Types of Animal Tissues, Functions and Examples - CBSE Tuts (2023)

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Structure and Types of Animal Tissues

Breathing is one of the most vital activities for humans. When we breathe, we can, in fact, feel the movement of our lungs and diaphragm. How do these parts move inside our body? For breathing, we have specialized cells in our bodies, called muscle cells. The contraction and relaxation of these muscle cells result in movement.

During breathing, we inhale oxygen gas (along with air). Where does this oxygen go? It goes to our two lungs and then is transported to all cells of our body through the blood. So, why do cells need oxygen? Mitochondria present in each and every cell of our body utilizes oxygen to generate energy in the form of ATP molecules. These ATP molecules provide energy for all sorts of activities performed by our body cells.

Blood is a fluid connective tissue. It flows and connects different parts of the body. It carries oxygen and food to all cells and in return collects metabolic wastes (e.g., carbon dioxide) from them. Thus, muscles and blood, both are examples of tissues found in our body. On the basis of the functions they perform in the body of multicellular animals, the animal tissues are classified as

1. Epithelial tissue
2. Muscle or muscular tissue
3. Connective tissue
4. Nervous tissue

Muscle forms the muscular tissue and blood is a type of connective tissue. These tissues are further differentiated as shown in the following chart:

Epithelial Tissue: Structure with Diagram, Function, Types

Nature: Epithelial tissue is the simplest tissue. It is the protective tissue of the animal body (as the epidermis of plants). The cells of this tissue are tightly packed and they form a continuous sheet. Cells of epithelium contain very little or no intercellular matrix. The tissue covers most organs and cavities within the body. It also forms a barrier to keep different body systems separate. The skin and lining of the buccal cavity, blood vessels, alveoli (of lungs), and kidney tubules are made of epithelial tissue. Epithelial cells lie on a delicate non-cellular basement membrane which contains a special form of matrix protein, called collagen. Epithelial tissue may be simple, i.e., composed of a single layer of cells, or stratified, i.e., made up of several layers of cells.

Functions of Epithelial Tissue:

• The cells of the body surface (i.e., epidermis) form the outer layer of skin.
• These cells protect the underlying cells from drying, injury, and chemical effects.
• They also protect the body from viral or bacterial infection.
• Inside the body, epithelial cells form the lining of the mouth and alimentary canal and protect these organs.
• Epithelial tissues help in the absorption of water and nutrients.
• Epithelial tissues help in the elimination of waste products.
• Some epithelial tissues perform the secretory function. They secrete a variety of substances such as sweat, saliva (mucus), enzymes, etc.

Types of Epithelial Tissue
Depending upon the shape and function of the cells, the epithelial tissues are classified as follows:

1. Squamous epithelium
2. Cuboidal epithelium
3. Columnar epithelium
4. Glandular epithelium
5. Ciliated epithelium

1. Squamous Epithelium
Nature: Squamous epithelium is made up of thin, flat, irregular-shaped cells which fit together like floor tiles to form a compact tissue. Squamous epithelium is also known as tessellated and pavement epithelium.

Occurrence: It forms the delicate lining of cavities (mouth, oesophagus, nose, pericardium, alveoli, etc.) and of blood vessels and covering of the tongue and skin.

Functions of Squamous Epithelium:
This epithelium protects the underlying parts of the body from mechanical injury, entry of germs, chemicals, and drying. It also forms a selectively permeable surface through which Alteration occurs.

Stratified Keratinized Squamous Epithelium
This is found in the skin and covers the external dry surface of the skin. Cells of this tissue are arranged in many layers. Cells forming different layers of this epithelium are not similar. Deeper layers of the tissue have cuboidal cells which become polygonal and finally flattened (squamous) towards the free surface.

The flattened cells of the superficial layer contain a fibrous protein, keratin, and become dead cells. Florny, scale-like remains of dead squamous cells, ultimately flake away. This epithelium is water-proof and highly resistant to mechanical injury.

2. Cuboidal Epithelium
Nature: It consists of cube-like (cubical) cells that are square in section but the free surface appears hexagonal.

Occurrence: The cuboidal epithelium is found in kidney tubules, thyroid vesicles and in glands (e.g., salivary glands, sweat glands, and exocrine pancreas). It forms the germinal epithelium of gonads (testes and ovaries).

Functions of Cuboidal Epithelium:
It helps in absorption, excretion, and secretion. It also provides mechanical support.

3. Columnar Epithelium
Nature: The columnar epithelium consists of cells that are taller than broader (i.e., pillar-like). The nuclei are towards the base and sometimes the free ends of cells have a brush border containing microvilli.

