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Tissues

Chapter: Tissues

Introduction:

The basic structural and functional unit of life in all the living organisms is the cell. In unicellular organisms like the amoeba, a single cell performs all the functions like intake of food, respiration and excretion. On the other hand in multicellular organisms there are several types of cells specialized to carry out various functions. Since each cell is specialized for a particular function, they do it efficiently. For example in human beings muscle cells are specialized to carry out movement and the nerve cells carry messages, transport of oxygen, food, hormones and waste material occurs due to blood flow. Vascular tissues in plants conduct food, hormones and waste material. Cell which carry out a same type of a function are often grouped together in the body. This indicates that a specific function is carried out by a cluster of cells at a specific place in the body. The cluster of cells is called as tissue. Tissue is designed and arranged in such a way that it can give highest possible efficiency of function. Blood, bone, muscle, phloem are few examples of tissue

Definition:

A group of cells that are similar in structure and work together to achieve a particular function is called a tissue.Let us learn about some basic differences between plant and animal tissue.

  1. Plants are stationary or fixed –they don’t move. Most of the plant tissues are dead while on the other hand animals can move around in search of food, mates and shelter and most of the animal tissues are living.
  2. The growth in plants is limited to certain regions, while this is not the case with animals. There are some tissues in plants that divide all through their life. These tissues are located in certain regions. Cell growth in animals is more uniform. So, there is no discrimination of dividing and non-dividing regions in animals.
  3. The structural organisation of organs and organ systems is far more specialised and localised in complex animals than even in very complex plants.

Plant Tissue:

Plants are made up of a number of tissues that function together. Vascular tissue conducts food and water from part to part in the plant. The tissues responsible for growth are located at the tips of root and stem. Depending on the dividing capacity of cells plant tissues are classified as meristematic tissue and permanent tissue.

1. Meristematic tissue:

A meristematic tissue is the only plant tissue that produces new cells by cell division. The cells of meristematic tissue are thin walled with striking nucleus and divide rapidly. Meristematic tissues are classified into two types based on the region of their location. The two types of meristematic tissues are apical meristem, lateral meristem and inter-calary meristem.

  1. Apical meristem is given that name as it is present at the growing tips of stems and roots and apical meristem increases the length of the stem and the root.
  2. Lateral meristem is also called cambium. Lateral meristem increases the girth and width of the stem or root.
  3. Inter-calary meristem is the meristem present at the base of the leaves or the internodes (on either side of the node) or twigs.

2. Permanent tissue:

Cells of meristematic tissue differentiate to form different types of permanent tissue. The cells formed by meristematic tissue take up a specific role and lose the ability to divide. As a result they form a permanent tissue. This process of taking up a permanent shape, size, and a function is called differentiation. There are two types of permanent tissues - i. Simple permanent tissue, ii. Complex permanent tissue

i. Simple permanent tissue: It is a type of permanent tissue made up of one type of cells. Depending on the functions it performs it is classified into the following types:

  1. Parenchyma: Its cells form the basic packing tissue. The cells are living possess nucleus and are thin walled and unspecialized. They are loosely packed, and therefore have intercellular spaces. This tissue stores food as in potato, beetroot, etc.
  2. Chlorenchyma: The parenchyma tissue in the leaf containing chloroplast is called as chlorenchyma. Photosynthesis is the function performed by the chlorenchyma tissue.
  3. Arenchyma: In aquatic plants large air cavities are formed because of the large inter cellular spaces. Aquatic plants float on the water because of these air cavities, these air cavities give buoyancy to the plants and help them float. Such a parenchyma tissue is called arenchyma.
  4. Collenchyma: The cells forming this tissue are live, elongated and thickened at the corners due to the deposition of cellulose. Leaf stalks below the epidermis contain this tissue. Collenchymas tissue plays a role in providing flexibility and thereby easy bending of various parts of plant like leaves, stems and twigs. It also provides mechanical support to the young plants.
  5. Sclerenchyma: Dead cells which are long and narrow form this tissue. Lignin is deposited in the cell walls making them thick. Lignin acts as cement making the cell walls hard. The hardness of the wall is so much that they have a reduced lumen. This tissue is localized in the stems around vascular bundles, in the hard covering of seeds and nuts and in the veins of leaves. The specific structure of this tissue makes the plant hard and stiff. It provides strength to parts of the plant, for example husk of the coconut is hard because of sclerenchyma tissue.
  6. Surface tissue: The entire surface of the plant consists of a single layer of cells called epidermis or surface tissue. Most of the cells forming surface tissue are relatively flat. The outer and lateral walls of the cell are mostly thicker than the inner walls. It protects all parts of the plant. In some plants like cactus, the epidermis may be thick since protection against water loss is very important. A waxy substance is secreted by the epidermal cells on the aerial part of the plant. This waxy layer protects the plant from the loss of water, against mechanical injury and invasion of parasitic fungi. In order to exchange gases with the atmosphere, epidermis has minute openings called stomata. Stomata are enclosed by two epidermal kidney shaped cells that guard the opening and closing of stomata. Transpiration (loss of water in the form of vapour also takes place through stomata.

