Part of the body that forms the supporting structure
A
skeleton
is the
structural frame
that supports the body of most
animals
. There are several types of skeletons, including the
exoskeleton
, which is a rigid outer shell that holds up an organism's shape; the
endoskeleton
, a rigid internal frame to which the
organs
and
soft tissues
attach; and the
hydroskeleton
, a flexible internal structure supported by the
hydrostatic pressure
of
body fluids
.
Vertebrates
are animals with an endoskeleton centered around an axial
vertebral column
, and their skeletons are typically composed of
bones
and
cartilages
.
Invertebrates
are other animals that lack a vertebral column, and their skeletons vary, including hard-shelled exoskeleton (
arthropods
and most
molluscs
), plated internal shells (e.g.
cuttlebones
in some
cephalopods
) or rods (e.g.
ossicles
in
echinoderms
), hydrostatically supported body
cavities
(most), and
spicules
(
sponges
). Cartilage is a rigid
connective tissue
that is found in the skeletal systems of vertebrates and invertebrates.
Etymology
[
edit
]
The term
skeleton
comes from
Ancient Greek
σκελετ??
(
skeletos
)
'dried up'.
[1]
Sceleton
is an archaic form of the word.
[2]
Classification
[
edit
]
Skeletons can be defined by several attributes. Solid skeletons consist of hard substances, such as
bone
,
cartilage
, or
cuticle
. 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.
[3]
Fluid or
hydrostatic skeletons
do not have hard structures like solid skeletons, instead functioning via pressurized fluids. Hydrostatic skeletons are always internal.
[4]
Exoskeletons
[
edit
]
An exoskeleton is an external skeleton that covers 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
moulting
or
ecdysis
as the animals grow. The
shells
of
molluscs
are another form of exoskeleton.
[4]
Exoskeletons provide surfaces 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
.
[5]
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
, a species of extremely large saltwater clam in the Pacific Ocean, has a shell that is massive in both size and weight.
Syrinx aruanus
is a species of sea snail with a very large shell.
Endoskeletons
[
edit
]
Endoskeletons are the internal support structure of an animal, composed of
mineralized tissues
, such as the bone skeletons found in most vertebrates.
[6]
Endoskeletons are highly specialized and vary significantly between animals.
[4]
They vary in complexity from functioning purely for support (as in the case of
sponges
), to serving as an attachment site for muscles and a mechanism for transmitting muscular forces. A true endoskeleton is derived from
mesodermal
tissue. Endoskeletons occur in
chordates
, echinoderms, and sponges.
Rigidity
[
edit
]
Pliant skeletons are capable of movement; thus, when
stress
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 shells
or the
mesoglea
of
cnidarians
such as
jellyfish
. Pliant skeletons are beneficial because only
muscle
contractions are needed to bend the skeleton; upon muscle relaxation, the skeleton will return to its original shape.
Cartilage
is one material that a pliant skeleton may be composed of, but most pliant skeletons are formed from a mixture of
proteins
,
polysaccharides
, and water.
[3]
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.
[7]
Rigid skeletons are not capable of movement when stressed, creating a strong support system most common in
terrestrial animals
. Such a skeleton type used by animals that live in water are more for protection (such as
barnacle
and
snail
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
(in arthropods),
calcium
compounds such as
calcium carbonate
(in
stony corals
and
mollusks
) and
silicate
(for
diatoms
and
radiolarians
).
Hydrostatic skeletons
[
edit
]
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.
[4]
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.
[8]
Cytoskeleton
[
edit
]
The cytoskeleton (
cyto-
meaning 'cell'
[9]
) 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
,
cilia
and
lamellipodia
, and transport within cells such as the movement of
vesicles
and
organelles
, 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.
[10]
Vertebrate skeletons
[
edit
]
In most vertebrates, the main skeletal component is bone.
[6]
Bones compose a unique skeletal system for each type of animal. Another important component is cartilage which in
mammals
is found mainly in the joint areas. In other animals, such as the
cartilaginous fishes
, which include the
sharks
, the skeleton is composed entirely of
cartilage
. 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.
[11]
[12]
Bones are rigid
organs
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
, 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
osteoblasts
,
osteoclasts
, or
osteocytes
.
[13]
Bone tissue is a type of dense
connective tissue
. One of the types of tissue that makes up bone tissue is
mineralized tissue
and this gives it rigidity and a
honeycomb-like
three-dimensional internal structure. Bones also produce
red
and
white blood cells
and serve as calcium and phosphate storage at the cellular level. Other types of tissue found in bones include
marrow
,
endosteum
and
periosteum
,
nerves
,
blood vessels
and cartilage.
