Animal Tissues:

Simple Epithelial Tissues:

• This tissue has a free surface, which faces either a body fluid or the outside environment and thus provides a covering or a lining for some part of the body.

• The cells are compactly packed with a little intercellular matrix.

• There are two types of epithelial tissues namely

  • Simple epithelium

  • Compound epithelium.

• Simple epithelium is composed of a single layer of cells and functions as a lining for body cavities, ducts, and tubes.

• The compound epithelium consists of two or more cell layers and has a protective function as it does in our skin.

• On the basis of the structural modification of the cells, the simple epithelium is further divided into three types.

Squamous Epithelium:

• It is made of a single thin layer of flattened cells with irregular boundaries.

• They are found in the walls of blood vessels and air sacs of lungs and are involved in functions like forming a diffusion boundary.

Cuboidal Epithelium:

• It is composed of a single layer of cube-like cells.

• This is commonly found in ducts of glands and tubular parts of nephrons in kidneys and its main functions are secretion and absorption.

• The epithelium of the proximal convoluted tubule (PCT) of the nephron in the kidney has microvilli.

Columnar Epithelium:

• It is composed of a single layer of tall and slender cells.

• Their nuclei are located at the base.

• The free surface may have microvilli.

• They are found in the lining of the stomach and intestine and help in secretion and absorption.

• If the columnar or cuboidal cells bear cilia on their free surface they are called the ciliated epithelium.

  • Their function is to move particles or mucus in a specific direction over the epithelium.

  • They are mainly present on the inner surface of hollow organs like bronchioles and fallopian tubes.

Glandular Epithelium:

• Some of the columnar or cuboidal cells get specialized for secretion and are called the glandular epithelium.

• They are mainly of two types:

  • Unicellular, consisting of isolated glandular cells (goblet cells of the alimentary canal)

  • Multicellular, consisting of a cluster of cells (salivary gland).

• On the basis of the mode of pouring of their secretions, glands are divided into two categories namely exocrine and endocrine glands.

  • Exocrine glands secrete mucus, saliva, earwax, oil, milk, digestive enzymes, and other cell products.

    • These products are released through ducts or tubes.

  • In contrast, endocrine glands do not have ducts.

    • Their products called hormones are secreted directly into the fluid bathing the gland.

Compound Epithelium:

• It is made of more than one layer (multi-layered) of cells and thus has a limited role in secretion and absorption.

• Their main function is to provide protection against chemical and mechanical stresses.

• They cover the dry surface of the skin, the moist surface of the buccal cavity, the pharynx, the inner lining of ducts of salivary glands, and pancreatic ducts.

Junctions:

• All cells in the epithelium are held together with little intercellular material.

• In nearly all animal tissues, specialized junctions provide both structural and functional links between their individual cells.

• Three types of cell junctions are found in the epithelium and other tissues.

• These are called tight, adhering, and gap junctions.

  • Tight junctions help to stop substances from leaking across a tissue.

  • Adhering junctions perform cementing to keep neighboring cells together.

  • Gap junctions facilitate the cells to communicate with each other by connecting the cytoplasm of adjoining cells, for rapid transfer of ions, small molecules, and sometimes big molecules.

Connective Tissue:

• They are most abundant and widely distributed in the body of complex animals.

• They are named connective tissues because of their special function of linking and supporting other tissues/organs of the body.

• They range from soft connective tissues to specialized types, which include cartilage, bone, adipose, and blood.

  • In all connective tissues except blood, the cells secrete fibres of structural proteins called collagen or elastin.

• The fibres provide strength, elasticity and flexibility to the tissue.

  • These cells also secrete modified polysaccharides, which accumulate between cells and fibres and act as a matrix (ground substance).

• Connective tissues are classified into three types:

  • Loose connective tissue.

    • Loose connective tissue has cells and fibres loosely arranged in a semi-fluid ground substance, for example, areolar tissue present beneath the skin.

      • Often it serves as a support framework for epithelium.

      • It contains fibroblasts (cells that produce and secrete fibres), macrophages and mast cells.

    • Adipose tissue is another type of loose connective tissue located mainly beneath the skin.

      • The cells of this tissue are specialised to store fats.

      • The excess nutrients which are not used immediately are converted into fats and are stored in this tissue.

  • Dense connective tissue.

    • Fibres and fibroblasts are compactly packed in dense connective tissues.

    • The orientation of fibres shows a regular or irregular pattern and is called dense regular and dense irregular tissues.

    • In the dense regular connective tissues, the collagen fibres are present in rows between many parallel bundles of fibres.

      • Tendons, which attach skeletal muscles to bones and ligaments which attach one bone to another are examples of this tissue.

    • Dense irregular connective tissue has fibroblasts and many fibres (mostly collagen) that are oriented differently.

      • This tissue is present in the skin.

  • Specialised connective tissue.

    • Cartilage is solid and pliable and resists compression.

      • Cells of this tissue (chondrocytes) are enclosed in small cavities within the matrix secreted by them.

      • Most of the cartilage in vertebrate embryos is replaced by bones in adults.

      • Cartilage is present in the tip of the nose, outer ear joints, and between adjacent bones of the vertebral column, limbs and hands in adults.

    • Bones have a hard and non-pliable ground substance rich in calcium salts and collagen fibres which give bone its strength.

      • It is the main tissue that provides a structural frame to the body.

      • Bones support and protect softer tissues and organs.

      • The bone cells (osteocytes) are present in the spaces called lacunae.

      • Limb bones, such as the long bones of the legs, serve weight-bearing functions.

      • They also interact with skeletal muscles attached to them to bring about movements.

      • The bone marrow in some bones is the site of the production of blood cells.

    • Blood is a fluid connective tissue containing plasma, red blood cells (RBC), white blood cells (WBC) and platelets.

      • It is the main circulating fluid that helps in the transport of various substances.

Muscle Tissue:

• Each muscle is made of many long, cylindrical fibres arranged in parallel arrays.

• These fibres are composed of numerous fine fibrils, called myofibrils.

• Muscle fibres contract (shorten) in response to stimulation, then relax (lengthen) and return to their uncontracted state in a coordinated fashion.

• Their action moves the body to adjust to the changes in the environment and to maintain the positions of the various parts of the body.

• In general, muscles play an active role in all the movements of the body.

• Muscles are of three types, skeletal, smooth, and cardiac.

Skeletal Muscle:

• It is closely attached to skeletal bones.

• In a typical muscle such as the biceps, striated (striped) skeletal muscle fibres are bundled together in a parallel fashion.

• A sheath of tough connective tissue encloses several bundles of muscle fibres

Smooth Muscle:

• They taper at both ends (fusiform) and do not show striations.

• Cell junctions hold them together and they are bundled together in a connective tissue sheath.

• The wall of internal organs such as the blood vessels, stomach and intestine contains this type of muscle tissue.

• Smooth muscles are ‘involuntary’ as their functioning cannot be directly controlled.

Cardiac Muscle:

• It is a contractile tissue present only in the heart.

• Cell junctions fuse the plasma membranes of cardiac muscle cells and make them stick together.

• Communication junctions (intercalated discs) at some fusion points allow the cells to contract as a unit, i.e., when one cell receives a signal to contract, its neighbours are also stimulated to contract.

Neural Tissue:

• Neural tissue exerts the greatest control over the body’s responsiveness to changing conditions.

• Neurons, the unit of the neural system are excitable cells.

• The neuroglial cell which constitutes the rest of the neural system protects and supports neurons.

• Neuroglia makes up more than one-half the volume of neural tissue in our body.

• When a neuron is suitably stimulated, an electrical disturbance is generated which swiftly travels along its plasma