ii. Complex permanent tissue:Tissues made of more than one type of cells are called as complex tissues. All the cells of this tissue coordinate with each other and perform a common function. Xylem and phloem are examples of such complex tissues. Both of them are conducting tissues and constitute a vascular bundle. Vascular or conductive tissue is a unique feature of the complex plants. Vascular conductive tissue present in stems, roots, and leaves help in conducting water and food material.

a. Xylem: Cells of this tissue are mostly dead and have thick walls. They are mainly of the following types:

  1. Tracheids: These are long narrow cells with walls that are not permeable to water. Tracheids die when they undergo and then their cytoplasm disintegrates. Disintegration of cytoplasm results in hollow cells which are connected with each other through which water can pass.
  2. Xylem vessels: These are tubular structures and are much wider than tracheids. They are arranged on top of one another like a pipeline. The death of the cells results in the disintegration of cell walls at both the ends because of which stacked vessels become continuous tubes. The water and minerals flow freely in vertical direction through these tubes
  3. Xylem parenchyma: Food is stored by xylem parenchyma
  4. Xylem fibres: Mainly support the plant.

b. Phloem: Phloem is made up of four types of elements: sieve tubes, companion cells, phloem fibres and phloem parenchyma. Sieve tubes are tubular cells with perforated walls. Unlike xylem materials can move in both the direction in Phloem. Phloem transports food from leaves to other parts of the plant. Phloem cells are living cells, the only exception is the phloem fibres.

Animal Tissues:

a. Animal body is made up of a number of organs that work together. Lungs, respiratory tubes and the contraction and relaxation of the special muscles help us to breathe.

b. Muscle cells are grouped together in fibres. These muscles are attached to bones. This helps in muscle and bones to move.

c. Blood is also a kind of tissue. This is so because blood flows and carries various substances from one part of the body to another. For example, it carries oxygen and nutrients to all cells.

d. Animal tissues are classified into different types based on the function they perform.

1. Simple tissue:

Simple animal tissue consists of one type of cells.

 

1 - Epithelial tissue:

i. Cells of this tissue are tightly packed and form a continuous sheet. They have no intercellular spaces. All epithelium is usually separated from the underlying tissue by an extra cellular fibrous basement membrane.

ii. The skin, the lining of the mouth, lining of blood vessels, lung alveoli and kidney tubules are all made of epithelial tissue.

iii. Epithelial tissue forms a protective covering of the animal body. It also acts as a barrier to keep different body systems separate from each other.

iv. The cells lack intercellular spaces, so the permeability of the cells play an important role in regulating exchange of material between the body and external environment

The different types of epithelial tissues are as follows:

a. Simple squamous epithelium:

  1. The cells forming this tissue are extremely thin flat, and form a delicate lining.
  2. These cells are situated in the lining of blood vessels, lungs alveoli, oesophagus, lining of the mouth.
  3. This tissue performs the function of transportation of substances through selectively permeable membrane.

b. Stratified squamous epithelium:

  1. The cells of this tissue are arranged to form layers.
  2. They are found in the outer layer of skin.
  3. This tissue protects the body organs, prevents wear and tear of the skin.

c. Columnar epithelium:

  1. The cells forming this tissue are tall and arranged like pillars.
  2. They are found in the inner lining of intestine.
  3. They secrete digestive enzymes and perform the function of absorption of nutrients from digested food.

d. Ciliated columnar epithelium:

  1. Columnar epithelial cells having hair like projections are called ciliated columnar epithelium.
  2. It is found in the respiratory tract.
  3. Due to the movement of cilia, the mucous and air is pushed forward to clear the respiratory tract.

e. Cuboidal epithelium:

  1. The cells forming this tissue are cube shaped.
  2. Cuboidal epithelium forms the lining of kidney tubules and ducts of salivary glands
  3. It helps in the absorption of useful material, from urine before it is passed out. It helps in the secretion of saliva.

f. Glandular epithelium:

  1. Sometimes a portion of epithelial tissue folds inward to form a multi-cellular gland. This structure is called glandular epithelium.
  2. It performs the function of secreting substances like sweat, oil, mucous from the skin.

