surface, therefore elongate if the root curves downward. Auxins on the other hand, stimulate the growth of the stem cells. The cells of the lower surface, elongate and stem curves upward. Nastic movements are due to some balance or ratio between growth inhibitors (abscisins) and growth stimulators (gibberellins). However, it has been observed that epinasty is due to auxins and hyponasty due to gibberellins.
SUPPORT AND MOVEMENTS IN ANIMALS
The skeleton is 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 organie substances or both. In protozoa it is secreted by a single cell, whereas in multicellular animals it is composed of specialized cells. There are three main types of skeletons in animals, hydrostatic skeleton, exoskeleton and endoskeleton.
1. Hydrostatic Skeleton
In animals that lack a hard skeleton, a fluid filled gastrovascular cavity or coelom can act as hydrostatic skeleton. Hydrostatic skeleton provides support and resistance to the contraction of muscles so that motility results. At is found in cnidarians, annelids and other soft-bodied invertebrates.
The sea anemone has hydrostatic skeleton. Its cavity is filled with sea water to extend its body and tentacles. The sea anemone closes its mouth and constricts its muscle fibers that are arranged in circles around its body. The contraction of these circular muscles puts pressure on the liquid in body cavity and that pressure forces the body to maintain upright stature.
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 wave of elongation and contraction move the earthworm through the soil, aided by paired setae in each segment.
2. Exoskeleton
An exoskeleton is hardened outer covering to which internal muscles are attached. The exoskeleton is inert and non-living. It is secreted by the ectoderm in animal cells. It is composed of two layers. The epicuticle is the outer most layer. Because it is made up of waxy lipoprotein, it is impermeable to water and serves as a barrier to microorganisms in insects. The bulk of exoskeleton is below the epicúticle and is called the procuticle. The procuticle is composed of chitin, tough, leathery, polysaccharide and several kinds of protein. It is further hardened by sclerotization and sometimes by impregnation with calcium carbonate.
The simplest example of an exoskeleton is the shell of mollusca, which generally consists of just one or two pieces. Some marine bivalvia and snail have shell composed of crystals of calcium carbonate. The shell of land snail generally lacks the hard minerals and are 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.
The most complex exoskeleton is found among the arthropods. The arthropods have made a variety of adaptations to allow them to live and grow within ther exoskeleton. The invagination of exoskeleton forms firm ridges and bars for muscle attachment. Another modification of exoskeleton is the formation of joints. The exoskeleton are thin, soft and flexible at joints, consequently joint move very easily. Other modifications of exoskeleton include sensory receptors called sensilla that are in the form of bristles, and lenses and the modification of the exoskeleton that permits gaseous exchange.
The exoskeleton in arthropoda protects the animals against their enemies and rough environment. It also protects them from drying.
However, it has one disadvantage and that is animals cannot grow larger. The animal, therefore, needs to shed its 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:
1. Enzymes, secreted from hypodermal glands, begin digesting the old endocuticle. This digestion separates hypodermis and the exoskeleton.
2. The digestion of endocuticle is followed by secretion of new procuticle and epicuticle.
3. The old exoskeleton is split and pores are formed.
4. Finally, the new exoskeleton is hardened by deposition of calcium carbonate.
During the hardening process, the arthropod is vulnerable to predators and remains hidden. All these changes are controlled by the nervous system and the hormone ecdysone.
Some major functions of the skeletal system are as follows:
(1) Support and shape: Bones support soft tissues and serve as attachment sites for most muscles and provide shape to the body.
(ii) Protection: Bones protect critical internal organs, such as brain, spinal cord, heart and lungs.
)س( Movement: Skeletal muscles attached to the bones help in moving the body.
(iv) Mineral homeostasis: Bones serve as store for calcium, phosphorus, sodium and potassium. Through negative feedback mechanisms, bones can release or take up minerals to maintain homeostasis.
(1) Blood cell production: Red and white blood cells are produced in bone marrow, a connective tissue found within certain bones.
3. Endoskeleton
The endoskeleton is primarily made up of two of types of tissues, bones and cartilage. Both bones and cartilage are types of rigid connective tissue. Both consists of living cells embedded in the matrix of protein called collagen.