Occurrence: It forms the lining of the stomach, small intestine, and colon, forming a mucous membrane. It also forms the lining of the gall bladder and oviducts and facilitates movement across the cells.

Functions of Columnar Epithelium:
Its main functions include absorption (e.g., stomach, intestine) and secretion (e.g., mucus by goblet cells or mucous membrane.

Differences between Squamous Epithelium and Columnar Epithelium

4. Glandular Epithelium
The columnar epithelium is often modified to form glands that secrete chemicals.

6. Ciliated Epithelium
Nature: Certain cuboidal or columnar cells have a free border that bears thread-like cytoplasmic outgrowths, called cilia. Such cells form the ciliated epithelium.

Occurrence: Ciliated cuboidal epithelium is found in the sperm ducts. The ciliated columnar epithelium lines the trachea (wind-pipe), bronchi (lungs), kidney tubules, and oviducts (Fallopian tubes).

Functions of Ciliated Epithelium:
The rhythmic, concerted beating of the cilia moves solid particles (e.g., mucus) in one direction through the ducts.

Structure, Functions, and Types of Muscular Tissue

The muscle tissues or muscles of the body form the contractile tissue and are made of muscle cells. Muscle cells are elongated and large-sized, so they are also called muscle fibers. The movements of the body or limbs are brought about by contraction and the relaxation of contractile proteins which are present in muscle cells. The movements of the internal organs such as the heart and alimentary canal, are all caused by muscle tissues. Muscle cells are typically arranged in a parallel arrangement allowing them to work together effectively.

On the basis of their location, structure, and function, there are following three types of muscle fibres:

• Striated muscles
• Smooth muscles
• Cardiac muscles

1. Striated Muscles
Nature: Striated muscles are also known as striped, skeletal or voluntary muscles. Since the entire muscle fibres show alternate dark and light stripes (striations or bands), they are called striped muscles. Since they are attached to the bones and are responsible for body movements, they are called skeletal muscles. And lastly, since these muscles work according to our will, they are also called voluntary muscles.

The striated muscle fibres (cells) are long or elongated, non-tapering, cylindrical, and unbranched. These cells have a number of nuclei (i.e., each muscle cell is multi-nucleated). Each muscle cell is enclosed in a thin but distinct plasma membrane, called sarcolemma. Its nuclei are peripheral in position, i.e., nuclei are located just beneath the sarcolemma. In the sarcoplasm (cytoplasm) of the muscle cell are embedded a large number of contractile elements, called sarcostyles or myofibrils.

Occurrence: Striated muscles occur in the muscles of limbs (e.g., biceps and triceps of arms), body wall, face, neck, etc. Striated muscles present in the tongue, pharynx, diaphragm, and upper part of the esophagus are called visceral striated muscles.

Functions of Striated Muscles:

• Striated muscles are powerful and undergo rapid Contraction. These muscles can get tired and may need rest.
• Striated muscles provide the force for locomotion and all other voluntary movements of the body.

2. Smooth Muscles
Nature: Smooth smooth muscles are also known as unstriated, visceral or involuntary muscles. Smooth muscles occur as bundles or sheets of elongated fusiform or spindle-shaped (pointed at both ends) cells or fibres. These are held together by loose connective tissue. Each muscle cell is enclosed in a plasma membrane. There is a single centrally located cigar-shaped nucleus in the centre of the cytoplasm or sarcoplasm (uninucleate cell). Delicate, contractile threads called myofibrils run longitudinally throughout the cell. These fibrils do not bear any bands, stripes, or striations across the muscle hence, called smooth or unstriated muscles.

Occurrence: Smooth muscles are found in the walls of the hollow (tubular) visceral organs except that of the heart, and so are called visceral muscles. Thus, they occur in the wall of the alimentary canal and internal organs, ducts of glands, urogenital ducts, and blood vessels. Smooth muscles are found in the stomach, intestine, ureter, bronchi, iris of eye, etc.

Functions Smooth Muscles:

• Smooth muscles do not work (contract) according to our will, so they are also called involuntary muscles. The movement of food in the alimentary canal, and opening and closing of tubes are involuntary movements.
• Smooth muscle contracts slowly but can remain contracted for long periods of time. Due to this fact, smooth muscles cause the characteristic peristaltic movements in the tubes. Peristaltic movements are the rhythmic progressive waves of muscular contraction and relaxation. Such wave-like peristaltic movements occur in the gastrointestinal tract and male genital tract.
• In some organs, smooth muscles contract throughout the organ to produce extrusive movements as in the urinary bladder, the gall bladder, and the uterus.