2. Complex Tissue:

Complex tissue is made up of more than one type of cells. They work together. Different types of connective tissue are as follows.

 

i. Connective tissue:

  1. Connective tissue consists of a matrix and the cells are embedded in the matrix. The nature, density and amount of the matrix differs depending on the function of the particular connective tissue.
  2. The matrix may be jelly-like, fluid, dense or rigid.

Different types of connective tissue are explained below:

a. Bone:

  1. It forms the supporting frame work of the body. It also anchors the muscles and supports the main organs of the body.
  2. It is a strong and non-flexible tissue.
  3. Cells forming bone are rooted in a hard matrix that is composed of calcium and phosphorus compounds.

b. Blood:

  1. Blood is a fluid connective tissue. It has a fluid matrix in which red blood corpuscles( RBC), white blood corpuscles (WBC) and blood platelets are suspended.
  2. The plasma contains proteins, salts and hormones.
  3. Transport of gases, digested food, hormones to different parts of the body occurs with the help of blood.

c. Ligament:

  1. It is a connective tissue that connects bones and is very elastic with considerable length. Ligament contains a very little matrix.
  2. It serves to hold the structures and keep them stable. For example ligament connects the shin bone to high bone, to provide strength to the knee.

d. Tendons:

  1. It is a connective tissue that connects muscles to the bones.
  2. Tendons are fibrous with great strength, but have limited flexibility.
  3. Tendons serve to move the bones or structures. For example in human beings muscles of calf are attached to the heel bone. Tendons also attach the structures such as eye balls to the bones.

e. Cartliage:

  1. Its cells are widely spaced. Cartilage smoothens the bone surface at joints.
  2. It is also present in the nose, ears, trachea and larynx.

f. Aerolar:

  1. It is found between skin and muscles, around blood vessels, nerves and in the bone marrow.
  2. The space inside the organs is filled by this tissue and areola tissue supports internal organs and helps in repair of tissue.

g. Adipose:

  1. This tissue is found beneath the skin, around the kidneys and between internal organs.
  2. The cells of this tissue are filled with fat globules as a result this tissue acts as an insulator.

ii - Muscular tissue:

  1. Muscular tissue consists of elongated cells, also called muscle fibres. This tissue is responsible for movement in our body.
  2. Muscles contain special proteins called contractile proteins, which undergo contraction and relaxation for causing movements.
  3. We can move some muscles by our conscious will. Muscle present in our limbs move when we want them to and stop when we decide. Such muscles are called voluntary muscles. Since voluntary muscles are mostly attached to bones and they help in body movement. They are also called as skeletal muscles.
  4. Under the microscope, these muscles show alternate light and dark bands or striations when stained properly.  As they show striations they are also called striated muscles. Cells forming this tissue are long, cylindrical and multinucleate.
  5. The movement of food in the alimentary canal or the contraction and relaxation of blood vessels are involuntary movements. We cannot really start or stop them simply by wanting to do so. Such movements are controlled by involuntary muscles or smooth muscles
  6. Involuntary muscles are also called as unstrained muscles as they do not show any striations when viewed under the microscope.
  7. These muscles are also found in the iris of the eye, in the uterus and in bronchii of the lungs.
  8. The cells of this muscle are long pointed (spindle shaped) and uninucleate.
  9. The muscles of the heart show hythmic contraction and relaxation in the entire life. These involuntary muscles are called cardiac muscles.
  10. Heart muscles are cylindrical, branched and uninucleate.

iii - Nervous tissue:

All of the cells are able to respond to stimulus, but the cell of nervous tissue are specialised for being stimulated and then for transmitting the impulses rapidly from place to place in the body. The brain, spinal cord and nerves, all of these are composed of the nervous tissue. The cells forming this tissue are called as nerve cells or neurons. A neuron consists of a cell body which contains cytoplasm in which nucleus is located. Long hair like projections called dendrites arises from the cell body. A single long portion called axon is present in each neuron. Nerve fibres combine to form nervous tissue.