Types of Animal Tissues – Their Structure & Functions

‘Types of Animal Tissues‘, if you have any doubt related to this topic, stay tuned with this article because it has covered everything about ‘What is Animal Tissue and its types ?‘.

But…

Before knowing more about ‘what are the four types of animal tissues ?‘, you should know ‘what is animal tissue ?‘ first.

What is Animal Tissue ?

Types of Animal Tissues

Based upon the structure and functions, simple tissues in multicellular animals are categorized into four types. These are :

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

Epithelial Tissue

(Epi means above and thelial means to grow)

This type of tissue always grows on some other type of tissue. It is characterized by the following properties :

• Cells are present end to end without or with negligible intercellular spaces.
• It always rests upon a non-cellular, non- living, gelatinous layer called basement membrane.
• There is no direct blood supply to this tissue.
• It is non-nervous in nature.
• The cells of this tissue may be flattened, cuboidal or columnar in shape.
• The cells may be present in a single layer (simple epithelium) or in many layers one above the other (compound epithelium).
• It covers all the organs and lines the cavities of hollow organs like stomach.

Functions of epithelial tissue

The epithelial tissue performs a number of functions, such as :

• It is primarily protective in function. It covers all the organs from outside e.g. epidermis of skin and forms the linings of hollow organs.
• It prevents the underlying cells from drying, injury and chemical effects. Epidermis of skin also protects the body from infections.
• Glandular epithelium secretes useful substances for the body.
• Sensory epithelium forms sense organs that help in perceiving the external stimuli.
• Germinal epithelium helps in the formation of gametes.
• In certain parts, epithelial tissue is absorptive, excretory or secretory in functions.

Depending upon the number of layers of cells the epithelial tissue is of two types i.e.,

(A) Simple (B) Compound

Simple Epithelial Tissue

In this type of epithelial tissue, the cells are present in a single layer so that all the cells rest upon basement membrane. Depending upon the shape of the cells, simple epithelial tissue is classified into five types :

(1) Squamous epithelium : It is also called pavement epithelium. It contains flattened cells arranged end to end like tiles of a floor. The cells are very thin polygonal in surface view and are slightly bulging in the centre where the nucleus is present.

Location : This type of tissue is found lining the body cavity, blood vessels, cheeks, etc.

Functions : It protects underlying parts from injury entry of germs, harmful chemicals and drying up.

(2) Cuboidal epithelium : In this type of epithelium, the cells are cube-like or isodiametric. They are polygonal when seen from the surface.

Location : This tissue is found lining the body of nephrons of kidneys, gonads, sweat glands, etc.

Functions : It is protective in function.

(3) Columnar epithelium : In this type of epithelium the cells are pillar or column-like. These are arranged side by side in such a way so that the longitudinal axis of the cells is at right angle to the free surface. The cells are polygonal in surface view.

Location : This tissue is found lining the alimentary canal, the sensory organs, etc.

Functions : It is protective, secretory, absorptive and sensory in function.

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(4) Pseudostratified epithelium : This type of epithelium contains two types of cells, i.e., long pillar-like and small triangular.

The pillar-like cells are broad on the free surface and narrow down towards the basement membrane. In between the narrow ends are present the small triangular cells.

Hence, it seems to be two-layered but is actually single-layered.

Location : It is found lining the trachea, bronchi, etc.

Function : It is protective in function.

(5) Ciliated epithelium : In this type of epithelium, the cells may be cuboidal, columnar or pseudostratified. On its free surface are present very fine hair-like cilia. These cilia are capable of beating.

Location : It is found lining the body of nephrons, bucco-pharyngeal cavity of frog and trachea. Functions : The cilia by their beating, set small structures, gases or liquids in motion over the epithelial surface.

Depending upon the functions performed by simple epithelial tissue, it is further classified into five types :

1. Glandular epithelium : Secretory in nature and found in glands.
2. Sensory epithelium: Capable of perceiving stimuli, it is found in all the sensory organs.
3. Germinal epithelium : Produces gametes and is found lining the gonads.
4. Pigmented epithelium : Contains coloured pigments and is found in the outer layer of the retina of the eye.
5. Ciliated epithelium : Causes movements by beating of cilia. It is found lining the trachea, ureters, etc.

Stratified epithelium or Compound epithelium

This type of epithelium is multilayered. The cells of the lowermost layer only, rest upon the basement membrane. It forms the epidermis of skin.

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Connective Tissue

As the name indicates, this tissue connects various tissues together in any organ. It is characterized by:

• (i) The few number of cells.
• Large intercellular spaces.
• Ground substance called matrix fills the intercellular spaces.
• White and yellow fibres present in the matrix.

Depending upon the structure and functions performed by the connective tissue, it is further classified into three types :

(A) Connective tissue proper (B) Skeletal tissue (C) Vascular tissue.

Connective Tissue Proper

It is most abundant type of connective tissue. It holds various tissues together in any organ. It is further divided into the following types :

Areolar tissue : This is the basic and most abundant type of connective tissue. In the matrix of this tissue are scattered three types of cells i.e., amoeboid cells – which are phagocytic in nature, fibrocytes-secrete fibres and mast cells – that secrete the matrix.

Two types of fibres are present in the matrix. These are inelastic white fibres and elastic yellow fibres.

Location : It is found below the skin, in the kidney, testis, between and around the muscles, etc.

