Organisation of cells animal tissues
|Philosophy of Science|
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NCERT Textbook Chapter Tissues
Concept #1 Animal tissues-Epithelial tissue
- Epithelial tissue perform the function of protection.
- Epithelium covers most organs and cavities within the body.
Notes for teachers
Blood and muscles are both examples of tissues found in our body. On the basis of the functions they perform we can think of four different types of animal tissues, such as epithelial tissue, connective tissue, muscular tissue and nervous tissue. Blood is a type of connective tissue, and muscle forms muscular tissue.
EPITHELIAL TISSUE The skin, the lining of the mouth, the lining of blood vessels, lung alveoli and kidney tubules are all made ofepithelial tissue. Epithelial tissue cells are tightly packed without intercellular spaces and form a continuous sheet.
Epithelial tissue is classified into 3types-Squamous epithelium, Columnar epithelium and Cuboidal epithelium.
Squamous epithelium In cells lining blood vessels or lung alveoli, where transportation of substances occurs through a selectively permeable surface, there is a simple flat kind of epithelium. This is called the simple squamous epithelium. Simple squamous epithelial cells are extremely thin and flat and form a delicate lining.The oesophagus and the lining of the mouth are also covered with squamous epithelium. The skin, which protects the body, is also made of squamous epithelium. Skin epithelial cells are arranged in many layers to prevent wear and tear. Since they are arranged in a pattern of layers,the epithelium is called stratified squamous epithelium.
Simple squamous epithelium
Where absorption and secretion occur, as in the inner lining of the intestine, tall epithelial cells are present called columnar epithelium. This columnar (meaning ‘pillar-like’) epithelium facilitates movement across the epithelial barrier. In the respiratory tract, the columnar epithelial tissue also has cilia, which are hair-like projections on the outer surfaces of epithelial cells. These cilia can move, and their movement pushes the mucus forward to clear it. This type of epithelium is thus ciliated columnar epithelium.
Simple columnar cells
Simple columnar cells in the digestive tract
ciliated columnar epithelium
Epithelium consisting of cube shaped cells is known as cuboidal epithelium.They form the lining for many ducts such as pancreatic duct, salivary duct and sweat ducts. Epithelial cells often acquire additional specialisation as gland cells, which can secrete substances at the epithelial surface. Sometimes a portion of the epithelial tissue folds inward, and a multicellular gland is formed. This is glandular epithelium.
Concept #2 Muscular tissue
- Muscular tissue is responsible for the movements of the body.
- The movement of the internal organs like heart,stomach are also caused by muscles.
Notes for teachers
Muscular tissue consists of elongated cells, also called muscle fibres.Muscles contain special proteins called contractile proteins, which contract and relax to cause movement.This property is responsible for movement of limbs and bending of body.The movement of internal organs like heart, stomach and alimentary canal are also caused by muscles.
Muscles are of three types, based on structure,function and location.They are;
- Striped muscles
- Unstriped muscles
- Cardiac muscles
Muscles present in our limbs can be moved or stopped by our conscious will. Such muscles are called voluntary muscles. These muscles are also called skeletal muscles as they are mostly attached to bones and help in body movement. Under the microscope, these muscles show alternatelight and dark bands or striations when stained appropriately. As a result, they are also called striated muscles. The cells of this tissue are long, cylindrical, unbranched and multinucleate.These muscles fatigue easily.
Longitudinal section of Striped muscles
T.S of Striped muscles
Unstriped muscles are made up of spindle shaped, elongated muscle fibres without striations. They are called smooth muscles.The movement of food in the alimentary canal or the contraction and relaxation of blood vessels are involuntary movements. We cannot control these movements on our own will. Smooth muscles or involuntary muscles control such movements. They are also found in the iris of the eye, in ureters and in the bronchi of the lungs.
The muscles of the heart show rhythmic contraction and relaxation throughout life. These involuntary muscles are called cardiac muscles. Heart muscle cells are cylindrical,elongated, branched and uninucleate. They do not fatigue easily.
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Concept #3 Connective tissue
Notes for teachers
This is the most widespread and abundant type of tissue in the human body. Its function is primarily to support, anchor and connect various parts of the body. Although connective tissue exists in a number of forms, all types have three basic structural elements -cells, fibres and ground substance. The cells of connective tissue are loosely spaced and embedded in an intercellular matrix . The matrix may be jelly like, fluid, dense or rigid.
Connective tissue is richly supplied with blood.It is also known as binding tissue since it connects or binds other tissues.
Connective tissues are classified into three types on the basis of nature of matrix.
- Loose Connective tissue
- Dense Connective tissue
- Fluid Connective tissue
Loose Connective tissue
The tissue in which the fibres in the matrix are loosely arranged is called loose Connective tissue.Areolar tissue,Adipose tissue and reticular tissue are included under this group.
