Posted by Table of Contents
Cells And Different Type Of Tissues
We know that cells are the building blocks of living matter that can maintain life and reproduce themselves. The human body, which is made up of different types of cells, begins as a single, newly fertilized cell.
Tissues:- However, tissues which are made up of similar kind of cells are more complex units. Hence, by definition, a tissue is termed as an organization of similar cells with varying amounts and kinds of non living, intercellular substance between them.
Organs:- An organ comprises of an organization of several different kinds of tissues so arranged that together they can perform a special function. For example, as we are all aware stomach, is a digestive organ. This organ consists of an organization of muscle, connective, epithelial, and nervous tissues. Muscle and connective tissues form its wall, epithelial and connective tissues make up its lining, while the nervous tissue extends throughout both its wall and its lining.
Organ Systems:- An organ system is composed of an intricate network of varying numbers and kinds of organs so arranged that together they can perform diverse functions of the body. Ten major systems exist and operate in the human body which include:
-
- Skeletal
- Muscular
- Nervous
- Endocrine
- Cardiovascular
- Lymphatic
- Respiratory
- Digestive
- Excretory
- Reproductive
Basic Structure Of An Animal Cell
A cell is a minute [ Jelly-Like ] mass of protoplasm containing a nucleus held together by a cell membrane. The structure of the cell is essential to relate its part to its function.
The protoplasm of the cell contains a centrally placed body, the nucleus and the cytoplasm or remainder of the protoplasm, which surrounds the nucleus. The cytoplasm contains the following essential cell organelles:
-
- The cell membrane which is selectively permeable, that is it allows only certain substances to pass through it and prevents other substances from getting access to it. Thus, it is most important in maintaining the correct chemical composition of the protoplasm.
-
- The nucleus or director of the cell is delimited from the cell’s cytoplasm by a double layered nuclear envelope. It contains a cell’s genetic material, the DNA. The DNA in the nucleus is tightly coiled around proteins, histones to form structures called chromosomes. This DNA also termed as chromosomal or genomic DNA is located in the nucleolus region of the nucleus, where the ribosomes are made. The chromosomes bear genes which determine the hereditary characteristics of an individual. It also controls the activities of the cell.
-
- Mitochondria are double membranous small rod-like structures closely connected with the catabolic or respiratory processes of the cell body. They store energy in the form of ATP and are called ‘Power House‘ of the cell.
-
- Golgi apparatus are canal-like structures which lie next to the nucleus and perform the secretory activities of the cell.
-
- Endoplasmic reticulum is composed of a network of membranes. It is of two types- Rough Endoplasmic Reticulum which bears ribosomes and Smooth Endoplasmic Reticulum without ribosomes. The rough endoplasmic reticulum is involved in protein biosynthesis while smooth endoplasmic reticulum is engaged in lipid biosynthesis.
-
- Lysosomes are single membrane bound, tiny circular structures filled with digestive enzymes. These are the suicidal bags of the cell which help in intracellular digestion.
-
- Ribosomes are composed of two subunits- 60S and 40S. They are not bound by any membrane and are concerned with protein synthesis.
-
- A vacuole stores cell sap which may be liquid or solid food or toxic by products.
-
- Centrosome is a minute dense part of the cytoplasm, lying close to the nucleus. It plays an important part during cell division.
Different Types Of Tissues
A tissue is a group of cells that usually have a common origin in an embiyo and functions together to carry out specialized activities.
The cell structure varies according to the function. Accordingly, the tissues are also different and are broadly classified into four types: [1] Epithelial tissue [2] Muscular tissue [3] Nervous tissue and [4] Connective tissue.
Epithelial Tissue
The epithelial tissue or epithelium has a free surface which faces either a body fluid or the outside environment and thus provides covering or lining for some part of the body. The cells are compactly packed with little intercellular matrix. There are two main classes of epithelial tissue, each containing several varieties. All epithelial cells lie on and are held together by a homogenous substance called a basement membrane.
Simple Epithelium
This class consists of a single layer of cells and functions as a lining for body cavities, ducts and tubes. Based on structural modification of the cells, it is subdivided into three types:
1- Squamous Epithelium: This type of epithelium is made up of a single thin layer of flattened cells with irregular boundaries. They are found whenever a very smooth surface is essential as in the air sacs of lungs, lining of the heart, lining of blood vessels and lymphatics. When lining these structures, the epithelial covering or lining is called endothelium. The epithelium is involved in functions like forming a diffusion boundary. They are wider than their height.
2- Cuboidal Epithelium: It comprises of a single layer of cube-like cells. It is commonly found in ducts of glands and tubular parts of nephrons in kidneys and its main functions are secretion and absorption.
3- Columnar Epithelium: Columnar epithelium is composed of a single layer of tall and slender cells. They are found in the lining of stomach and intestine and help in the secretion and absorption. The nuclei in these kind of epithelia are located at the base and at the free surface they may bear microvilli.
If the columnar or cuboidal cells bear cilia on their free surface they are called ciliated epithelium. Their function is to move particles or mucus in a specific direction over the epithelium. Ciliated epithelium is mainly present in the inner surface of hollow organs like bronchioles and fallopian tubes.
