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Friday, October 22, 2010

Kinds of Connective Tissue

Friday, October 22, 2010 - 7 Comments

Connective tissues are diverse group of tissues that serve various binding and supportive functions. Connective tissue is composed of smaller cells, large number of fibres suspended in extra cellular ground substance. Cells that are present in connective tissue are fibroblasts and macrophages. Fibroblasts produce protein ingredients of extra cellular fibres. Macrophages are amoeboid cells that move around in the network of fibres and engulf bacteria and debris of dead cells by phagocytosis.
Ground substance is matrix that is secreted by cells of connective tissue and may be liquid, jelly like or solid in nature. The nature and chemical composition of matrix determine the functional properties of various connective tissues. Fibrous network of connective tissue consists of three types of fibres namely collagenous fibre, elastic fibre and reticular fibre. All these fibres are proteins in nature strong flexible collagenous fibre consists of protein collagen that has great tensile strength and is produced by fibroblast. Elastic fibres are long threads made up of protein elastin. These fibres are elastic in nature and resist stretching. Reticular fibres are very thin and branched and are also made up of collagen. All three fibres are tightly interwoven and form a fabric like structure that joins connective tissue with adjacent tissue. Connective tissues may be classified on basis of loose and dense arrangement of these fibres. Major types of connective tissue in vertebrates are loose connective tissue, fibrous connective tissue, adipose tissue, cartilage, bone and blood.

(1)        Loose Connective Tissue:
In loose connective tissue strong, flexible fibres of protein collagen that are interwoven with fine, elastic and reticular fibres. Collagen fibres are made up of protein collagen that has great tensile strength and is produced by fibroblast. All these compounds give loose connective tissue its elastic consistency and make it excellent binding tissue. It is widely distributed under the epithelia of human body and binds the skin of underlying muscle tissue. Loose connective tissue also wraps different organs and act as cushions for these organs.

(2)        Fibrous Connective Tissue:
In fibrous connective tissue the collagen fibres are densely packed and may lie parallel to one another creating very strong cords. Number of fibroblast in fibrous connective tissue is higher than loose connective tissue. It is located in the dermis of skin, subnucosa of digestive tract, fibrous capsules of organs and joints where it provides structural strength. Tendons and ligaments are two important forms of fibrous connective tissue. The tendons connect muscles to bones or to other muscles while ligaments not only lie between two bones but also help in joint formation.

(3)        Adipose Tissue:
Adipose tissue is specialized form of loose connective tissue that consists of large sized cells known as adipocytes or adipose cells distributed throughout matrix. Each adipose cell contains large fat droplets that push the nucleus and cytoplasm towards one side closer to plasma membrane. Adipose cells swell when fat is stored and shrinks when it is used as fuel by the body. These cells accumulate in large number to form material commonly known as fat. It is located in the dermis of skin, in bones, breasts and mesenteries in abdomen and around the kidney and heart. It provides reverse fuel in the form of lipids. It acts as insulating pad and helps in prevention of heat loss. They also support and protect organs.

(4)        Cartilage Tissue:
It is hard but flexible tissue comprising of few cells, a ground substance and large number of fibres. In the cartilaginous tissue there are present large numbers of small spaces called lacunae. The rubbery matrix the chondrion which is secreted by chondroblasts surrounds the lacunae. Chondrion is protein carbohydrate complex and it along with collagen fibres gives cartilage its strength and elasticity. Cartilage lacks blood supply; all nutrients and waste material must diffuse through the ground substance from the surrounding tissue.
Fishes like sharks, rays and skates have cartilaginous skeleton other vertebrates also have the cartilaginous skeleton during embryonic development which is replaced by bony skeleton. Cartilagenous skeleton is located as flexible support in outer ear, nose and rings supporting wind pipe, dises that act as cushion between vertebrate and caps on ends of some bones.

(5)        Hyaline Cartilage:
In Hyaline cartilage cells are located in lacunae surrounded by inter cellular material containing fine collagen fibres. Hyaline cartilage forms embryonic skeleton, covers the ends of long bones and form cartilage of nose, trachea and larynx. It also maintains the shape of these structures and also provides support.