Areolar connective tissue is found between the skin and muscles, around blood vessels and nerves and in the bone marrow. It fills the space inside the organs, supports internal organs and helps in repair of tissues.
The three different fibres of areolar connective tissue are arranged in no particular pattern but run in all directions and form a loose network in the intercellular material.
- Collagen fibres or white fibres are predominant. They usually appear as broad pink bands.
- Yellow fibres or elastic fibres, which appear as thin, dark fibres are also present.
- Reticular fibres.
Figure showing different types of fibres and cells in areolar tissue
The different types of cells embedded within the areolar tissue include:
- Fibroblasts:They are large,flat,highly branched.They secrete and maintain fibres.
- Plasma Cells:They are oval in shape.They produce antibodies.
- Adipocytes:Adipose tissue consists of fibrous connective tissue and fat cells called Adipocytes.
- Mast Cells:They are large cells having spherical nucleus with granular cytoplasm.
- Macrophages.:They are ameoboid in shape and are wandering cells.They defend against micro-organisms and ingest them and hence are called phagocytes.
Fat-storing adipose tissue is found below the skin and between internal organs. The cells of this tissue are filled with fat globules. The tissue stores fat which are used as and when the body requires.Storage of fat provides insulation against cold and protects the body like a shock absorber.
Obesity in animals, including humans, is not dependent on the amount of body weight, but on the amount of body fat - specifically adipose tissue. In mammals, two types of adipose tissue exist: white adipose tissue (WAT) and brown adipose tissue (BAT).
Adipose tissue with fat cells
Figure showing the location of adipose tissue below epidermis
Reticular connective tissue is a type of connective tissue.It has a network of reticular fibers, made of type III collagen.Reticular connective tissue is found around the liver, the kidney, the spleen, and lymph nodes, as well as in bone marrow.Reticular connective tissue resembles areolar connective tissue, but the only fibers in its matrix are reticular fibers, which form a delicate network along which fibroblasts called reticular cells lie scattered. Although reticular fibers are widely distributed in the body, reticular tissue is limited to certain sites. It forms a labyrinth-like stroma or internal framework, that can support many free blood cells (large lymphocytes) in lymph nodes, the spleen, and red bone marrow.It forms the architectural framework of: liver, adipose tissue, bone marrow, spleen and basement membrane, to name a few.
Tendons and ligaments
A tendon is a fibrous connective tissue which attaches muscle to bone or cartilage.Tendons contain more of white fibres. Tendons may also attach muscles to structures such as the eyeball. A tendon serves to move the bone or structure. A ligament is a fibrous connective tissue which attaches bone to bone, and usually serves to hold structures together and keep them stable.They contain more of elastic fibres.
Figure showing tendons and ligaments
Dense connective tissue
Connective tissue consisting of hard matrix called dense connective tissue.The cells are embedded in a dense matrix which is composed of calcium carbonate and calcium phosphate.
Dense connective tissue is of two types:cartilage and bone
Cartilage is a flexible connective tissue found in many areas in the bodies of humans and other animals, including the joints between bones, the rib cage, the ear, the nose, the bronchial tubes and the intervertebral discs. It is not as hard and rigid as bone but is stiffer and less flexible than muscle.
Cartilage is composed of specialized cells called chondrocytes that produce a large amount of extracellular matrix composed of collagen fibers, abundant ground substance rich in proteoglycan, and elastin fibers. Cartilage is classified in three types, elastic cartilage, hyaline cartilage and fibrocartilage, which differ in the relative amounts of these three main components.Chondroblasts that get caught in the matrix are called chondrocytes. They lie in spaces called lacunae with up to eight chondrocytes per lacuna.
Cartilage forms 'C' shaped rings in the trachea,bronchi.
Cartilage present in larynx,epiglottis,wall of the eustachian tube and pinna contains elastic fibres in the matrix.It is called elastic cartilage.
Cartilage found in between the vertebrae in the vertebral column chiefly contains collagen fibres.They are called fibrous cartilage.They bring about bending and stretching movements.
Bone tissue is a type of connective tissue that contains lots of calcium and phosphorous salts. About 25% of bone tissue is water, another 25% is made up of protein fibers like collagen. The other 50% of bone tissue is a mixture of mineral salts, primarily calcium and phosphorous.
Bone tissue consists of cells embedded in a firm calcified matrix.The matrix chiefly consists of collagen fibres,protiens and inorganic salts like calcium phosphate,chlorides of potassium,sodium and magnesium.
The long and strong bones of the limbs are filled with a fluid called bone marrow,which consists of fat and blood vessels.It produces blood cells.