During
embryonic development
, 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.
[14]
Cartilage is also used in vertebrates to resist stress at points of articulation in the skeleton. Cartilage in vertebrates is usually encased in
perichondrium
tissue.
[15]
Ligaments
are elastic tissues that connect bones to other bones, and
tendons
are elastic tissues that connect muscles to bones.
[16]
Amphibians and reptiles
[
edit
]
The skeletons of turtles have evolved to develop a
shell
from the ribcage, forming an exoskeleton.
[17]
The skeletons of
snakes
and
caecilians
have significantly more vertebrae than other animals. Snakes often have over 300, compared to the 65 that is typical in lizards.
[18]
Birds
[
edit
]
The skeletons of birds are adapted for
flight
. 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
ossifications
.
[19]
Because of this, birds usually have a smaller number of bones than other terrestrial vertebrates. Birds also lack teeth or even a true
jaw
, instead having evolved a
beak
, which is far more lightweight. The beaks of many baby birds have a projection called an
egg tooth
, which facilitates their exit from the amniotic egg.
Fish
[
edit
]
The skeleton, which forms the support structure inside the fish is either made of cartilage as in the
Chondrichthyes
, or bones as in the
Osteichthyes
. 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
, 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.
[4]
Mammals
[
edit
]
Marine mammals
[
edit
]
To facilitate the movement of
marine mammals
in water, the hind legs were either lost altogether, as in the whales and
manatees
, or united in a single
tail fin
as in the
pinnipeds
(seals). In the whale, the
cervical vertebrae
are typically fused, an adaptation trading flexibility for stability during swimming.
[20]
Humans
[
edit
]
The skeleton consists of both fused and individual bones supported and supplemented by ligaments, tendons,
muscles
and cartilage. It serves as a scaffold which supports organs, anchors muscles, and protects organs such as the brain,
lungs
,
heart
and
spinal cord
.
[21]
The biggest bone in the body is the
femur
in the upper leg, and the smallest is the
stapes
bone in the
middle ear
. In an adult, the skeleton comprises around 13.1% of the total body weight,
[22]
and half of this weight is water.
Fused bones include those of the
pelvis
and the
cranium
. Not all bones are interconnected directly: There are three bones in each
middle ear
called the
ossicles
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
, 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 bones
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 bones
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.
[
citation needed
]
These bones are organized into a longitudinal axis, the
axial skeleton
, to which the
appendicular skeleton
is attached.
[23]
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.
[24]
[25]
Invertebrate skeletons
[
edit
]
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
and
earthworms
, have hydrostatic skeletons.
[26]
Arthropods
[
edit
]
The skeletons of
arthropods
, including
insects
,
crustaceans
, and
arachnids
, are cuticle exoskeletons. They are composed of
chitin
secreted by the
epidermis
.
[27]
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
, developing a new exoskeleton, digesting part of the previous skeleton, and leaving the
remainder
behind. An arthropod's skeleton serves many functions, working as an
integument
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.
[5]
Echinoderms
[
edit
]
The skeletons of
echinoderms
, such as
starfish
and
sea urchins
, are endoskeletons that consist of large, well-developed
sclerite
plates that adjoin or overlap to cover the animal's body. The skeletons of
sea cucumbers
are an exception, having a reduced size to assist in feeding and movement. Echinoderm skeletons are composed of
stereom
, made up of
calcite
with a
monocrystal
structure. They also have a significant
magnesium
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
animals.
[28]
The skeletons of echinoderms are
mesodermal
, 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.
[29]
Molluscs
[
edit
]
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
.
[4]
Sponges
[
edit
]
The skeleton of
sponges
consists of microscopic
calcareous
or
silicious
spicules
. The
demosponges
include 90% of all species of sponges. Their "skeletons" are made of spicules consisting of fibers of the protein
spongin
, the mineral
silica
, or both. Where spicules of silica are present, they have a different shape from those in the otherwise similar
glass sponges
.
[30]
Cartilage
[
edit
]
Cartilage is a connective skeletal tissue composed of specialized cells called
chondrocytes
that in an
extracellular matrix
. This matrix is typically composed of Type II
collagen
fibers,
proteoglycans
, and water. There are many types of cartilage, including
elastic cartilage
,
hyaline cartilage
,
fibrocartilage
, and lipohyaline cartilage.
[15]
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
[
edit
]
References
[
edit
]
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External links
[
edit
]
Media related to
Skeletons
at Wikimedia Commons