3. Cardiac Muscles
Nature: Cardiac muscles show characteristics of both smooth and striated muscles. Cardiac muscles are composed of branched fibres and the branches join to form a network. Each fibre or cell is surrounded by sarcolemma, a cytoplasm (sarcoplasm) with longitudinal myofibrils, and a centrally located nucleus (i.e., each cell is uninucleated). The intercellular spaces of cardiac muscles are filled with abundant loose connective tissue supplied with blood capillaries.

Cardiac muscles have stripes of light and dark bands. In addition, these muscle fibers show densely stained cross-bands called intercalated impulses (Act as impulse boosters). These are regions of interdigitations of plasma membranes (sarcolemmas) consisting of adjacent muscle cells or fibres (i.e., each fibre of cardiac muscle is formed due to the joining of an individual uninucleate muscle cell to one another in a linear arrangement).

Occurrence: The cardiac muscles occur in the heart (i.e., in the walls of the heart).

Functions of Cardiac muscles:

• Cardiac muscles contract and relax rapidly, rhythmically, and tirelessly throughout a lifetime. They contract endlessly from the early embryonic stage until death.
• The contraction and relaxation of the heart muscles help to pump and distribute blood to various parts of the body.

Comparison of Smooth, Skeletal, and Cardiac Muscles

Connective Tissue – Definition, Components, & Functions

The connective tissue is specialized to connect and anchor various body organs. The tissues can connect bones to each other, muscles to bones, bind tissues, and can also give support to various parts of the body by creating a packing around organs. The packing would prevent the organs from getting displaced by body movements. Thus, the main functions of connective tissue are binding, supporting, and packing together different organs of the body.

The cells of connective tissue are living, separated from each other (i.e., loosely spaced), and low in number. The homogeneous, gel-like intercellular substance called medium or matrix forms the main bulk of the connective tissue. Thus, the space between cells is filled with a non-living matrix which may be solid as in bone and cartilage and fluid as in the blood. Matrix is fibrous in nature and binds other tissues. In fact, the nature of the matrix decides the function of connective tissue.

Cells of Connective Tissue
Connective tissue contains the following types of cells:

• Fibroblasts: They form ground substances and fibres (e.g., collagen).
• Adipose cells: They store fats (lipids) in their vacuoles.
• Macrophages: These may be free-moving or fixed phagocytes (leucocytes or WBCs). They are involved in the destruction and removal of invading bacteria, foreign bodies and damaged cells from tissues.
• Mast cells: They secrete substances such as heparin (anticoagulant), histamine (vasodilator), and serotonin (vaso-constrictor). They promote inflammation of the infected area.
• Immunocytes: These include cells such as lymphocytes and plasma cells both producing antibodies for the immune response.

Protein fibres of the matrix
The matrix of connective tissue is secreted by the component cells. It chemically contains GAGs (i.e., glycosaminoglycans or mucopolysaccharides). The matrix also contains the following three main types of protein fibres:

• White fibres of collagen
• Yellow fibres of elastin
• Reticular fibres of reticulin

Types of Connective Tissue
In animals, there are following five types of connective tissues:

• Areolar (or loose) connective tissue
• Dense regular connective tissue
• Adipose tissue
• Skeletal tissue;
• Fluid connective tissue

1. Areolar or Loose Connective Tissue
Nature: As the name suggests, this tissue is a loose and cellular connective tissue. Its matrix consists of two kinds of fibres: 1. White collagen fibres (which change into gelatin on boiling in water) and 2. Yellow elastic fibres or elastin. Also scattered in the matrix are several kinds of irregular cells (e.g., fibroblasts), some of which can engulf bacteria and prevent infection (e.g., macrophages).

Occurrence: It is the simplest and most widely distributed connective tissue. It joins skin to muscles, fills spaces inside organs, and is found around muscles, blood vessels, and nerves.

Areolar Connective Tissues:

• It acts as a supporting and packing tissue between organs lying in the body cavity. The Matrix of this tissue is important in the diffusion of oxygen and nutrients from small blood vessels.
• It helps in the repair of tissues after an injury.
• It also helps in combating foreign toxins.
• It fixes skin to underlying muscles.

2. Dense Regular Connective Tissue
It is a fibrous connective tissue. It is characterized by an ordered and densely packed collection of fibres and cells. Dense regular connective tissue is the principal component of tendons and ligaments and aponeuroses.

