What is Skeleton? The skeleton is a tough and rigid framework of the body of animals which provides protection, shape and support to the body organs. It is composed of inorganic or organic substances or both, In protozoa, it is secreted by a single cell, whereas in multicellular animals, it comprises by specialized cells.
TYPES OF SKELETON SYSTEM
There are three main types of a skeleton in animals i.e.
- Hydrostatic Skeleton
1. HYDROSTATIC SKELETON
In animals that lack a hard skeleton, a fluid-filled gastrovascular cavity or coelom can act as a hydrostatic skeleton. The hydrostatic skeleton provides support and resistance to the contraction of muscles so that motility results.
Hydrostatic skeleton is found in cnidarians, annelids and other soft-bodied invertebrates.
- i) Sea anemone: The sea anemone has a hydrostatic skeleton. Its cavity is filled with seawater to extend its body and tentacles. The sea anemone closes its mouth and constricts its muscle fibres that are arranged in circles around its body. The contraction of these circular muscles puts pressure on the liquid of body cavity and those pressure forces the body to maintain upright stature.
- Earthworm: In earthworm, the hydrostatic skeleton consists of fluid filled compartments separated by septa. Contraction of circular muscle causes compartments to elongate and contraction of longitudinal muscle causes a compartment to shorten. Alternating waves of elongation and contraction move the earthworm through the soil, aided by paired setae in each segment.
An exoskeleton is hardened outer surface to which internal muscles can be attached.
Characteristics. It has following characteristics:
- Inert and nonliving: The exoskeleton is inert and non-living.
- Secreted by ectoderm; It ‘is secreted by the ectoderm in animal cells.
- Composition: It is composed of two layers. The epicuticle is the outermost layer. Because it is made up of waxy; lipoprotein, it is impermeable to water and serves as a barrier to microorganisms and insects. The bulb of the exoskeleton is below the epicuticle and is called the procuticle. In crustaceans, the procuticle is sometimes called endocuticle. The procuticle is composed of chitin, tough, leathery, polysaccharide and several kinds of proteins. It is further hardened by sclerotization (process of hardening of exoskeleton) and sometimes by impregnation with calcium carbonate.
- Protection: The exoskeleton protects the animals against their enemies.
1) Shell of molluscs: It generally consists of just one or two pieces; Some marine Bivalvia and snail have a shell composed of crystals of calcium carbonate. The shell of land snail generally lacks the hard minerals and is much lighter. Molluscan shell can grow as the animal grows and growth rings are apparent on the shell. The soft parts of the molluscan body have a hydrostatic skeleton as well.
2) Exoskeleton of Arthropods: It is the most complex exoskeleton. Modifications or adaptations of the exoskeleton in arthropods. The arthropods have made a variety of adaptations to allow arthropods to live and grow within their exoskeleton.
a) Firm ridges and bars: The invagination of exoskeleton form firm ridges and bars for muscle attachment.
b) Formation of joints: The exoskeleton is thin soft and flexible at joints, consequently joints move very easily.
c) Sensilla: These are sensory receptors that are in the form of bristles, and lenses.
d) Gas exchange: The exoskeleton is modified to permit gas exchange.
e) Protection: It protects them from drying and rough environment and also against their enemies.
f) Moulting or ecdysis: Arthropods have one disadvantage of exoskeleton hat is when animals have to grow they need to shed exoskeleton periodically and replace it with one of the larger size. This process is known as “Ecdysis or moulting.” Ecdysis is divided into four stages:
i) Enzymes secreted from hypodermal glands, begin digesting the old endocuticle. This digestion separates hypodermis and the exoskeleton.
ii) The digestion of endocuticle is followed by secretion of new procuticle and epicuticle.
iii) The old exoskeleton is split and pores are formed.
iv) Finally, the new exoskeleton is hardened by deposition of calcium carbonate.
During the hardening process, the arthropods are vulnerable ‘to predators and remain hidden. All these changes are controlled by the nervous system and the hormone “Ecdysone.”
The skeleton -that lies internally to the muscles is called endoskeleton. The vertebrates have developed endoskeleton that has many advantages or functions:
i) Support: Bones support soft tissues and serve as attachment sites for most muscles.
ii) Shape: Bones provide proper shape to the body.
iii) Protection: Bones protect critical internal organs, such as a brain, spinal cord, heart lungs, and reproductive organs.
iv) Movement: Skeletal muscles attached to the bones help move the body.
(v) Mineral homeostasis: Bones serve as a repository (storehouse) for calcium, phosphorus, sodium, and potassium. Through negative feedback mechanisms, bones can release or take up minerals to maintain homeostasis.
(vi) Blood cell production: Red and white blood cells are produced in cartilage ligament bone marrow, a connective tissue found within certain bones.
COMPOSITION OF ENDOSKELETON
The endoskeleton is primarily made up of two types of tissues.
1) Bones 2) Cartilage
Both bones and cartilage are types of rigid connective tissue. Both consist of living cells in the matrix of a protein called collagen.
Bone has following characteristics:
a) Rigid connective tissue: Bone is the most rigid form of connective tissue.
b) Collagen fibres: The collagen fibres of bone are hardened by deposition of calcium phosphate.
c) Bones of arms and legs: Bones supporting arms and legs consist of an outer shell of compact bone, with spongy bone in the interior.
i) Compact bone: Compact bone is dense and strong and provides an attachment site for muscle.
ii) Spongy bone: Spongy bone is light, rich in blood vessels, and highly porous. The cavities of spongy bone contain bone marrow where blood cells are formed.
d) Cells associated with a bone: There are three types of cells associated with a bone:
i) Bone-forming cells (Osteoblasts),
ii) Mature bone cells (Osteocytes),
Bone dissolving cells (Osteoclasts)e) Development of bone: Early development, when bone is replacing cartilage, the osteoclasts invade and dissolve the cartilage. Then osteoblasts replace it with bone. As bones grow the matrix of bone is hardened and the Osteoblasts are gradually entrapped within it.
it has following characteristics:
a) Soft connective tissue: Cartilage is much softer than bone. It is a form of connective tissue.
b) Covers various parts: It covers ends of the bone at the joint, and also supports the flexible portion of nose and external ears.
c) Chondrocytes: The living cells of cartilage are called chondrocytes. These cells secrete flexible, elastic, non-living matrix collagen that surrounds the chopdrocytes.
d) No blood vessels: No blood vessels penetrate into the cartilage.
e) Main types: There are two main types of cartilage.
i) Hyaline Cartilage: It is the most abundant type in the human body. It is found in the moveable joints.
ii) Fibro Cartilage; it has matrix-containing bundles of collagen fibres. It forms external pinnae (outer ear) of ear and epiglottis.