(6)        Elastic Cartilage:
It consists of fine collagenous fibres and many elastic fibres in its inter cellular material. It is located in external ear and epiglottis where it maintains the shape of these structures also provides great flexibility.

(7)        Fibro Cartilage:
Fibro cartilage contains many large collagenous fibres in it inter cellular material. Fibroblasts are present in fibro-cartilage. They are present in inter verbal dise, Public symphysis and discs of knee joint. It is responsible for absorbing compression shock.

(8)        Bone Tissue:
It is the strongest of vertebrate connective tissue containing mineralized collagen fibres. Bone cells also known as osteocytes are located within lacunae but the matrix around them is heavily impregnated with calcium phosphates? Making this kind of tissue hard and ideally suited for its functions of support and protection.
Bone matrix is deposited in concentric layers around central canal, the osteonic canal, located in bones and support, protects and provides lower system for muscles to act on, stores calcium and fat from blood cells.

(9)        Blood:
Blood is also a kind of connective tissue having the fluid matrix in the form of plasma, suspends specialized red blood cells, white blood cells and platelets. Matrix plasma consists of water, salts and variety of dissolved proteins. Suspended in the plasma are red blood cells or erythrocytes which carry oxygen to different parts of the body. Other type of blood cells is called white blood cells or leucocytes that fight against viruses, bacteria and other invaders. Platelets are the cell fragments and are involved in blood clothing. It is located within blood vessels in higher organisms. It transports substances like oxygen, carbon dioxide, nutrients, wastes, hormones, minerals, vitamins etc throughout the body of animal.

Kinds of Epithelial Tissues

Epithelial tissue exists in many structural forms and mostly either covers or lines an organ or a duct. These cells typically consist of renewable sheets of cells that have surface specialization adapted for their specific roles. All epithelial cells are supported by underlying basement membrane which is condensed form of connective tissue. Epithelial tissues are classified on the basis of shape and number of layers present. Epithelium can be simple, stratified. Individual epithelial cells can be flat, cube shaped or column like. Epithelial tissues absorb, transport, excrete, protect and contain nerve cells for sensory reception. The size, shape and arrangement of the epithelial cells are related to these specific functions.
(1)        Simple squamous epithelium:

It consists of single layer of tightly packed, flattered cells with disc shaped central nucleus. They are located in air sacs of the lungs, kidney glomeruli, lining of heart, blood vessels and lymphatic vessels. It allows passive diffusion of gases and tissue fluids into and out of cavities.
(2)        Simple Cuboidal epithelium:

It consists of single layer of tightly packed. These cells are smaller in size cube or box with almost central nucleus and have pentagonal or hexagonal outline. They are located in kidney tubules, small glands and surface of the ovary. These cells are responsible in secretions and absorption.
(3)        Simple columnar epithelium:

It consists of single layer of elongated cells with an elongated nucleus located near the basal end of the cell. These cells often bear minute finger like projection called microvilli that increase its absorptive surface area. In some cases they also develop cilia and become ciliated cells as in the lining of female reproductive tract. Some are specialized as the global cells that secrete mucus. They are located in the lining of digestive tract, gall bladder, female reproductive tract and excretory ducts of some glands. They are highly absorptive in nature and as such are present along the intestinal tract of most animals. They are also responsible for absorption of different material and enzyme secretion.
(4)        Pseudostratified ciliated columnar epithelium:

It consists of columnar cells with a tuft of cilia at the top except the goblet cells. The cells of psendostratified ciliated columnar epithelium appear stratified or layered as each cell has two or more nuclei. They are located in the lining of branch, uterine, tubes and some regions of uterus. These cells help the reproductive cells and mucous to move by ciliary action.
(5)        Stratified squamous epithelium:

It consists of many layers of cells. They form the lining of oral cavity, oesophagus, digestive canal and vagina. Some keratinized cells also line the surface of the skin. They protect the underlying tissues against abrasion. The basal layers of cell undergo division, pushing the cells towards the surface where they are sloughed off and replaced by new cells form beneath.