Some of the columnar or cuboidal become specialized for secretion and are known as glandular epithelium. They are mainly of two types: unicellular consisting of isolated glandular cells [ goblet cells of the alimentary canal ] and multicellular, consisting of cluster of cells [ salivary gland ].
Compound Epithelium
Compound epithelium is multilayered. Its main function is to provide protection against chemical and mechanical stresses. They cover the dry surface of the skin, the moist surface of buccal cavity, pharynx, inner lining of ducts of salivary glands and pancreatic ducts, the lower part of the urethra, the anal canal and the vagina, and also the surface of the cornea. In these areas it does not become cornified. The compound epithelia may be stratified and transitional.
Stratified Epithelium: It has many layers of epithelial cells in which the innermost layer is made up of columnar or cuboidal cells. This epithelium is classified on the basis of the shape of the cells present in the superficial layers. It is of four types [1] Stratified squamous epithelium [2] Stratified cuboidal epithelium [3] Stratified columnar epithelium [4] Stratified ciliated columnar epithelium.
Transitional Epithelium: This is a compound stratified epithelium consisting of 4-6 layers of cells. It lines the urinary bladder, the pelvis of the kidney, the ureters and the upper part of the urethra. The deeper layers of cells in transitional epithelium are of the columnar or cuboidal type of cells with rounded ends which make them pyriform or pear-shaped. As the cells in the deeper layers multiply by dividing, the superficial layers of cells are cast off. The superficial cell layers in transitional epithelium are less scale-like than those of stratified epithelium. Because of its distribution, the transitional epithelium is also termed as urothelium.
Functions Of Epithelial Tissues
-
- Protection: The epithelial tissue protects the underlying tissues from [1] Mechanical Injury [2] Entry of germs (infection) [3] Harmful chemicals and also [4] Drying up.
- Selective Barriers: They prevent the absorption of harmful or unnecessary substances.
- Absorption: The epithelium lining the uriniferous tubules [Nephrons], stomach and intestine is absorptive.
- Conduction: Ciliated epithelia like those of the respiratory and genital tracts serve to conduct mucus or other fluids in the ducts they line.
- Excretion: The epithelium of uriniferous tubules is specialized for urine formation for excretion.
- Sensation: Sensory epithelia of sense organs, for example, that of olfactory epithelium etc. help to receive various stimuli from the atmosphere and convey them to the brain.
- Regeneration: In case of any injury in the epithelial tissue, the regeneration is very rapid than any other tissues, thus facilitating the rapid healing of wounds.
- Exchange Of Gases: Epithelium of alveoli of the lungs help in the exchange of gases between blood and air.
- Pigmentation: Pigmented epithelium of the retina darkens the cavity of the eyeball.
- Secretion: Epithelium also forms glands that produce and secrete secretions like mucus, gastric juice and intestinal juice.
- Reproduction: Germinal epithelium of the ovaries and seminiferous tubules of the testes produce ova and sperms respectively.
- Exoskeleton: This tissue also produces exoskeletal structures like scales, feathers, hair, nails, claws, horns and hoofs.
Connective Tissue
The most abundantly distributed tissues of the human body, the connective tissues derive their name because of the special function of linking and supporting other tissues/organs of the body. These tissues range from soft connective tissues to specialized types which include cartilage, bone, adipose and blood. In all the connective tissues except blood, the cells secrete fibers of structural proteins called collagen or elastin which provide strength, elasticity and flexibility to the tissue. These cells also secrete modified polysaccharides that accumulate between cells and fibers and act as the ground substance or matrix. The connective tissue can be classified into three types.
1- Loose connective tissues which has cells and fibers loosely arranged in a semi fluid ground substance, for example, areolar tissue which is present beneath the skin. This tissue serves as a support framework for the epithelium.
Another type of loose connective tissue is adipose tissue which is located mainly beneath the skin. The adipose tissue forms a protective covering around the body. Additionally, it acts as a store of water and of fat which when required can be re absorbed.
2- Dense connective tissue has compactly packed fibres and fibroblasts. These fibres may be arranged in a regular [ Dense Regular ] or irregular [ Dense Irregular tissues ] pattern.
3- Specialized connective tissue. Tendons, which attach skeletal muscles to bones and ligaments which attach one bone to another are examples of this type. On the other hand, dense irregular connective tissue has fibroblasts and many fibres that are oriented differently. This tissue is present in the skin. Cartilage, bones and blood are various types of specialized connective tissues.
Cartilage: Cartilage is a dense, clear blue-white substance very firm though less than the bone. It is principally present at joints and between bones. The bones of the embryo are first cartilage, then only the growing centers persist as cartilage and when adult age is reached cartilage is found just covering the bone ends. Cartilage does not contain blood vessels but is covered by a membrane, the perichondrium, from which it derives its blood supply. There are three main varieties of cartilage which demonstrate the characteristic of this substance-firmness, flexibility and rigidity.
