What is permanent tissue
What is permanent tissue
The term tissue was coined by N Grew. It is defined as a group of cells that are structurally similar and constitute together to perform a particular function.i e.g., blood, phloem, muscle, etc.
Tissues are broadly categorised into plant tissues and animal tissues. Branch of biology which deals with the study of tissues is known as histology.
Plant Tissue
Most of the tissues present in plants are dead and provide more mechanical strength to them than the living cells because the dead cells of plant do not need so much of maintenance.
On the basis of the functions they perform, plant tissue can be categorised as
Meristematic Tissue
It includes group of cells, which remain in continuous state of division or retain their power of division, i.e., they actively divide throughout their life. The meristematic tissues divide to form new cells, which differentiate to give rise to permanent tissues. These tissues are generally found in growing parts of the plants like receptacle axis of stem to which the floral organs are attached, sepal and stigma of the flower, apex of the root, cambium, etc. Cells produced by meristem are similar in appearance as those of the older meristematic cells itself, but as the newly formed cells start growing and becomes mature, the characteristics of the newly formed cells change slowly.
Characteristics of Meristematic Tissue
- The cells of meristematic tissues may be rounded, oval, polygonal or rectangular in shape without any intercellular spaces.
- These are living cells which bring an increase in the length and girth (thickness) of an the plant.
- Meristematic tissues have thin cell wall made up of cellulose. They also lack vacuoles.
- Cells of meristematic tissues contain clear and dense protoplasm and large nuclei.
- Plastids can also be seen in the form of protoplastids.
Types of Meristematic Tissue
Meristematic tissues are of three types, depending upon the position in the plant body
Apical Meristems ::-
These are found at the apices (growing tips) of main and lateral shoots and roots. It is responsible for linear growth of an organ like the height or length of the plants, which is called primary growth.
Intercalary Meristems ::-
These are found at the base of the leaves or internodes on twigs (on either side of the node). These meristems help in longitudinal growth of the plant by adding primary tissue. e.g., in stems of various grasses and wheat or at the base of the leaf de (as in pinus) and at the nodes (as in mint).
Lateral Meristems (Cambium) ::-
These are also found on the lateral sides of the stem and roots. The meristem consists of initials which divide mainly in one plane and results in the increase in the diameter. e.g., cork cambium, vascular cambium, etc. These are also responsible for the growth in thickness, i.e., to increase stem or root in diameter called secondary growth. They occur in dicotyledon plants due to the presence of cambium but do not occur in monocotyledons.
Permanent Tissue or Mature Tissue
These are made up of mature cells which have lost their ability of cell division and are already adopted or accommodated to perform some specific work. These tissues originate from meristematic tissues, when they loose the ability to divide. This process of taking up permanent shape, size and function is known as differentiation. The permanent tissue may be dead or living, having thick or thin cellular walls.
These tissues are of following two types
Simple Permanent Tissue
These are composed of similar type of cells, which have common origin and function.
These are further classified under three categories
1.Parenchyma ::-
These are the most simple and unspecialised type of permanent tissue, whose cells are usually isodiametric (equally expanded on all sides) with intercellular spaces among them, which allows the exchange of gases. These are living cells of oval, round, polygonal or elongated shape.
The cell wall is made up of cellulose or calcium pectate. Each cell possesses a nucleus and a vacuole. It is found in epidermis, cortex, pith, pericycle, mesophyll of leaves, pulp of fruits, endoderm of seeds and in meristematic tissues. If parenchymatous cells have chloroplast in some situations, then the phenomenon of photosynthesis takes place in them, and hence parenchyma is also called chlorenchyma (chlorotissue).
It gives rise to secondary meristem in the form of cork cambium and vascular cambium. These tissue cells also store the food stuffs in the protoplasm of the cells.
2. Collenchyma ::-
It consists of living cells. There is no intercellular space as these are elongated in shape due to irregular thickening of cellulose and pectin in the cellular walls. It is basically a mechanical tissue which provides mechanical support and elasticity to the plants. Thus, its existence always provides rigid support to the plants. It is found in outer region of cortex. e.g., angles of stems, midrib of leaves, etc.
3. Sclerenchyma ::-
It consists of dead cells and are devoid of protoplasm cells of rigid tissue. Cell walls of such cells are greatly thickened due to lignin or cellulose (or both). It gives mechanical strength, support and rigidity to the plant body. Sclerenchyma tissues are the components of hard coating of several seeds and nuts. e.g., husk of a coconut.