Saturday, October 16, 2010

Structure and Function of Cytoplasmic Organelles of Cell

Saturday, October 16, 2010 - 0 Comments

Cytoplasm of cell consists of Endoplasmic reticulum, Golgi apparatus, Golgi bodies, Mitochondria, Ribosomes, Vacuoles and Vesicles, etc.

(1)        Ribosomes:
They are non membrane bound structures that are the sites for protein synthesis. They contain almost equal amount of protein and rRNA. Some ribosomes attach to endoplasmic reticulum and some float freely in cytoplasm. Clusters of ribosomes connected in strand of mRNA are called polysomes.

(2)        Endoplasmic reticulum:
It is complex membrane bound labyrinth of flattered sheets, sacs and tubules that branches and spreads throughout the cytoplasm. The ER is continuous from the nuclear envelope to plasma membrane. They are series of channels that help various materials to circulate throughout the cytoplasm. ER with attached ribosomes is rough ER and ER without attached ribosomes is smooth ER. Smooth ER is site for lipid production, detoxification of wide variety of organic molecules and storage of calcium ions in muscle cells.

(3)        Golgi apparatus:
It is composed of flattered stacks of membrane bound disternae. Golgi apparatus sorts, packages and secrets proteins and lipids. Proteins that ribosomes synthesize are sealed off in little packets called transfer vesicles. Transfer vesicles pass from ER to Golgi apparatus and fuse with it. In Golgi apparatus proteins are concentrated and chemically modified. Proteins are packaged into secretary vesicles which are released into cytoplasm close to plasma membrane. When the vesicles reach plasma membrane they fuse with it and release their contents to the outside of cell by exocytosis.
Golgi apparatus are most abundant in cells that secrete chemical substances. Golgi apparatus also produces lysosomes.

(4)        Lysosomes:
They are membrane bound spherical organelles that contain enzymes called acid hydrolases which are capable of digesting organic molecules (lipids, proteins, nucleic acids and polysaccharides) under acidic conditions. Enzymes are synthesized in ER, transported to Golgi apparatus for processing and then secreted by Golgi apparatus in the form of lysosomes or as vesicles that fuse with lysosomes. Lysosomes fuse with phagocytic vesicles, thus exposing the vesicle’s contents to lysosomal enzymes.

(5)        Mitochondria (Power generators):
Mitochondria are double membrane bound organelles that are spherical to elongate in shape. Small space separates outer membrane form inner membrane. Inner membrane folds and doubles in on itself to form incomplete partitions called cristae. The cristae increase the surface area available for chemical reaction that trop usable energy for the cell. The space between cristae is the matrix. The matrix contains ribosomes, circular DNA and other material. Because they convert energy to usable form, mitochondria are called, ‘Power generators’ or ‘Power house’ of the cell. Mitochondria usually multiply when a cell needs to produce more energy.

(6)        Centrioles and Microtubule organizing centres:
The specialized non membranous regions of cytoplasm near nucleus are microtubule organizing centres. These centres of dense material give rise to large number of microtubules with different functions in cytoskeleton. For example one type of centre gives rise to Centrioles that lie at right angles to each other. Each centriole is composed of nine triplet microtubules that radiate from the centre like the spokes of a wheel. The centroils are duplicated preceding cell division are involved with chromosomes movement and help to organize they cytoskeleton.

(7)        Vacuoles:
They are membranous sacs that are part of cytomembrane system. Vacuoles occur in different shapes and sizes and have various functions. For example some protozoa and sponges have contractile vacuoles that collect water and pump it outside to maintain the organism’s internal environment. Other protozoa and sponges have vacuole for storing food.

