Types of skeletons
Exoskeletons are external, and are found in many invertebrates; they enclose and protect the soft tissues and organs of the body. Some kinds of exoskeletons undergo periodic moulting as the animal grows, as is the case in many arthropods including insects and crustaceans. Exoskeletons are made of different materials including chitincalcium compounds (in corals and mollusks) and silicate (for diatomsradiolarians). (in arthropods), and
The exoskeleton of insects is not only a protection but also serves
as a surface for muscle attachment, as a watertight protection against
drying and as a sense organ to interact with their environments. The shell of mollusks also performs all of the same functions, except that in most cases it does not contain sense organs.
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.
Endoskeleton of an elephant.
A beaver skeleton on display at The Museum of Osteology, Oklahoma City, Oklahoma.
Endoskeletons is the internal support structure of an animal, composed of mineralized tissue and are typical of many vertebrates. They vary in complexity such as functioning purely for support (as in the case of sponges),
but often serves as an attachment site for muscle and a mechanism for
transmitting muscular forces. A true endoskeleton is derived from mesodermal tissue. Such a skeleton is present in echinoderms and chordates.
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.
The skeleton of the echinoderms, which include, among other things, the starfish, is composed of calcite and a small amount of magnesium oxide. It lies below the epidermis in the mesoderm
and is within cell clusters of frame-forming cells. This structure
formed is porous and therefore firm and at the same time light. It
coalesces into small calcareous
ossicles (bony plates), which can grow in all directions and thus can
replace the loss of a body part. Connected by joints, the individual
skeletal parts can be moved by the muscles.
In humans and generally in most vertebrates, the main skeletal component is referred to as bone. Another important component is cartilage. In mammals they are 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.
Bones in addition to supporting the body also serve, at the cellular level, as calcium and phosphate storage.
The skeleton of the fish is either made of cartilage (cartilage fish) or bones (bony fishes). The main features of the fish, the fins, are bony fin rays and, except the caudal fin, have no direct connection with the spine. They are supported only by the muscles. The ribs attach to the spine.
The bird skeleton is highly adapted for flight. It is extremely lightweight but strong enough to withstand the stresses of taking off, flying, and landing. One key adaptation is the fusing of bones into single ossifications, such as the pygostyle.
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.
To facilitate movement of marine mammals in the water, the hind legs were either lost altogether, as in the whales and manatees, or united in a single tail fin (eg, seals).
The simplest form of skeleton is the Hydrostatic skeleton found in many cold-bloodedcoelom. Invertebrate such as starfishs, sea urchins, jellyfish and earthworms has this form of skeleton. The hydrostatic skeleton have some similarities to muscular hydrostats.
organisms and soft-bodied animals. The pressure of the fluid and action
of the surrounding muscles are used to change an organism's shape and
produce movement. This fluid filled cavity is the known as the
The cytoskeleton (gr. kytos
= 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, cilia and lamellipodia), and plays important roles in both intracellular transport (the movement of vesicles and organelles, for example) and cellular division.
Main article: Human skeleton
The human 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, lungsheart.
Although the teeth do not consist of tissue commonly found in other
bones, the teeth are considered bones and are a member of the skeletal
system. 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 14% of the total body weight, and half of this weight is water. and
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 ossicleshyoid 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. that articulate only with each other. The
There are 206 bones in the adult human skeleton (this number depends
on whether the pelvic bones (the os innominatum 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. These bones are organized into a longitudinal axis, the axial skeleton, to which the appendicular skeleton is attached.
The human skeleton takes 20 years before it is fully developed. In many animals, the skeleton bones contain marrow, which produces blood cells.
Much of the human skeleton maintains the ancient segmental pattern present in all vertebrates
(mammals, birds, fish, reptiles and amphibians) with basic units being
repeated. This segmental pattern is particularly evident in the
vertebral column and in the ribcage.
Bones and cartilage
Main article: Bone
Bones are rigid organs that form part of the endoskeleton of vertebrates. They function to move, support, and protect the various organs of the body, produce red and white blood cells and store minerals. Bone tissue is a type of dense connective tissue. Because bones come in a variety of shapes and have a complex internal and external structure they are lightweight, yet strong and hard, in addition to fulfilling their many other functions. One of the types of tissue that makes up bone is the mineralized osseous tissue, also called bone tissue, that gives it rigidity and a honeycomb-likemarrow, endosteum and periosteum, nerves, blood vessels and cartilage. There are 206 bones in the adult human bodyand 270 in an infant. three-dimensional internal structure. Other types of tissue found in bones include
Main article: Cartilage
Cartilage is a stiff and inflexible connective tissue found in many areas in the bodies of humans and other animals, including the joints between bones, the rib cage, the ear, the nose, the elbow, the knee, the ankle, the bronchial tubes and the intervertebral discs. It is not as hard and rigid as bone but is stiffer and less flexible than muscle.
Cartilage is composed of specialized cells called chondrocytes that produce a large amount of extracellular matrix composed of Type II collagen (except Fibrocartilage which also contains type I collagen) fibers, abundant ground substance rich in proteoglycan, and elastin fibers. Cartilage is classified in three types, elastic cartilage, hyaline cartilage and fibrocartilage, which differ in the relative amounts of these three main components.
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.