Tendons: Tendons are cord-like, strong, inelastic, structures that join skeletal muscles to bones. A tendon is a white fibrous tissue that has great strength but limited flexibility. It consists of parallel bundles of collagen fibres, between which are present, rows of fibroblasts (called tendinocytes). Collagen fibres are bounded by areolar connecting tissue.

Ligaments: They are elastic structures that connect bones to bones. A ligament is highly elastic and has great strength but contains very little matrix. In the ligament, some elastic and many collagen fibres are bound together by areolar connective tissue. Fibroblasts are compressed in between regular rows of fibres.

Ligaments strengthen the joint and permit normal movement but prevent over-flexing or over-extension. A sprain is caused by excessive pulling (stretching) of ligaments.

Differences between Tendon and Ligament

Aponeuroses: These are broad sheets of dense, fibrous, collagenous connective tissues that cover, invest and form the terminations and attachments of various muscles.

3. Adipose Tissue
Nature: Adipose tissue is basically an aggregation of fat cells or adipocytes. Each fat cell is rounded or oval in shape and contains a large droplet of fat that almost fills it. The fat cells are arranged into lobules separated by partitions of collagen and elastin fibres. These partitions carry blood vessels of lobules.

Occurrence: The adipose tissue is abundant below the skin, between the internal organs (e.g., around the kidneys) and in the yellow bone marrow.

Functions of Adipose Tissue:

• Serves as a fat reservoir.
• Provides shape to the limbs and the body.
• Keeps visceral organs in position. It forms shock-absorbing cushions around the kidneys and eyeballs.
• Acts as an insulator. Being a poor conductor of heat, it reduces heat loss from the body, i.e., it regulates body temperature.

4. Skeletal Tissue
The skeletal or supporting tissue includes cartilage and bone which form the endoskeleton of a vertebrate body.

(a) Cartilage
Nature: The cartilage is a specialized connective tissue that is compact and less vascular. It has widely spaced out cells. Its extensive matrix is composed of proteins and is slightly hardened by calcium salts. Its matrix is produced and maintained by the chondrocytes. Matrix is solid, cheese-like, and firm but also slightly elastic. This accounts for its flexible nature.

The matrix of cartilage has a delicate network of collagen fibres and living cells, chondrocytes. Chondrocytes are present in fluid-filled spaces known as lacunae. Blood vessels are absent in the matrix. Chondrocytes multiply by mitosis and help in the internal growth of cartilage. Thus, cartilage is capable of continuous and rapid growth.

Occurrence: Cartilage is located in the following body parts: ear pinna, nose tip, epiglottis, intervertebral discs, end of long bones, lower ends of ribs, and rings of the trachea (wind-pipe).

Functions of Cartilage:
It provides support and flexibility to the body parts. It smoothens the surface at joints.

(b) Bone
Nature: Bone is a very strong and non-flexible tissue. Like cartilage, bone is a specialized connective tissue. It is porous, highly vascular, mineralized, hard and rigid. Its matrix is made up of proteins (e.g., osteonectin, osteocalcin, proteoglycan, and collagen). The matrix of bone is rich in salts of calcium and magnesium such as phosphates and carbonates of calcium and magnesium (e.g., hydroxyapatite). These minerals are responsible for the hardness of the bone. The matrix of bone is in the form of thin concentric rings, called lamellae.

Bone cells, called osteoblasts or osteocytes, are present between the lamellae in fluid-filled spaces called lacunae. All lacunae of the bone communicate with each other by a network of fine canals, called canaliculi. Each canaliculus is filled with a delicate cytoplasmic process of the bone cell. Through canaliculi, each bone cell of each lacuna receives food and oxygen and eliminates waste.

Functions of Bone:
Bone forms the endoskeleton in human beings and other vertebrates except for sharks. It serves the following functions:

• Provides shape to the body.
• Provides skeletal support to the body.
• Protects vital body organs such as the brain, lungs, etc.
• Serves as a storage site for calcium and phosphate.
• Anchors the muscles.

Differences between Bone and Cartilage

5. Fluid Connective Tissue
Fluid connective tissue links the different parts of the body and maintains continuity in the body. It includes blood and lymph.