Bone Structure: Bone is the hardest of the connective tissues of the body. It is composed of nearly 50% water and the remaining solid parts are divided into a composition of mineral matter, principally calcium salts 67% and cellular matter 33%.
Bone consists of two kinds of tissue; compact and cancellous tissue. Compact bone tissue is hard and dense, it is found in flat bones and in the shaft of the long bones, and as a thin covering over all bones.
Cancellous bone tissue is spongy in structure. It is found principally in the end of the ling bones, in the short bones, and as a layer in between two layers of compact tissue in the flat bones such as the scapula, cranium, sternum and ribs.
Blood: Blood is a fluid tissue containing plasma, red blood cells [ RBC ], white blood cells [ WBC ] and platelets. It is the main circulating fluid that helps in the transport of various substances.
Nervous Tissue
The nervous tissue consists of three kinds of matter, [1] Grey Matter, forming the nerve cells, [2] White Matter, the nerve fibres and [3] Neuralgia, a special kind of supporting cell, found only in the nervous system, which holds together and supports nerve cells and fibers. Each nerve cell with its processes is called a neuron.
Nerve cells are composed of highly specialized granular protoplasm, with large nuclei and cells and fibers. Various processes arise from the nerve cells; these processes carry the nerve impulses to and from the nerve cells.
Muscular Tissue
Muscle is a tissue which is specialized for contraction by means of which, movements are performed. It is composed of cylindrical fibres which correspond to the cells of other tissues. These are bound together into little bundles of fibres by a form of connective tissue which contains a highly specialized contractile element.
There Are Three Types Of Muscles
Striped [ Striated, Skeletal, Or Voluntary Muscle ]: The individual muscle fibers are transversely striated by alternate light and dark markings. Each fiber is made up of a number of myofibrils and enclosed in a fine membrane-the sarcolemma [ Muscle Sheath ]. A number of fibers are assembled together to form bundles, many of these bundles are bound together by connective tissue to form large and small muscles. When a muscle contracts, it shortens and each individual fiber participates in the movement by contracting. This type of muscle contracts only when stimulated to do so by the nervous system.
Unstriped [ Un-striated, Smooth Or Involuntary Muscle ]: The muscles will contract without nervous stimulation although in most parts of the body their activity is under the control of the autonomic [Involuntary] nervous system. These type of muscles are composed of elongated spindle-shaped muscle cells which retain the appearance of a cell.
Involuntary muscle is found in the coats of blood and lymphatic vessels, in the walls of the digestive tract and the hollow viscera, trachea and bronchi in the iris and ciliary muscle of the eye and in the involuntary muscles in the skin.
A sphincter muscle is composed of a circular band of muscle fibers situated at the internal or external openings of a canal or at the mouth of an orifice, tightly closing it when contracted. Examples include the cardiac and pyloric sphincters at the openings of the stomach, the ileocolic sphincter or valve, the internal and external sphincters of the anus and urethra.
Cardiac muscle is found only in the muscle of the heart. It is striated like involuntary muscle. But it differs in that its fibers branch and anastomose with each other; they are arranged longitudinally as in striated muscle, are characteristically red in color and not controlled by the wall. Cardiac muscle possesses the special property of automatic rhythmical contraction independent of its nerve supply. This function is described as myogenic as distinct from neurogenic. Normally the action of the heart is controlled by its nerve supply.
Muscular Contraction: When a muscle is stimulated a short latent period follows, during which it is taking up the stimulus. It then contracts, when it becomes short and thick and finally it relaxes and elongates.
In the case of a striped [voluntary] muscle fiber, the contraction lasts only a fraction of a second and each contraction occurs in response to a single nerve impulse. Each single contraction is of the same force. The force with which a whole muscle contracts is adjusted by varying the number of fibers contracting and the frequency with which each fibers contracts. When contracting vigorously, the individual fibers may contract more than 50 times each second.
Certain factors influence the force with which a muscle fiber contracts. It contracts more forcibly when it is stretched and cold conditions weaken the power to contract.
Unstriped muscle fibers contract much more slowly and are not dependent on nervous impulses, although these alter the force of their contraction.
Muscle Tone: Muscle is never completely at rest; it may appear to be but it is always in a condition of tone, which means ready to respond to stimuli. For instance, the knee-jerk obtained by sharply tapping the patellar tendon results in contraction of the quadriceps extensors of the thigh and slight extension of the knee joint. This is a reflex produced by stimulation of the nerves. Posture is determined by the degree of muscle tone.
The energy of the muscular contraction is provided by the conversion of adenosine diphosphate [ADP] into adenosine triphosphate by energy provided by the breakdown of glycogen. In the presence of adequate supplies of oxygen, this breakdown is aerobic and produces CO2 and water. If there is not enough O2, the glycogen is only broken down to lactic acid [Anaerobic Glycogen] and the content of lactic acid vigorous in the blood increases. This is a normal occurrence in vigorous athletes, but occurs too readily in patients whose heart or circulation does not supply the working muscles with enough blood.
Related Posts
Bhakti Yoga
Karma Yoga
Aims And Objectives Of Yoga
Misconceptions About Yoga
True Nature Of Yoga