Structure and Function of Nucleus in the Cell

In the centre of eukaryotic cell, very important cell organelle is located which is named as nucleus. The nucleus is differentiated from the cytoplasm due to the present of a membranous structure called nuclear membrane or nuclear envelope. In prokaryotic cells the nuclear envelope is absent, thus no distinct nucleus is present. The shape of the nucleus is generally spherical but it may slightly irregular. Nucleus contains DNA and is the control and information centre for eukaryotic cell. It has two major functions. The nucleus directs chemical reactions in cells by transcribing genetic information from DNA to RNA, which then translates this specific information into proteins like enzymes that determine the cell’s specific activities. Nucleus also stores genetic information and transfers it during cell division from one cell to the next and from one generation of organisms to the next. Nucleus comprises of nuclear envelope, chromosomes and nucleolus.
Structure of Cell:

Nuclear envelope: It is gateway to nucleus. The nuclear envelope is a structure that separates the nucleus from the cytoplasm that is continuous with endoplasmic reticulum at number of points. It acts as a barrier between the contents of the nucleus and cytoplasm. The nuclear envelope is made up of two layers, other nuclear membrane and inner nuclear membrane. The structure and chemical composition of these membranes is the same as that of the cell membrane. There is present space between two nuclear envelopes layers the cisternae. Outer layer may be continuous with endoplasmic reticulum, cytoplasm and adjacent cells or exterior.
Nuclear pore:
Nuclear membrane has at places small pores called nuclear pores that are formed by the fusion of two layers of nuclear envelope. In addition to fusion of two layers of nuclear envelope, the pore is composed of an ordered array of globular and filamentous granules forming nuclear pore complex. These granules are made up of protein. These nuclear pores control the transport of different molecules into and out of the nucleus.
Size of pore is also important as it allows specific sized molecules to pass through. Generally it presents the movement of DNA but permits RNA to be moved out. These pores also provide direct contact of nucleus to the cytoplasm, endoplasmic reticulum or event to the exterior through endoplasmic reticulum. Number of pores present in the nucleus is variable and depends upon specific function of that particular cell. The nucleus of undifferentiated cells like eggs may have over thirty thousand pores while the differentiated cell like eukaryocytes may have only three or four pores in single nucleus. In majority of cells nuclear pores may exceed over three thousand in single nuclear envelope.
Genetic Containers:
Inside the nuclear envelope there is present a fluid material called nucleoplasm. In non dividing cells the nucleoplasm contains nucleoli, chromosomes and enzymes for the synthesis of DNA and RNA. In addition it also performs a number of other functions as well. Genetic material is in the form of network of threads called chromatic or chromatin network. Chromatin consists of uncoiled, tangled mass of chromosomes that are coloured bodies containing hereditary information in segments of DNA called genes chromosomes are self duplicating and carry the hereditary instructions. During cell division each chromosome coils tightly which makes the chromosome visible when viewed through light microscope. Chromosomes are made up of bead like structure, the nucleosomes. Nucleosomes are connected with one another by means of a strand of DNA called the linker DNA or linker that consists of 50 nucleotides. A nucleosome is made up of an octamere of histones surrounded by two turns of DNA ribbon that consists of about 200 nucleotides. The octamere is formed by eight different types of histones called H2A, H2B, H3 and H4. Another histone H1 fixes DNA helix over histone octamere and parents from uncoiling. The number of chromosomes in all individuals of the same species remain constant generation after generation e.g. in man each cell has 46 chromosomes, frog cell has 26, chimpanzee has 48 and fruit fly has 8 chromosomes.
It is permeable point for ribosomes. Nucleus contains one or two discrete non membrane bound structure called nucleolus in the nucleoplasm of non-dividing cells. Sometimes the number of nucleoli may be two or more, even in thousands in case of amphibian egg. They can be readily stained with basophilic dyes; chemically the nucleoli are composed of nucleic acids, especially the ribnucleic acid (RNA) and some proteins. Nucleolus is pre assembly points for ribosomes in many stages of synthesis and assembly. Assembly of ribosome is completed after they leave the nucleus through pores of nuclear envelope into cytoplasm where they are helpful in protein synthesis.

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