Animal Tissues

Animal tissues are groups of similar cells, with intercellular substances, that together perform a specific function. All complex animals are built from only four basic animal tissues: epithelial, connective, muscular and neural. This page covers the kinds of epithelium and glands, cell junctions, loose, dense and specialised connective tissues, the three types of muscle, and neural tissue. NEET asks where each tissue is found and what it does, often through match-the-list and statement questions, so learn every tissue with its location and function.

1. What Is a Tissue?

• In unicellular organisms, a single cell performs all functions, such as digestion, respiration and reproduction.
• In multicellular animals, these functions are carried out by different groups of cells in a well-organised manner.
• The body of Hydra has different types of cells, each type numbering in thousands; the human body has billions of cells.

• The structure of cells varies with their function, so tissues differ from one another.
• ★ Exam imp Tissues are broadly classified into four types: epithelial, connective, muscular and neural.
• Tissues arranged in a specific proportion and pattern form organs, such as the stomach, lung, heart and kidney.
• Two or more organs that perform a common function by physical and/or chemical interaction form an organ system, such as the digestive and respiratory systems.
• Cells, tissues, organs and organ systems split up the work; this division of labour ensures the survival of the body as a whole.

2. Epithelial Tissue

2.1 General features

• An epithelial tissue is commonly called an epithelium (plural: epithelia).
• It has a free surface, which faces either a body fluid or the outside environment.
• It therefore provides a covering or a lining for some part of the body.
• Its cells are compactly packed with little intercellular matrix.
• Epithelia are sheet-like tissues; their cells are structurally and functionally connected at junctions.

2.2 Simple epithelium

• Based on the structural modification of its cells, simple epithelium is of three types: squamous, cuboidal and columnar.

• ★ Exam imp The cuboidal epithelium of the proximal convoluted tubule (PCT) of the nephron has microvilli.
• The free surface of columnar cells may have microvilli.
• Ciliated epithelium: columnar or cuboidal cells that bear cilia on their free surface.

2.3 Glandular epithelium

• Glandular epithelium: columnar or cuboidal cells that are specialised for secretion.
• Unicellular glands are isolated glandular cells, such as the goblet cells of the alimentary canal.
• Multicellular glands are clusters of cells, such as the salivary gland.
• Based on how they pour out their secretions, glands are exocrine or endocrine.

2.4 Compound epithelium

• Compound epithelium is made of more than one layer (multi-layered) of cells.
• ★ Exam imp So it has only a limited role in secretion and absorption.
• Its main function is protection against chemical and mechanical stresses.
• It covers the dry surface of the skin and the moist surface of the buccal cavity and pharynx.
• It also forms the inner lining of the ducts of salivary glands and of pancreatic ducts.

2.5 Cell junctions

• All cells in an epithelium are held together with little intercellular material.
• In nearly all animal tissues, specialised junctions provide both structural and functional links between cells.
• ★ Exam imp Three types of cell junctions occur in the epithelium and other tissues: tight, adhering and gap junctions.

3. Connective Tissue

3.1 General features

• ★ Exam imp Connective tissues are the most abundant and widely distributed tissues in the body of complex animals.
• They are named for their special function of linking and supporting other tissues and organs.
• They bind together, support, strengthen, protect and insulate other tissues.
• They range from soft connective tissues to specialised types such as cartilage, bone, adipose tissue and blood.
• In all connective tissues except blood, the cells secrete fibres of structural proteins called collagen or elastin.
• The fibres give the tissue strength, elasticity and flexibility.
• The cells also secrete modified polysaccharides, which collect between cells and fibres as the matrix (ground substance).
• Soft connective tissues consist of protein fibres and a variety of cells arranged in a ground substance.

1. Loose connective tissue: areolar tissue and adipose tissue.
2. Dense connective tissue: dense regular and dense irregular.
3. Specialised connective tissue: cartilage, bones and blood.

3.2 Loose connective tissue

• Loose connective tissue: cells and fibres loosely arranged in a semi-fluid ground substance.
• Areolar tissue, present beneath the skin, is an example; it often serves as a support framework for epithelium.
• ★ Exam imp Areolar tissue 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.
• Its cells are specialised to store fats. Excess nutrients not used immediately are converted into fats and stored here.
• Adipose tissue is thus a reservoir of stored energy.

3.3 Dense connective tissue

• In dense connective tissues, fibres and fibroblasts are compactly packed.
• The fibres show a regular or an irregular pattern, giving dense regular and dense irregular tissues.

