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Animal Tissues

On the basis of the structure of cells and their function, animal tissues are classified into four major types:

1. Epithelial Tissue
2. Muscular Tissue
3. Nervous Tissue
4. Connective Tissue

Epithelial Tissue:

• The covering or protective tissues in the animal body are animal tissues.
• The cells of this tissue are tightly packed and it forms continuous sheet. Indeed cells of epithelium contain very little or no intercellular matrix.
• The skin and lining of 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.

Functions:

• Epithelial cells protect the underlying cells from mechanical and chemical injuries and bacterial or viral infection.
• It covers most organs and cavities within the body. It also forms a barrier to keep different body system separate.
• Epithelial tissues help in absorption of water and nutrients
• Epithelial tissues help in elimination of waste products.
• Some epithelial tissues secrete secretion, such as sweat, saliva etc.

Note: 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.
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

Differences between different types of epithelial tissues:

S. No.	Characteristic property	Squamous epithelium	Cuboidal epithelium	Columnar or glandular epithelium	Ciliated epithelium
	Shape of cells	It consists of thin, flat. Disc like polygonal or irregular-shaped cells with round and flat nucleus.	It consists of cube-like cells of almost equal height and width.	It consists of tall, cylindrical, pillar-like cells. Basal part of cell bears oval nucleus	It consists of tall cells with cytoplasmic hair like cilia at free ends.
	Appearance of cells	Adjacent cells fit together like tiles on a pavement or floor.	Cells appear square-like in vertical section but their free surface seems to be hexagonal.	The free end of the cells consists of finger-like projections called microvilli.	Cells may be cuboidal or columnar and are, therefore , also called ciliated cuboidal epithelium or ciliated columnar epithelium.
	Place of occurrence	Forms the lining of nose, pericardial cavity, blood vessels, lung alveoli  etc.	Present in kidney tubules, salivary glands etc.	Present in the inner surface of stomach, intestine, gall bladder etc.	Present in the lining of trachea, fallopian tube, nasal passage etc.
	Functions	Protects the underlying parts of the body from mechanical injury. Prevent the entry of germs inside our body. Prevent desiccation of organs. Facilitates diffusion of gases.	Provide mechanical support to the organs. Secretion of gastric juices. Absorption and excretion.	Helps in absorption of nutrients. Secretion of gastric juices. Provide mechanical support to the organs.	Causes movement of small solid particles or mucus in a specific direction through the ducts. Causes movement of ovum and zygote towards the uterus. Helps in removing unwanted particles from trachea.

Muscular tissue:

• Muscular tissue constitutes all the muscles of the body of an animal.
• Muscle cells are elongated and large sized, so they are called muscle fibres.
• Muscle cells are typically arranged in parallel arrangement allowing them to work together effectively.
• This tissue is responsible for movement in our body. Muscles contain special proteins called contractile proteins, which contract and relax to cause movement.
• On the basis of their location, structure and function, there are following three types of muscle fibers:

1. Striated muscles (stripped, skeletal or voluntary muscles)
2. Smooth muscles (unstriated, visceral or involuntary muscles)
3. Cardiac muscles

S.No.	Unstriated muscles	Striated muscles	Cardiac muscles
1.	Present in the wall of alimentary canal, blood vessels, respiractory tract, urinary bladder etc.	Present in limbs, tongue, body wall and pharynx.	They are present in the wall of heart.
2.	Muscle fibres are spindle- shaped.	Muscle fibres are cylindrical.	Muscle fibres are cylindrical.
3.	Fibres are unbranched.	Fibres are unbranched.	Fibres are branched.
	Muscle cells are multinucleate.	Muscle cells are uninucleate.	Muscle cells are uninucleate.
	Nerve supply from autonomous nervous system.	Nerve supply from central nervous system.	Nerve supply from both autonomous and central nervous system.
	Cross striations absent.	Dark and light bands (cross striations) present.	Cross striations and intercalated disc present.
	Exhibit slow contraction.	Exhibit rapid contraction.	Exhibit rapid contraction.
	Involuntary.	Voluntary.	Involuntary.
	Do not get fatigued.	Get fatigued.	Do not get fatigued.
	Function: Cause contraction and mobility in visceral organs and involuntary muscles.	Function: Cause movement of limbs and locomotion.	Function: cause heartbeat.