Complex Permanent Tissue
These are another type of permanent tissue, composed of more than one type of cells having common origin.
Complex tissues are also of two main types
Xylem ::-
It is the chief conducting tissue of vascular plants responsible for the conduction of water and inorganic solutes (minerals) from roots to apical parts of the plant. Its main function is to provide mechanical strength and rigidity to the plant. The components of xylem include tracheids, vessels, xylem parenchyma and xylem fibres. It exists in the vascular pores of the roots, stems and leaves of the plants Xylem consists of dead hollow cells (such as wood) in the form of fibres and sclereids. Out of all different types of cells of xylem, vessels are the most important cells.
Phloem ::-
It is also the chief conducting tissue of vascular plants responsible for the conduction of organic solutes (food). This conduction may be bi-directional, i.e., from leaves to storage organs or may be from storage organs to growing parts of plants. The components of phloem include sieve tubes, companion cells, phloem parenchyma and phloem fibres. Out of all the phloem components, phloem fibres are the dead cells. The cells of seive tubes (having numerous pores like holes in the wall) are responsible for the transformation and distribution of food to various organs. Phloem is also known as base because the phloem fibres of some plants are used for binding purposes, e.g., flax and hemp.
Animal Tissues
Animal tissues have organised layers or masses of structurally similar cells of common embryonic origin and same function. All complex animals consist of four basic types of tissues
- epithelial,
- muscular
- connective
- neural (nervous tissue).
Epithelial Tissue or Epithelia
It is composed of one or more layers of closely packed cells covering the external surface and internal lining of the body organs. The epithelium lines all the organs of the body cavity as well as ducts. It also forms a barrier to keep the different body systems separately. Epithelial tissues are of ectodermal, mesodermal or endodermal (germ cells) origin.e.g., skin, lining of mouth, lining of blood vessels, lung alevoli, kidney tubules, etc.
Characteristics of Epithelial Tissue
- Epithelial tissue forms a continuous sheet due to tightly packed cells, hence, they do not have inter cellular spaces between them i.e., have very small amount of cementing material between them.
- The epithelial tissue rests on a non-cellular basement membrane separating it from the underlying connective tissue.
- These tissues help to protect organisms from invading microorganisms, injury to internal organs and fluid loss.
- The outer layer of epithelium is exposed to the external environment, while inside layer acts as a basement membrane.
- The moist nature of the linings of some hollow organs or cavities is due to the mucous secreted by the epithelial tissue.
- Epithelial cells have no blood vessels.
- Epithelium tissues use diffusion as a process for exchange of materials between the epithelial cells and vessels of connective tissue.
Functions of Epithelial Tissue
- Helps in the absorption of water and nutrients.
- Provides protection to the underlying cells from injury, drying, infection and a from the harmful effects of chemicals.
- Helps in the elimination of waste products from the body.
Types of Epithelial Tissues
On the basis of cell layers and shape of cells, epithelial tissues are classified as
Simple Epithelial Tissue
It is formed from a single layer of cells, resting on the basement membrane. These are mainly found on the secretory and absorptive surfaces.
On the basis of cell layers and shape of cells, simple epithelial tissues are of the following types
1.Squamous Epithelium
It is composed of a single layer of flat, thin and disc-like cells. The cells have irregular boundaries and closely attached to each other like tiles of the floor hence, also known as pavement epithelium. The nuclei of the cells are flattened and often lie at the centre of the cells and cause bulging of cell surface.
Location Present in the skin, lining of the mouth, oesophagus, terminal bronchioles. alveoli of the lungs, membranous labyrinth (internal ear), coelomic cavities and rete testis.
Function Provide protection and helps in the secretion of coeloric fluid, also helps in the exchange of gases between the cells.
2.Cuboidal Epithelium
It consists of tall, polygonal (in outline), wide and cube-shaped cells around the nucleus located in the centre of the cells.
Location Present in the small salivary glands, pancreatic ducts, thyroid vesicles, distal convoluted tubules of the nephrons of kidneys, ovaries, seminiferous tubules, testes and iris of eyes. Other sites of cuboidal epithelium are the inner surface of the lens and the pigment cell layer of the retina of the eye.
Functions These cells help in the protection, secretion, absorption, excretion and also in gamete formation.