The basic unit of Compact Bone is an osteon, which is also known as a Haversian System. Each Haversian System has a cylindrical structure that consists of four parts:
- A central tube called a Haversian Canal, which contains blood vessels and nerves.The Haversian Canal is surrounded by alternate layers of:
- Lamellae(the word lamellae literally means "little plates") are concentric rings of a strong matrix formed from mineral salts including calcium and phosphates and collagen fibres. The mineral salts result in the hardness of the bone structure, while the collagen fibres contribute its strength.
- Lacunae are the small spaces between the lamellae in which contain the bone cells (called "osteocytes") are located.
- The lacunae are linked together by minute channels called canaliculi.The canaliculi provide routes by which nutrients can reach the osteocytes and waste products can leave them.
The haversian canals of adjoining haversian systems are interconnected by transverse canals called volkmann's canal.
Cross section of long bone showing both spongy and compact osseous tissue
Fluid connective tissue
It is a type of connective tissue in which the matrix is in the liquid form. It is known as plasma. The plasma encloses different types of cells. Fibres are absent.Fluid connective tissue is distinguished into the following two types:
Blood is a bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells.
In vertebrates, it is composed of blood cells suspended in blood plasma. Plasma, which constitutes 55% of blood fluid, is mostly water (92% by volume),and contains dissipated proteins, glucose, mineral ions, hormones, carbon dioxide, and blood cells themselves. Albumin is the main protein in plasma, and it functions to regulate the colloidal osmotic pressure of blood. The blood cells are mainly red blood cells (also called RBCs or erythrocytes) and white blood cells, including leukocytes and platelets.
The most abundant cells in vertebrate blood are red blood cells. These contain hemoglobin, an iron-containing protein, which facilitates transportation of oxygen. In contrast, carbon dioxide is almost entirely transported extracellularly dissolved in plasma as bicarbonate ion. Red blood cells are the body's oxygen transport systems. They are not true cells, as they do not have a nuclei or organelles.Their life span is 100-120 days. Their color ranges from bright red to dark red, depending on how much oxygen they are carrying. In the lungs, the oxygen binds to hemoglobin, a protein with four iron-containing sites for oxygen to bind to. Hemoglobin is also what gives the cells their red color.The number of RBC varies from 4.5 to 5,5 million cells/cubic mm of blood.
White blood cells, or leukocytes, are the defense army within the blood tissue. They are the only true cells in the blood and are not limited to the blood stream. When called to defeat an infection or to dispose of a dead cell, they can squeeze through openings in the vessel walls. There, they can move through amoeboid action, meaning that they sprout cytoplasmic extensions that can move them. Leukocytes, along with platelets, usually make up less than 1% of blood.Their number varies from 6000 to 10000 cells/cubic mm of blood.
Platelets are actually cell fragments that help in blood clotting. When a tear occurs in the blood vessels, platelets stick to the exposed collagen at the site of injury. They produce fibrous protein that traps other blood cells in a clot that stops blood from flowing and helps to prevent excess blood loss. Some platelets also secrete substances that signal other platelets to the break, substances that constrict blood vessels, or substances that promote an inflammatory response. Once the clot is formed, the platelets contract and pull the opening back together, thus healing the break.They are about 2,50,000 platelets in 1cc of blood.
Most of the blood cells contained in blood tissue don't last very long. Red blood cells only last about 100-120 days before they are disassembled, recycled, and disposed of. Platelets last about 10 days if they do not need to clot blood. The bone marrow makes and releases blood cells into the blood on a regular basis to replace old blood cells. Thus, the tissue is constantly being restored.
Lymph is a colourless fluid similar to blood in structure.It differs from blood only in the absense of RBC and some plasma protiens.Lymph flows in a set of lymphatic capillaries.The lymph system is a network of organs, lymph nodes, lymph ducts, and lymph vessels that make and move lymph from tissues to the bloodstream. The lymph system is a major part of the body's immune system.
Lymph is a clear-to-white fluid made of:
- White blood cells, especially lymphocytes, the cells that attack bacteria in the blood.
- Fluid from the intestines called chyle, which contains proteins and fats.
Lymph nodes are soft, small, round- or bean-shaped structures. They usually cannot be seen or easily felt. They are located in clusters in various parts of the body, such as the neck, armpit, groin, and inside the center of the chest and abdomen.
Lymph nodes make immune cells that help the body fight infection. They also filter the lymph fluid and remove foreign material such as bacteria and cancer cells. When bacteria are recognized in the lymph fluid, the lymph nodes make more infection-fighting white blood cells, which causes the nodes to swell. The swollen nodes are sometimes felt in the neck, under the arms, and groin.
The lymphatic system includes the tonsils, adenoids, spleen, and thymus.
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Notes for teachers
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