Nervous tissue:

• A tissue which is specialized to transmit messages in our body is nervous tissue. Brain, spinal cord and nerves are all composed of nervous tissue.
• Nervous tissue contains highly specialized unit cells called nerve cells or neurons.
• These cells are specialized for the conduction of impulse over great distance at great speed.
• A neuron consists of a cell body (cyton or soma) with a nucleus and cytoplasm, from which long thin hair- like parts arise called dendrons.
• Dendrons further branched out to form dendrites. From the distal part of cyton arises a very long process called axon.

Functions:

• The nervous tissue is responsible for the reception and transmission of information between different parts of the body.
• The dendrites receive impulses and the axon takes impulses away from the cell body.

Connective Tissue:

• The connective tissue is specialized to connect and anchor various body organs.  As such, it connects one bone with another and a bone with a muscle.
• Three components are present in all the connective tissues. These are intercellular medium, connective tissue cells and fibers.
• The cells of connective tissue are loosely spaced and embedded in an intercellular matrix. The matrix may be jelly like, fluid, dense or rigid.
• The nature of matrix decides the function of connective tissue.
• General functions:
• Connective tissue binds other tissues together in the organs.
• Connective tissue also provides the structural framework and mechanical support to different tissues.
• It is also concerned with body defense, fat storage, repair etc.
• The main functions of connective tissue are binding, supporting and packing together different organs of the body.

Types of connective tissue:

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

1. Areolar (loose) connective tissue
2. Dense connective tissue
3. Adipose connective tissue
4. Skeletal tissue
5. Fluid connective tissue

Aerolar (loose) connective tissue:

• It is a loose and cellular connective tissue. Its matrix consists of two kinds fibers: white collagen fibers and yellow elastic fibers.
• Aerolar connective tissue is found between the skin and muscles, around blood vessels and nerves and in the bone marrow.
• It fills the spaces between different tissues and organs, hence called packing tissue.

Functions:

• It acts as supporting and packing tissue between organs lying in the body cavity.
• It provides rapid diffusion of oxygen and nutrients from blood vessels.
• It helps in repair of tissues after an injury.
• It helps in fighting foreign antigen and toxin.

Dense connective tissue:

• It is a fibrous connective tissue. It is characterized by ordered and densely packed collection of fibers and cells.
• It is the chief component of ligaments and tendons.
• Ligaments: These are elastic structures made up of yellow elastic fibrous tissue s which connect bone to another. It has considerable strength. Ligaments contain very little matrix. Ligaments strengthen the joint and they prmit normal movement but prevent over-flexing or over-extension. Sprain is caused by excessive pulling (stretching) of ligaments.
• Tendons: Te dons are cord like, strong inelastic structures that join skeletal muscles to bones. They are composed of white collagen fibrous tissue.
• It has great strength but its flexibility is limited.

Adipose tissue:

• It consists of large number of oval and rounded adipose cells (adipocytes) filled with fat globules.
• The adipose tissue is abundant below the skin, between the internal organs (e.g., around the kidney) in yellow bone marrow.

Functions:

• It serves as a fat reservoir.
• Adipose tissue acts as food reservoir by storing fat.
• It acts as an insulator and regulates body temperature.

Skeletal tissue:

• Skeletal connective tissue forms the endoskeleton of the body of vertebrates. It includes cartilage and bone.

S.No.	Bone	Cartilage
1.	They are hard and flexible endoskeleton.	They are soft and flexible endoskeleton.
2.	Porous in nature.	Non-porous in nature.
3.	Blood vessels are present.	Blood vessels are absent.
4.	Matrix not arranged in lamellae.	Matrix (chondrin) arranged in lamellae.
6.	Bone cells are known as osteocytes.	The cells in matrix are called chondriocytes.
7.	Matrix contains protein and mineral salts.	Matrix made up mainly of protein.
8.	Long bones contain bone marrow in hollow, narrow cavity.	Bone marrow absent. Cartilage is always solid.
9.	Bones are present in the whole body forming internal skeletal framework.	Cartilage are present at the joints of bones, in external ear (pinna), nose tip, epiglottis, trachea etc.
10.	Functions: It provides shape to the body. It provides skeletal support to body. It protects vital body organs such as brain, lungs etc.	Functions: Cartilage provides support and flexibility to the body parts. It smoothens body surfaces at joints.

Fluid connective tissue: Fluid connective tissue links the different parts of the body and maintains continuity in the body. It includes blood and lymph.
Blood:

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

Red blood corpuscles (RBC) or erythrocytes
White blood corpuscles (WBC) or leucocytes
Platelets
RBCs and WBCs are living, while plasma and platelets are non-living.


Posted by:- Aaryan Sekh