3.Columnar Epithelium
It possess elongated cells which are placed side by side like a tall column. The outer free surface of each cell is slightly broader. The nuclei are somewhat elongated along the axis of the cells and lie near the bases of the cells. Certain cells contain mucous and are goblet (or mucous) cells.
Location It lines the inner surface of the stomach, intestine, gall bladder and bile duct. It also forms the gastric glands, intestinal glands and pancreatic lobules (present in the pancreas) where it helps in the secretion and absorption of various digestive juices,
Functions Protection, secretion and absorption.
4.Glandular Epithelium
The columnar cells which get specialised for the purpose of secretion are referred to as glandular epithelium. These tissues are generally found in various glands of the animals. These can be columnar or cuboidal cell.
These are mainly of two types
- Unicellular Glands They have isolated glandular cells, generally found in intestine and mucous membrane. e.g., globlet cells of alimentary canal.
- Multicellular Glands They have cluster of glandular cells and are found deep inside e.g., salivary gland.
Multicellular glands are further divided as
- Tubular (when gland retains its shape as a tube throughout) e.g., sweat gland and oil gland of the skin.
- Alveolar (having sac-like secretory portion and an obvious lumen), e.g.. salivary glands, mammary glands, etc.
5.Ciliated Epithelium
It has numerous cubical or columnar cells in it bearing many delicate hair-like outgrowths, (the cilia), arising from the basal granules.
Location It lines the inner surfaces of most of the respiratory tract, fallopian tubes, ventricles of the brain and the central canal of the spinal cord tube.
Functions Provides protection and helps in the movement of mucus, urine, eggs and cerebrospinal fluid in a particular direction.
6.Pseudo-Stratified Epithelium
This epithelium is one-cell thick but look like two layered (as some cells are smaller in size, than the others i.e., unequal in size) hence, named as pseudostratified. The cells are columnar, nuclei are present in all the layers. The long cells have oval nuclei, possess cilia and extend up to free surface. The short cells have round nuclei, lack cilia and do not reach the outer free surface.
Location It occurs in the large ducts, certain glands such as parotid salivary gland, the urethra of the human male and in the olfactory mucosa. They occur in the trachea and large bronchi. (The movements of the cilia propel the mucus and foreign particles towards the larynx).
Functions Provides protection and secretion. Also helps in the movement of secretions from glands, urine and semen in the male uerthra and mucus loaded with dust particles from the trachea towards the larynx.
Compound Epithelia (Multilayered Epithelia)
The compound epithelia consists of a few to several layers of cell. Therefore, they thicker and stronger than the simple epithelia. Being multilayered, they have little role in the secretion and absorption but have major role in providing protection to the underlying tissues against chemical, mechanical,thermal or osmotic stresses.
Compound epithelia may be of the following two types
Stratified Epithelium
These consist of many layers of epithelial cells, the deepest layer is made up of loose columnar or cuboidal cells but the outer surface may vary in nature. On the basis of the form of cells found in the outer layer, it is classified into four types i.e., stratified squamous, stratified cuboidal, stratified columnar and stratified ciliated columnar epithelium.
Transitional Epithelium
It differs from stratified epithilium in having fewer layers (i.e., four to six layers) of cells. The cells of basal layer are columnar or cuboidal. The cells of middle layer are polyhedral or pear-shaped. While, the cells of the surface layer are large and globular or umbrella shaped.
Location It lies in the renal calyces, renal pelvis, ureter, urinary bladder and part of the urethra. Because of its distribution mainly in the urinary system, it is also called urothelium.
Functions It permits distention. Thus, urinary bladder can be stretched considerably without being damaged and regain its original shape under normal conditions.
Connective Tissue
These are most abundant and widely distributed in the body of complex animals. These tissues arise from the mesoderm of the embryo. They are named so because of their special function of linking and supporting other tissues or organs of the body. The basic components of connective tissue are intercellular medium, connective tissue cells and fibres. The cells of connective tissue are loosely packed, living, embedded in an intercellular matrix. The matrix, may be jelly-like fluid, dense or rigid in nature.
Functions Attachment of one tissue to another, supporting cartilage and bone, storage and insulation (by fat storage) transporting materials, etc.
Muscular Tissue
Muscle cells have the feature of contractility. The muscle tissue is composed of cells, called myocytes which can shorten considerably and return to the original relaxed state. Muscles also have the property of electrical excitability, which is due to the energy provided by the electrical potential difference across the plasma membrane.