(a) Blood
Blood is a fluid connective tissue. In this tissue, cells (or corpuscles) move in a fluid or liquid matrix or a medium called blood plasma. Blood plasma does not contain protein fibres but contains cells called blood corpuscles or blood cells. These blood corpuscles or cells are:

• Red Blood Corpuscles (RBCs) or Erythrocytes
• White Blood Cells (WBCs) or Leucocytes
• Platelets

RBCs and WBCs are living, while plasma and platelets are non-living. Plasma forms 55 percent of the total volume of blood. It is a complex fluid and contains inorganic salts and organic compounds. Organic substances of plasma are soluble proteins such as albumins (maintain osmotic pressure of blood), globulins (some act as antibodies), fibrinogen (used in blood clotting); and glucose, amino acids, lipids, vitamins, enzymes, hormones, and waste materials (urea, uric acid).

Red blood corpuscles (RBCs) are large in number and have iron-containing red respiratory pigment, the haemoglobin. The erythrocytes of most vertebrates are oval-shaped, nucleated, and biconvex. However, erythrocytes of mammals are circular, biconcave, disc-like, and lack nuclei. In this way, mammalian erythrocytes have an increased surface area for gaseous exchange and they accommodate much more haemoglobin in them than RBCs of other animals. Erythrocytes play a vital role in the transport of oxygen.

White blood cells (WBCs) are of two main kinds: phagocytes and immunocytes. Phagocytes are capable of phagocytosis and they carry out the function of body defence by engulfing bacteria and other foreign substances.

Phagocytes are of two types: 1. Granulocytes which have irregular-shaped nuclei and cytoplasmic granules with specific staining properties. They include neutrophils, basophils, and eosinophils. 2. Agranular leucocytes have no cytoplasmic granules and include monocytes. Monocytes have a large nucleus indented on one side and a large amount of cytoplasm. They ultimately migrate to body tissues and transform into macrophages and histiocytes. Immunocytes produce antibodies and are involved in the immune response. They include lymphocytes which have a nearly spherical nucleus and little cytoplasm with no granules. Some lymphocytes later on transform into plasma cells.

Blood platelets are minute, anucleated, fragile fragments of giant bone marrow cells, called megakaryocytes.

Formed Elements of Blood

*Percentage of white blood cells.

Difference between Red Blood Corpuscles (Erythrocytes) and White Blood Cells (Leucocytes)

Occurrence: Blood occurs in blood vessels called arteries, veins, and capillaries which are connected together to form the circulatory system. The extensive branching network of vessels enables blood to reach every part of the body.

Functions of Blood:

• Blood transports nutrients, hormones, and vitamins to the tissues and transports excretory products from the tissues to the liver and kidney.
• The red blood corpuscles (RBCs) carry oxygen to the tissues for the oxidation of foodstuff.
• The white blood cells (WBCs) fight disease either by engulfing and destroying foreign bodies or by producing antitoxins and antibodies that neutralize the harmful effects of germs.
• Blood platelets disintegrate at the site of injury and help in the clotting of blood.

(b) Lymph
Nature: Lymph is a colorless fluid that is filtered out of the blood capillaries. Since it is a part of blood, its composition is similar to that of blood except that red blood corpuscles and some blood proteins are absent in it. In the lymph, white blood cells are found in abundance.

Functions of Lymph:

• Lymph transports the nutrients (oxygen, glucose) that may have been filtered out of the blood capillaries back into the heart to be recirculated in the body.
• It brings CO2 and nitrogenous wastes from tissue fluid to blood.
• Being loaded with WBCs such as lymphocytes, the lymph protects the body against infection. It forms the defence or immune system of the body.

Nervous Tissue – Characteristics, Structure, Function

Nature: Nervous tissue is a tissue that is specialized to transmit messages within our body. The brain, spinal cord, and nerves are all composed of nervous tissue. Nervous tissue contains highly specialized unit cells called nerve cells or neurons. Neurons have the ability to receive stimuli from within or outside the body and to conduct (send) impulses (signals) to different parts of the body. The impulse travels from one neuron to another neuron.

Each neuron has the following three parts:

• The cyton or cell body contains a central nucleus and cytoplasm with characteristic deeply stained particles, called Nissl’s granules (i.e., clumps of ribosomes).
• The dendrons are short processes arising from the cyton and further branching into dendrites; and
• The axon is a single, long cylindrical process of uniform diameter. It forms fine branches terminally.

Each such twig-like branch of the axon ends in a swollen structure, called a synaptic knob or bouton. Bouton contains acetylcholine-filled vesicles. Acetylcholine (ACh) is an important neurotransmitter (i.e., a substance that plays an important role in the transmission of nerve impulses within the nervous system). Axon is also called nerve fibre.

Functions of Nervous Tissue:
The dendrites receive impulses and the axon takes impulses away from the cell body.

Difference between Axon and Dendrite