Functions of Areolar tissue :

• It helps to hold various tissues together in any organ.
• It helps in the repair of body tissues after any injury.
• Histiocytes (amoeboid cells) of this tissue eat upon the foreign particles and protect against diseases.
• Mast cells in this tissue are concerned with allergy.
• It binds skin with the underlying tissues.

White fibrous connective tissue : The matrix of this tissue contains abundant white fibres forming layers or bundles making it inelastic.

Location :

• Sheets of this tissue are found covering the bones, cartilages, kidneys, etc.
• Bundles of this tissue called tendons, attach muscles to the bones.

Functions of white fibrous tissue :

• It forms protective coverings around kidney, liver, bones, cartilages, etc.
• Tendons join muscles with the bones.

Yellow fibrous connective tissue : This tissue is very elastic due to the presence of a network of very elastic yellow fibres in its matrix.

Location :

• Sheets of this tissue are found covering the blood vessels.
• Bundles of this tissue are called ligaments. They attach the bones to each other at the joints.

Functions of yellow fibrous connective tissue :

• It forms protective and elastic coverings around blood vessels.
• Ligaments hold bones together at the joints.

Muscular Tissue

All types of movements are brought about in the body with the help of muscular tissue. It is characterized by the following properties :

• It contains long, fibre-like cells called muscle fibres.
• Muscle fibres in a muscle may be present in the form of sheets or bundles.
• The muscle fibres are held together by the connective tissue to form muscles.
• The muscle fibres are capable of contractions or relaxations.
• The muscle fibres contain long thread-like proteinaceous fibres called myofibrils. These may be smooth or striated because of the presence of dark and light bands.

The muscular tissue is of three types:

1. Striped or striated or skeletal or voluntary muscle fibres.
2. Unstriped or non-striated or smooth-walled or visceral muscle fibres.
3. Cardiac muscle fibres.

Striped or striated or skeletal or voluntary muscle fibres

These are the most abundant type of muscle fibres found attached to all the bones, hence, called skeletal muscle fibres.

They are voluntary in their activity, i.e., they are under the control of our will power, e.g., movements of arms, legs, etc.

These muscle fibres are bounded from the outside by an elastic but tough covering called sarcolemma. Bounded by the sarcolemma is a clear fluid called sarcoplasm which contains contractile fibrils called myofibrils.

The myofibrils show alternate dark and light bands giving a characteristic striped or striated appearance, hence, its name.

Location : It is found attached to all the bones. But, the most common parts to find such muscles are arms, legs, body wall, face and neck.

Functions :

• Striped muscle fibres are voluntary in their activity. They bring about movements according to the will of an organism.
• Locomotory movements are brought about by striped muscles.
• Tongue movements are brought about by striped muscles.

Unstriped or non-striated or smooth- walled or visceral muscle fibres

These muscle fibres are spindle-shaped, i.e., pointed at the ends and broad in the middle.

They are usually present in the form of thin sheets, in the walls of hollow organs of the body, hence, called visceral muscle fibres.

They are bounded from the outside by simple plasma membrane. Myofibrils in these muscle fibres, do not show any light and dark bands.

That is why…

They are smooth walled or unstriped or non-striated. They are uninucleated. They are involuntary in their activity.

Location : Such muscles are found in the walls of hollow organs, such as stomach, intestines, urinary bladder, etc.

Functions :

• They are involuntary and are not under the control of will power.
• Peristaltic (wave like) contractions in the alimentary canal are brought about by the smooth muscles. These contractions push the food ball in the alimentary canal.
• In some organs, smooth walled muscles contract throughout the organ to produce extrusive movements, such as in the urinary bladder, gall bladder and the uterus.

Cardiac muscle fibres

These muscle fibres are found in the walls of heart only, hence, their name. Their structure is in between the striped and unstriped muscle fibres.

They show light and dark bands in their myofibrils. They are uninucleated. They also possess certain specific characters, such as, they are branched.

The branching forms a network. They possess dark discs called intercalated discs. They are involuntary in their activity and contract in a rhythmic manner.

Location : In the walls of heart.

Functions : The pulsating movements and rhythmicity of heart are controlled by these muscles. They never undergo fatigue.

Nervous Tissue

It is a highly specialized tissue due to which the animals are able to perceive and respond to stimuli. It is characterized by the following properties :

• It contains specialized cells called neurons.
• The neurons usually have a star-shaped cell body called cyton or perikaryon.
• The cells are capable of perceiving the external and internal stimuli.
• The neurons are capable of transmitting and conducting nerve impulses from one part of the body to the other.

Nerve cell or Neuron

The unit of nervous tissue is called nerve cell or neuron. It has a star-shaped cell body known as cyton. The cyton has one to many highly branched projections.

The cyton or cell body is covered by simple plasma membrane. Bounded by the plasma membrane is the ground substance called neuroplasm. It has a single nucleus in the centre.

The projections which arise from the cell body are of two types : i.e.,

(i) Dendrites or dendrons : which are one to many, highly branched small projections. They carry messages towards the cell body and

(ii) Axon : It is a single long projection of uniform thickness. It carries messages away from the cyton. The highly branched end of the axon is called terminal arborization.

The terminal arborization of axon of one neuron is very closely placed to the dendrons of another neuron to carry impulses from one neuron to another neuron. This close proximity is called synapse.

Location : Neurons are found in the brain and spinal cord.

Functions :

• Nervous tissue controls all the body activities.
• Nervous tissue co-ordinates between various body parts during any body function.
• Dendrons carry nerve impulses towards the cyton whereas axon carries impulses away from the cyton.

SUMMARY