3.4 Specialised connective tissue

• Cartilage, bones and blood are specialised connective tissues; cartilage and bone are structural materials, and blood is a fluid tissue with transport functions.

• Most cartilages of vertebrate embryos are replaced by bones in adults.
• ★ Exam imp In adults, cartilage is present in the tip of the nose, outer ear, joints, between adjacent bones of the vertebral column, and in the limbs and hands.
• Calcium salts and collagen fibres give bone its strength; bone is the main tissue that provides the structural frame of the body.
• Limb bones, such as the long bones of the legs, serve weight-bearing functions.
• Bones interact with the skeletal muscles attached to them to bring about movements.
• ★ Exam imp The bone marrow in some bones is the site of production of blood cells.
• Blood is a fluid connective tissue made of plasma, red blood cells (RBC), white blood cells (WBC) and platelets.

4. 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) when stimulated, then relax (lengthen) and return to their uncontracted state in a coordinated way.
• Their action moves the body to adjust to changes in the environment and keeps the parts of the body in position.
• Muscles play an active role in all movements of the body.
• ★ Exam imp Muscles are of three types: skeletal, smooth and cardiac.

• We usually cannot make smooth muscle contract merely by thinking about it, as we can with skeletal muscle.
• ★ Exam imp In cardiac muscle, communication junctions (intercalated discs) at some fusion points let the cells contract as a unit.
• When one cardiac cell receives a signal to contract, its neighbours are also stimulated to contract.
• Skeletal muscle is attached to bones, smooth muscle is a component of internal organs, and cardiac muscle forms the contractile walls of the heart.
• Connective tissue covers the muscle tissues: sheaths of connective tissue enclose bundles of skeletal and smooth muscle fibres.

5. Neural Tissue

• Neural tissue exerts the greatest control over the body's responsiveness to changing conditions.
• ★ Exam imp Neurons, the units of the neural system, are excitable cells.
• Neuroglial cells make up the rest of the neural system; they protect and support the neurons.
• Neuroglia make up more than one-half the volume of neural tissue in our body.

1. A neuron is suitably stimulated.
2. An electrical disturbance is generated and travels swiftly along its plasma membrane.
3. The disturbance arrives at the neuron's endings, the output zone.
4. This triggers events that may cause stimulation or inhibition of adjacent neurons and other cells.

6. Exam Essentials

Pairs to Match

Numbers to Remember

• 4 basic types of tissue.
• 1 layer of cells in simple epithelium; 2 or more in compound epithelium.
• 3 types of simple epithelium: squamous, cuboidal, columnar.
• 3 types of cell junctions: tight, adhering, gap.
• 3 types of connective tissue: loose, dense, specialised.
• 3 types of muscle: skeletal, smooth, cardiac.
• Neuroglia: more than one-half the volume of neural tissue.

7. Quick Revision

• Tissue: similar cells with intercellular substances; four basic types: epithelial, connective, muscular, neural.
• Epithelium: free surface, compact cells, little matrix; simple (one layer) or compound (many layers).
• Squamous: blood vessels and air sacs (diffusion); cuboidal: gland ducts and nephron tubules; columnar: stomach and intestine.
• Ciliated epithelium moves particles or mucus: bronchioles and fallopian tubes.
• Goblet cells are unicellular glands; the salivary gland is multicellular.
• Exocrine glands secrete through ducts; endocrine glands are ductless and secrete hormones.
• Compound epithelium protects: skin, buccal cavity, pharynx, salivary and pancreatic ducts.
• Junctions: tight (no leaks), adhering (cement), gap (cytoplasmic connection).
• Connective tissue: most abundant; fibres of collagen or elastin in all types except blood.
• Loose: areolar (fibroblasts, macrophages, mast cells) and adipose (stores fat).
• Dense regular: tendons and ligaments; dense irregular: skin.
• Cartilage: chondrocytes, pliable matrix; bone: osteocytes in lacunae, calcium salts; bone marrow makes blood cells.
• Muscle: myofibrils; skeletal striated, smooth fusiform and involuntary, cardiac with intercalated discs.
• Connective tissue sheaths enclose bundles of skeletal and smooth muscle fibres.
• Neural tissue: excitable neurons; neuroglia protect, support and form over half the volume.

8. Solved Examples

9. Practice Questions

Common Mistakes to Avoid