Tissues Part 2

Chapter-6 Tissues Part 2 Animal Tissues You already know that tissue is a cellular organizational level intermediate between cells and a complete

organism. Hence, a tissue is an ensemble of cells, not necessarily identical, but from the same origin, that together carry out a specific function. Organs are then formed by the functional grouping together of multiple tissues. Multicellular (large) organisms function more efficiently if cells become specialized for specific functions. There are four types of tissues found in animals: epithelial, connective, nerve, and muscle tissue. Sponges do not have tissues.

Epithelial Tissue: Epithelial tissue covers the whole surface of the body. It is made up of cells that are closely packed and are composed of one or more layers. This tissue is specialised to form the covering or lining of all internal and external body surfaces. Epithelial tissue that occurs on surfaces on the interior of the body is known as endothelium. Properties:  

Epithelial cells are packed tightly together, with almost no intercellular spaces and only a small amount of intercellular substance. Epithelial tissue, regardless of the type, is usually separated from the underlying tissue by a thin sheet of extracellular and fibrous basement membrane. The basement membrane provides structural support for the epithelium and also binds it to neighbouring structures.

Functions of Epithelial Tissue  Protection: Epithelial cells from the skin protect underlying tissue from mechanical injury, harmful chemicals, invading bacteria and from excessive loss of water.  Secretion: In glands, epithelial tissue is specialised to secrete specific chemical substances such as enzymes, hormones and lubricating fluids.  Absorption: Certain epithelial cells lining the small intestine absorb nutrients from the digestion of food.  Excretion: Epithelial tissues in the kidney excrete waste products from the body and reabsorb needed materials from the urine. Sweat is also excreted from the body by epithelial cells in the sweat glands.  Diffusion: Simple epithelium promotes the diffusion of gases, liquids and nutrients. Because they form such a thin lining, they are ideal for the diffusion of gases (eg. walls of capillaries and lungs).  Cleaning: Ciliated epithelium assists in removing dust particles and foreign bodies which have entered the air passages.



Reduces Friction: The smooth, tightly-interlocking, epithelial cells that line the entire circulatory system reduce friction between the blood and the walls of the blood vessels.

Connective Tissue It is an animal tissue that is characterized by the abundance of extracellular components (such as fibers and intercellular substances). The tissue derives its name from its function in connecting, supporting, surrounding or binding cells and tissues. Connective tissue is composed of:  cells  extracellular matrix Extracellular matrix is a special feature that distinguishes connective tissue from the other tissues of the body. This matrix may be jelly-like, fluid, dense or rigid. The nature of matrix differs according to the function of that particular connective tissue.

Type of connective tissue

Matrix

Type of cells

Location and function

Blood (liquid connective tissue)

Fluid Matrix called Plasma

Plasma contains RBC, WBC and Platelets

Location: Present all over the body in blood vessels.

Lymph (liquid connective tissue)

Fluid Matrix

Cells called lymphocytes

Location: Present in lymph vessels, unidirectional flow

Bone

Hard matrix composed of calcium and phosphorous compounds

Cells called osteocytes embedded in matrix , have slender projections

Flexible matrix due to the presence of proteins and sugars

Chondrocytes

Tendons

fibrous

__

Location & Function: connects muscles to bones, not very flexible, very strong

Ligaments

fibrous

__

Location & Function: connects bones to bones, flexible and elastic, contain very little matrix, limited strength

Areolar Tissue

Fluid (semi-solid)

Fibroblasts, plasma cells, macrophages, mast cells etc.

Location & Function: Packing tissue, have collagen fibres that give firmness to skin, fills space inside organs, supports internal organs, helps in repair of tissues

Adipose Tissue

Fluid (semi-solid)

Adipocytes

Location: Present below skin and between the internal organs

Cartilage

(white to pale yellow)

Function: Transportation of gases, digested food, hormones and waste materials to different parts of the body.