Cartilaginous Joints: These joints allow little or no movement. Hyaline Cartilage forms joint between growing bone. The bones held together by fibrous Cartilage are found between vertebrae at the point where coxal bones meet in front of the pelvis.
3.
Synovial Joints: These joints contain a cavity filled with fluid and are adapted mo reduce friction between the moving joints. The joint is surrounded by a layer of connective tissue called "fibrous capsule" and their inner layer the synovial membrane. Some parts of capsule may be modified to form distinct ligament, holding the bones Mogether.
Based on structure and movements allowed, the synovial joints can be classified further into major categories.
(i) Hinge Joint: The joint that allows the movements in two directions. These are at elbow and knee. At these joints, pair of muscles are arranged in the same plane as that of joints. One end of each muscle, the origin is fixed to the immovable bone on one side of joint and the other end of muscles, the insertion is attached to the far side of the joint.
(ii) Ball & Socket Joint: The joint that allows the movement in several directions. Such joints have at least two pairs of muscles present perpendicular to each other. They provide maximum flexibility. Hip joint and shoulder joint are the examples of ball and socket joints.
DEFORMITIES OF SKELETON
Human skeleton supports an upright body. Sometimes our skeletal system becomes weak and results in deformations. The causes of the deformations are variable
e.g.
Genetic Causes
1.
Cleft palate, a condition in which palatine processes of maxilla and palatine fail to fuse. The persistent opening between the oral and nasal cavity interferes with sucking. It can lead to inhalation of food into the lungs causing aspiration pneumonia.
Microcephaly, the small sized skull is caused by some genetic defect. Arthritis covers over 100 different types of inflammatory or degenerative diseases that damage the joints. Osteoarthritis is the most common chronic arthritis, which is joint disease also caused by genetic defect.
2.
Hormonal Causes
Osteoporosis is a group of diseases in which bone resorption out paces bone deposit. In this case bone mass is reduced and chemical composition of the matrix remains normal. Osteoporosis mostly occurs in aged women, which is related to decreased estrogen level. Other factors which may contribute înclude, insufficient exercise, diet poor in calcium and protein, smoking. etc.
Estrogen replacement therapy (ERT) offers the best protection against osteoporotic bone fractures.
REPAIR OF BROKEN BONES L
Despite remarkable strength, the bones may break. During youth, most fractures result from trauma that may twist or break the bones such as sports injuries, automobile accidents, falls etc. In old age, bones become thin and weak and hence fractures occur more frequently.
A fracture is treated by reduction followed by realignment of the broken bone ends. There are two types of reduction: closed and open reduction. In closed reduction the bone end is coaxed back to their normal position by physician's hand. In open reduction surgery is performed and the bone ends are secured together with pins or wires. After broken bone is reduced, it is immobilized by a cast or by traction to allow the healing process to begin. Healing time is 8-12 weeks, but it is much longer for large weight bearing bones and for bones of elderly people (because of their poorer blood circulation).
The repair process of a simple fracture takes place in four phases:
1. Hematoma Formation: When a bone breaks, the blood vessels in the bone itself, and perhaps in surrounding are torn resulting hemorrhage. As a result, a hematoma, a mass of clotted blood, forms at the fracture site. Soon after, bone cell deprived of food begin to die and the tissue at the fracture site becomes swollen and hence painful.
2. Callus Formation: Next "soft callus" begins to form in few days. Capillaries grow into the hematoma and clear up the debris. Fibroblasts and osteoblasts migrate into the fracture site and begin to construct bone.
3. Bony Callus Formation: Osteoblasts and osteoclasts continue to migrate inward, multiply rapidly and gradually convert the soft callus into bony callus. Bone formation begins few days after injury and continues until a firm bony union is formed within 2-3 months later.
4. Remodeling: After several months bony callus is remodeled by the excess material on the outside of the bone. Final structure of remodeled area resembles that of the original unbroken bone because it responds to the same set of mechanical stimuli.
MUSCLES
Many multicellular animals have evolved specialized cells for movement. These cells contain numerous filaments of special protein actin and myosin. The vertebrate possess three kinds of muscle cells, Smooth muscles, skeletal muscles and cardiac muscles (Fig 16.7)
