletons. They are produced by proteins and minerals secreted from the animal's
mantle.
Sponges
The skeleton of
sponge
Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the diploblasts. They are multicellular organisms that have bodies full of pores and channels allowing water to circulate through t ...
s consists of microscopic
calcareous
Calcareous () is an adjective meaning "mostly or partly composed of calcium carbonate", in other words, containing lime or being chalky. The term is used in a wide variety of scientific disciplines.
In zoology
''Calcareous'' is used as an adje ...
or
silicious
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...
spicules
Spicules are any of various small needle-like anatomical structures occurring in organisms
Spicule may also refer to:
*Spicule (sponge), small skeletal elements of sea sponges
*Spicule (nematode), reproductive structures found in male nematodes ( ...
. The
demosponge
Demosponges (Demospongiae) are the most diverse class in the phylum Porifera. They include 76.2% of all species of sponges with nearly 8,800 species worldwide (World Porifera Database). They are sponges with a soft body that covers a har ...
s include 90% of all species of sponges. Their "skeletons" are made of spicules consisting of fibers of the protein
spongin
Spongin, a modified type of collagen protein, forms the fibrous skeleton of most organisms among the phylum Porifera, the sponges. It is secreted by sponge cells known as spongocytes.
Spongin gives a sponge its flexibility. True spongin is found ...
, the mineral
silica
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...
, or both. Where spicules of silica are present, they have a different shape from those in the otherwise similar
glass sponges.
[.]
Cartilage
Cartilage is a connective skeletal tissue composed of specialized cells called
chondrocytes that in an
extracellular matrix
In biology, the extracellular matrix (ECM), also called intercellular matrix, is a three-dimensional network consisting of extracellular macromolecules and minerals, such as collagen, enzymes, glycoproteins and hydroxyapatite that provide stru ...
. This matrix is typically composed of Type II
collagen
Collagen () is the main structural protein in the extracellular matrix found in the body's various connective tissues. As the main component of connective tissue, it is the most abundant protein in mammals, making up from 25% to 35% of the whole ...
fibers,
proteoglycan
Proteoglycans are proteins that are heavily glycosylated. The basic proteoglycan unit consists of a "core protein" with one or more covalently attached glycosaminoglycan (GAG) chain(s). The point of attachment is a serine (Ser) residue to whic ...
s, and water. There are many types of cartilage, including
elastic cartilage
Elastic cartilage, fibroelastic cartilage or yellow fibrocartilage is a type of cartilage present in the pinnae (auricles) of the ear giving it shape, provides shape for the lateral region of the external auditory meatus, medial part of the aud ...
,
hyaline cartilage
Hyaline cartilage is the glass-like (hyaline) and translucent cartilage found on many joint surfaces. It is also most commonly found in the ribs, nose, larynx, and trachea. Hyaline cartilage is pearl-gray in color, with a firm consistency and has ...
,
fibrocartilage
Fibrocartilage consists of a mixture of white fibrous tissue and cartilaginous tissue in various proportions. It owes its inflexibility and toughness to the former of these constituents, and its elasticity to the latter. It is the only type of ...
, and lipohyaline cartilage.
Unlike other connective tissues, cartilage does not contain blood vessels. The chondrocytes are supplied by diffusion, helped by the pumping action generated by compression of the articular cartilage or flexion of the elastic cartilage. Thus, compared to other connective tissues, cartilage grows and repairs more slowly.
See also
*
Bonesetter
Traditional bone-setting is a type of a folk medicine in which practitioners engaged in joint manipulation. Before the advent of chiropractors, osteopaths and physical therapists, bone-setters were the main providers of this type of treatment. T ...
*
Endochondral ossification
*
Intramembranous ossification
Intramembranous ossification is one of the two essential processes during fetal development of the gnathostome (excluding chondrichthyans such as sharks) skeletal system by which rudimentary bone tissue is created.
Intramembranous ossification is a ...
*
Exoskeleton
An exoskeleton (from Greek ''éxō'' "outer" and ''skeletós'' "skeleton") is an external skeleton that supports and protects an animal's body, in contrast to an internal skeleton (endoskeleton) in for example, a human. In usage, some of the ...
*
Osteoblast
Osteoblasts (from the Greek language, Greek combining forms for "bone", ὀστέο-, ''osteo-'' and βλαστάνω, ''blastanō'' "germinate") are cell (biology), cells with a single Cell nucleus, nucleus that synthesize bone. However, in the p ...
*
Osteometric points
Osteometric points (bone measuring points) are an agreed upon set of locations on the skeleton, or more commonly the skull, that aid in the study of osteology. These points were developed to help researchers make comparative measurements between ...
*
Skeletal system of the horse
The skeletal system of the horse has three major functions in the body. It protects vital organs, provides framework, and supports soft parts of the body. Horses typically have 205 bones. The pelvic limb typically contains 19 bones, while the ...
*
Skeletonization
Skeletonization is the state of a dead organism after undergoing decomposition.The Australian Museum. (2018). Decomposition-Body Changes. Retrieved from: https://australianmuseum.net.au/about/history/exhibitions/death-the-last-taboo/decomposit ...
References
Bibliography
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External links
3-D Viewerof a male
American mastodon
American(s) may refer to:
* American, something of, from, or related to the United States of America, commonly known as the "United States" or "America"
** Americans, citizens and nationals of the United States of America
** American ancestry, p ...
skeleton, with bones labelled, at the
University of Michigan
, mottoeng = "Arts, Knowledge, Truth"
, former_names = Catholepistemiad, or University of Michigania (1817–1821)
, budget = $10.3 billion (2021)
, endowment = $17 billion (2021)As o ...
br>
Mammutidae digital fossil repositoryInteractive viewsof various
primate
Primates are a diverse order of mammals. They are divided into the strepsirrhines, which include the lemurs, galagos, and lorisids, and the haplorhines, which include the tarsiers and the simians (monkeys and apes, the latter including huma ...
skeletons at eSkeletons.org (associated with the
University of Texas at Austin
The University of Texas at Austin (UT Austin, UT, or Texas) is a public research university in Austin, Texas. It was founded in 1883 and is the oldest institution in the University of Texas System. With 40,916 undergraduate students, 11,075 ...
)
{{Authority control
Animal anatomy