Function: immune system of the body, collects extracellular fluid. Location: Forms endoskeleton of body, Function: forms a framework that supports the body, anchors the muscles and supports the main organs

Location: ear, tip of nose, trachea, larynx, endoskeleton of fish like shark Function: smoothens the bone surfaces at joints, forms the endoskeleton in some animals.

(cells filled with fat globules)

Function: Insulation and protection

Muscular Tissue Muscles of the body are made up of elongated muscle cells also known as muscle fibre. The movement of the body is brought about by the contraction and relaxation of contractile protein present in muscle cells. These contractile proteins are actin and myosin. There are three types of muscle fibres 

Striated Muscles: also known as Skeletal muscle or "voluntary muscle"( since their contraction is under the control of the will), it is anchored by tendons to bone and is used to effect skeletal movement such as locomotion and in maintaining posture. Striated muscle cells are long, cylindrical,

unbranched and multinucleate. These muscle fibres are found in the muscles of the limbs, body wall, face, neck etc. The function of these muscles is to provide locomotion and all other voluntary movements to the body. 

Smooth muscle or "involuntary muscle" is found within the walls of organs and structures such as the oesophagus, stomach, intestines, bronchi, uterus, urethra, bladder, blood vessels, and the arrector pili in the skin (in which it controls erection of body hair). Unlike skeletal muscle, smooth muscle is not under conscious control. Each muscle fibre is a long, narrow, spindle shaped tapering cell. The cell is uninucleate. These muscles are found in the walls of the alimentary canal, and internal organs. In the alimentary canal they cause movement of the food and in the blood vessels they help the blood to flow.



Cardiac muscle is also an "involuntary muscle" but is more akin in structure to skeletal muscle, and is found only in the heart. They show characteristics of both striated and unstriated muscles. They are composed of non-tapering cells with faint cross-striations. Each cell contains one or two nuclei. The cells are cylindrical and branched. The function of the cardiac muscle is to rhythmically contract and relax throughout life without fatigue and to pump the blood and distribute it to the various parts of the body.

Nervous Tissue All living cells have the ability to react to stimuli. Nervous tissue is specialised to react to stimuli and to conduct impulses to various organs in the body which bring about a response to the stimulus. Nerve tissue (as in the brain, spinal cord and peripheral nerves that branch throughout the body) are all made up of specialised nerve cells called neurons. Neurons have many different shapes and sizes. However, a typical neuron in a human consists of four major regions: a cell body, dendrites, an axon, and synaptic terminals. Like all cells, the entire neuron is surrounded by a cell membrane. The cell body is the enlarged portion of a neuron that most closely resembles other cells. It contains the nucleus and other organelles (for example, the mitochondria and endoplasmic reticulum). The dendrites and axon are thin cytoplasmic extensions of the neuron. The dendrites, which branch out in treelike fashion from the cell body, are specialized to receive signals and transmit them toward the cell body. The single long axon carries signals away from the cell body. In humans, a single axon may be as long as 1 meter (about 3 feet). Some neurons that have cell bodies in the spinal cord have axons that extend all the way down to the toes.

Axons generally divide and redivide near their ends and each branch gives rise to a specialized ending called a synaptic knob (synaptic terminal). It is the synaptic terminals of a neuron that form connections either with the dendrites or cell body of another neuron or with effector cells in muscles or glands. Once an electrical signal has arrived at the end of an axon, the synaptic terminals release a chemical messenger called a neurotransmitter, which relays the signal across the synapse to the next neuron or to the effector cell. Many nerve fibres bind together by connective tissue make up a nerve. Muscles do not move on their own. They move only when they receive a nerve impulse. When we decide to move, the brain sends a message to the muscles that must contract. This message is in the form of an electrical impulse that travels along a nerve. Nerves are bit like electrical wires. They run from the brain, along the spinal cord in the back, and spread out all over the body. When a nerve impulse from the brain gets to a muscle, chemicals are released that cause the muscle fibres to contract.