Structurally, they consist of long, narrow cells called muscle fibres and the cytoplasm which is known as sacroplasm. The endoplamic reticulum present in it is called sacroplasmic reticulum (SR). The sacroplasm is largely occupied by fine parallel threads of proteins, called myofibril.
In muscle tissue cells, mitochondria is abundantly present in between the myofibrils and termed as sarcosomes and it is also the site where the glycogen granules are present which provide energy for the contraction (by oxidation of glucose).
Functions of Muscle Tissue
- It brings about movement of the body parts and locomotion of the organisms.
- Muscles support bones and other structures.
- Muscles are responsible for the heart beat, flow of blood and lymph, movement of food through alimentary canal, air flow through respiratory tract, sound production, propulsion of waste products, etc.
- Facial expressions and gestures are also dependent on muscles.
Types of Muscle Tissue
There are three types of muscle tissue
1. Striated (Striped) Muscle
The striated muscle consists of 80% or more of the mass of soft tissues in vertebrates. The cells of this tissue are long, cylindrical, unbranched and multinucleate (having many nuclei). They have light deep coloured strips on their outer surface which seem to be in the alternative forms. Due to this property, this muscle tissue is called striped or striated muscle tissue.
Location These are found in body walls, limbs, tougue, pharynx and beginning of oesophagus in the form of bundles that are wrapped by connective tissue sheaths. They form the various body muscles such or biseps, triceps, etc.
General Features of Striated Muscles
- The striated muscles contract rapidly but not for long time i.e., soon get fatigued.
- It is innervated by nerves from the brain and spinal cord.
- The contraction of striated muscle is under the control of person’s conscious will. Hence, called voluntary muscles.
- Most of the striated muscles are joined to the bones by tendons.
- Blood capillaries are also present in the surface of the fibres.
. Non-striated (Smooth) Muscle
The cells of non-striated muscle tissue are thin, long with pin pointed ends (spindle shaped) and uninucleate (i.e., have single nucleus). The conical shaped nucleus is found in the centre of every fibre. These cylindrical shaped fibres are arranged in the form of parallel branched bundle.
Functionally, smooth muscles are of following two main types
- Single unit present in the walls of hollow organs such as digestive tract, uterus, ureters, urinary bladder.
- Multi unit present in the dermis of skin, iris of eye and walls of large blood vessels.
Location The non-striated (smooth) muscles are present in the iris of eye, walls of the internal organs (viscera) such as alimentary canal, genital tracts, ducts, blood vessels, urinary bladder, ureter, etc. Thus, they are also called visceral muscles.
Structurally, striated muscles have two types of myofilaments
- Primary myofilament (composed of myosin protein).
- Secondary myofilament (composed of actin, tropomyosin and troponin proteins).
General Features of Smooth Muscle Tissue
- The visceral muscle contracts slowly but can stay contracted for long time without getting fatigued as compared to the striated muscles.
- Contraction is not under persons conscious will, (control) hence called involuntary muscles.
Cardiac Muscle
These muscles are mainly confined to the wall of the heart. The muscle cells of the heart are cylindrical, branched and uninucleate. The middle of cell contains one or two nuclei. Location It is found in the walls of the pulmonary veins and superior vena cava.
Functions
- It generates its own wave of excitation that can pass through fibre to fibre.
- It accounts for the rhythmic contractions of the heart muscle by its own throughout the life. Hence, known as involuntary muscle.
Neural Tissue
This tissue in an animal receives stimuli and conduct impulses for controlling and coordinating body functions. It forms central nervous system (CNS), peripheral nervous system (PNS) and autonomic nervous system (ANS). Nervous tissue is mainly composed of nerve cells and glial cells. The main function of neural tissue is to accommodate the working of almost all organs according to the specific requirements.
Components of Neural Tissue
The different components of neural tissue are described below
Neurons
Structural and functional unit of neural tissue, longest cell of the body, composed of cell body, an axon (single long part) and dendrites (small branched part).
Neurosecretory Cells
Specialised nerve cells, function as an endocrine organ, release neuro hormones from their axons into blood instead of synaptic cleft.
Neuroglia
Non-nervous cells, present along with neurons in CNS, ganglia and retina, hold neurons at their position in CNS, they are further of 2 types;
- microglia
- macroglia.
Ependymal cells
Arranged as epithelial layer, lines the brain ventricles (cavities) and central canal of spinal cord, free surface possess microvilli (for the absorption of cerebro spinal fluid) and cilia.
