Structural Organization In Animals
A group of cells which is formed to carry out specific task is called a tissue.
Formation of tissues is the first step towards division of labour in multicellular animals. Subsequently, a group of tissues forms an organ and a group of organs forms the organ system.
There are four major types of animal tissues, viz. epithelial, connective, muscular and neural tissues.
Tissues which provide covering to the inner and outer linings of various organs are called epithelial tissue. The cells in the epithelial tissue are compactly packed. There is little intercellular matrix in the epithelial tissue.
The epithelial tissues are further divided into two main types, viz. simple epithelium and compound epithelium.
Simple Epithelium: The simple epithelium is composed of a single layer of cells. It is present in the lining of body cavities, ducts and tubes.
Compound Epithelium: The compound epithelium is composed of more than one layer of cells. The compound epithelium serves the protective function. Compound epithelium is present in skin, in the lining of buccal cavity, pharynx, ducts of salivary glands and pancreatic ducts.
The simple epithelium can be further divided into three types, viz. squamous, cuboidal and columnar.
Squamous Epithelium: The squamous epithelium is made up of flattened cells with irregular boundaries. Squamous epithelium is present in the walls of blood vessels and air sacs of lungs. They form a diffusion boundary.
Cuboidal Epithelium: The cuboidal epithelium is composed of cube-like cells. The cuboidal epithelium is usually found in ducts of glands and tubular parts of nephrons. The main functions of cuboidal epithelium are secretion and absorption.
Columnar Epithelium: The columnar epithelium is composed of tall and slender cells. The nuclei of the columnar cells are located at the base. Microvilli may be present on the free surface. Columnar epithelium is present in the lining of stomach and intestine. They help in secretion and absorption.
Ciliated Epithelium: Sometimes, cilia may be present in columnar or cuboidal epithelium. In that case, they are called ciliated epithelium. The cilia move particles or mucus in a specific direction over the epithelium. They are present in the inner surface of hollow organs; like bronchioles and fallopian tubes.
The connective tissues are most abundant and widely distributed in the body of complex animals. The connective tissues support and link other tissues or organs of the body. The cells of the connective tissue secrete fibres of structural proteins; called collagen or elastin. But blood is an exception; in which no structural fibre is secreted. The structural fibres provide strength, elasticity and flexibility to the tissue. The cells of the connective tissue also secrete polysaccharides. These polysachharides accumulate between cells and fibres and act as matrix (ground substance).
There are three types of connective tissues, viz. loose connective, dense connective and specialized connective tissues.
Loose Connective Tissue: The cells and fibres are loosely arranged in a semi-fluid matrix; in loose connective tissue. Areolar tissue is an example of loose connective tissue. Areolar tissue is present beneath the skin. It usually makes the support framework for epithelium. It contains fibroblasts, macrophages and mast cells. Adipose tissue is another example of loose connective tissue. Adipose tissue is usually located beneath the skin. The cells of the adipose tissue are specialized to store fat.
Dense Connective Tissue: The cells and fibres are compactly packed in dense connective tissue. The orientation of fibres can be regular or irregular. On this basis, the dense connective tissue is called dense regular or dense irregular connective tissue. In dense regular connective tissue, the collagen fibres are present in rows between many parallel bundles of fibres, e.g. tendons and ligaments. Tendons attach muscles to bones, while ligaments attach one bone to another. In dense irregular tissue, fibroblasts and fibres are oriented in irregular pattern. Dense irregular tissue is present in skin.
Specialised Connective Tissue: Cartilage, bones and blood are the specialized connective tissues.
Cartilage: The matrix of cartilage is solid and pliable and resists compression. The cells of cartilage are called chondrocytes. Chondrocytes are enclosed in small cavities within the matrix. Most of the cartilages in vertebrae embryo are replaced by bones in adulthood.
Bones: The matrix of bones is hard and non-pliable. The matrix is rich in calcium salts and collagen fibres; which give strength to the bone. The bone cells are called osteocytes. Osteocytes are present in spaces; called lacunae. The bone marrow in some bones is the site of production of blood cells.
Blood: Blood is a fluid connective tissue. It contains plasma, blood cells and platelets. Blood is the main circulating fluid which helps in transport of various substances.
A muscle is made of many long, cylindrical fibres which are arranged in parallel rays. The muscle fibres are composed of very fine fibrils; called myofibril. The muscle fibres can contract and relax in a coordinated fashion. Thus, muscles play an important role in movement and locomotion.
There are three types of muscles, viz. skeletal, smooth and cardiac muscles.
Skeletal Muscle: The skeletal muscle is closely attached to skeletal bones. Skeletal muscle fibres are bundled together in a parallel fashion. Several bundles of muscle fibres are covered by a sheath of tough connective tissue. Striations are present in skeletal muscles and hence they are also called striated muscles. The voluntary movements are facilitated by skeletal muscles. Skeletal muscles quickly get tired.
Smooth Muscle: The smooth muscle fibres are spindle-shaped. Striations are absent on smooth muscle. Various smooth muscle fibres are held together by cell junctions. They are bundled together in a sheath of connective tissue. Smooth muscles are present in those organs which are not under our conscious control. Smooth muscles can carry on their activity for a longer duration, as compared to skeletal muscles.
Cardiac Muscle: The muscles in the heart are called cardiac muscles. Cardiac muscles are multinucleated and have striations. The muscle fibres are held together by cell junctions. Communication junctions (intercalated discs) are present at some fusion points. These communication junctions allow the cells to contract as a unit. Due to this, when one cell receives a signal to contract, its neighbours are also stimulated to contract. Cardiac muscles keep on contracting and relaxing continuously throughout the life.
Neural tissue is made of neurons. Neurons are excitable cells. The neuroglial cell protects and supports neurons. Neuroglia make up more than half of the volume of the neural tissue in human body.
When a neuron is stimulated, an electrical disturbance is generated. The electrical disturbance travels along its plasma membrane. When a disturbance arrives at a neuron’s endings, it triggers events which may cause stimulation of inhibition of adjacent neurons and other cells. Thus, neural signals are transmitted to different parts of the body.
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