Inderbir singh histology pdf


    Textbook of HUMAN HISTOLOGY (With Colour Atlas & Practical Guide) SIXTH EDITION Textbook of HUMAN HISTOLOGY (With Colour Atlas & Practical Guide) . The book Inderbir Singh's Textbook of Human Histology with Colour Atlas and Practical Guide gives comprehensive information about human. Inderbir Singh's Textbook of Human Histology with Colour Atlas and Practical Guide {United VRG}.pdf - Ebook download as PDF File .pdf), Text File .txt) or read.

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    Inderbir Singh Histology Pdf

    This books (Inderbir Singh s Textbook of Human Histology [PDF]) Made by Neelam Vasudeva About Books none To Download Please Click. Read Download Inderbir Singh's Textbook of Human Histology: With Colour Atlas and Practical Guide |PDF books PDF Free Download Here. Download the Medical Book: Inderbir Singh's Textbook of Human Histology 7th Edition For Free. This Website we Provide Free Medical Books for all St.

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    Post to Cancel. Post was not sent - check your email addresses! Sorry, your blog cannot share posts by email. This site uses cookies. By continuing to use this website, you agree to their use. To find out more, including how to control cookies, see here: Cookie Policy. Pyramidal cell layer 4. Molecular layer 2. Nerve fibres. Piamater W. White matter Fig. Sensory ganglia 1.

    Autonomic ganglia A44 Color Atlas. Nerve fibres Fig. Pseudounipolar neurons 2. Nerve fibres 3. The neuron is sur- rounded by a ring of satellite cells. Multipolar neurons 2. Smooth muscle. There is no cartilage in their walls. It consists of a lining of mesothelium resting on a layer of connective tissue. Alveoli B. Key A.

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    Bronchiole A45 Color Atlas. The theca interna and externa are collectively called theca folliculi.

    Membrana granulosa 3. Capsule A46 4. Large follicles have a C. Deep to the epithelium there is a layer of connective tissue that constitutes the tunica albuginea. Ovarian follicles present. Discus proligerus 8. Maturing oocyte 5. Zona pellucida 6. Ovary a developing ovum surrounded Key by flattened follicular cells are 1. Stroma Color Atlas. Antrum folliculi 2. Medulla several layers of follicular cells. Cortex follicular cavity surrounded by M.

    Cumulus oophoricus 7. Anterior white column 7. The stain used for the slide is Luxol Fast Blue. Anterior median sulcus. The fibres in front of the grey commissure form the anterior white commissure 8.

    Central canal lying in grey commissure. Posterior white column 3. Lateral white column 5. The grey matter lies towards the centre and is surrounded all round by white matter.

    Posterior median septum 2. Anterior grey column 6. Lateral motor tracts MN. The grey matter consists of a centrally placed mass and projections horns that pass forwards and backwards. It is made up of white matter made up mainly of myelinated fibres. Posterior grey column 4. Multipolar neurons in grey matter A47 Color Atlas. Anterior motor tracts LT. Spinal cord A.

    The core of each folium is formed by pink staining fibres of the white matter. Purkinje cells 3. Peripheral nerve A48 Color Atlas. Granule cell layer 4. White matter P. Perineurium 2. The myelin sheaths. Endoneurium Fig. Cerebellum Key 1. All routine histological techniques involve the bright field microscopy. The brightness of light can be adjusted.

    In light microscopy the tissue is visualized against a bright background so it also referred to as bright field microscopy and is best suited to view stained tissue sections. Illuminating Device Most of the advanced microscopes come with a built-in illuminator using low voltage bulbs for transmitted light. The condenser is close to the stage and has an aperture iris diaphragm that controls the amount of the light coming up through the condenser.

    His invention of this primitive microscope detected small structures like bacteria. Older monocular microscopes used mirrors to reflect light from an external source. Collects and focuses light from the light source onto the specimen being viewed.

    The light enters from the periphery and scattered light enters the objective lens showing a bright specimen against a dark background. Optical Parts The functioning of the microscope is based on the optics of the lenses.

    Antonie Philips van Leeuwenhoek in invented the concept of using combination of convex lenses to magnify small structures and visualise them. Chapter 1 Light Microscopy and Tissue Preparation —Sabita Mishra A light microscope is an optical device that uses visible light for illumination and lenses to magnify a specimen or tissue section for detailed visualisation.

    In contrast a dark field microscopy is where unstained specimens. Simple microscopes utilise a single lens while compound microscopes have a number of lenses in combination. Mechanical knobs that bring the specimen into general focus. The arm connects the body tube to the base of the microscope. In a binocular microscope diopter adjustments are present to adjust the focusing of the eye piece Non-Optical Components of Microscope Body Tube Head A cylindrical tube that connects the eyepiece to the objective lenses.

    The objective lens collects maximum amount of light from the object to form a high quality magnified real image. Standard length mm. A standard microscope has three. These lenses are attached to the nosepiece of the microscope. It is the final stage of the optical path of the microscope and produces a magnified virtual image which is seen by the eye. The eye piece has a power of 10X. Textbook of Human Histology Fig. Eye pieces may be monocular.

    The viewer looks through the eyepiece to observe the magnified image. The type and quality of an objective lens influences the performance of a microscope. The base supports the microscope. The illuminator with its power switch is located on the base. The viewer rotates the nosepiece to select different objective lenses. An aperture in the middle of the stage allows light from the illuminator to reach the specimen. Resolution of a microscope is dependent on a.

    This distance is called resolution of a normal eye. Most specimens are mounted on slides. Stage Control Knobs move the stage left and right or up and down. Numerical aperture 3 Chapter Specimen or slide: The specimen is the object being examined. By assembling a combination of lenses the distance can be increased and the eye can visualise objects closer than 0.

    Principles of a conventional bright field microscope The word compound refers to the fact that two lenses. A rotating turret that houses the objective lenses. Stained tissue sections are mounted on a glass slide with a coverslip placed over it. The flat platform where the slide is placed and lies perpendicular to the optical pathway.

    Resolution Given sufficient light an unaided eye can distinguish 2 points lying 0. Numerical aperture NA: The slide is placed on the stage for viewing. Fine tunes the focus and increases the detail of the specimen.

    It also allows the specimen to be labelled. This allows the slide to be easily inserted or removed from the microscope. The stage also has a vernier caliper attached to it so that the viewer can come back to any reference point by the help of the caliper.

    Thus the viewer observes a magnified virtual image. The best and most expensive lenses have the least aberrations. Light from the light source enters the condenser. Working of a light microscope Fig. The real magnified image formed by the objective lens is further magnified by the eyepiece.

    IB Singh – Textbook of Human Histology, 6th Edition [PDF]

    Working of a light microscope Schematic representation Axial Aberrations When light passes through the lens it suffers a number of aberrations which result in image degradation. The optical parts are the condenser. Flowchart 1. Production of a coloured spectrum of white light. This is corrected by using a compound lens. Images produced by dark-field microscopy are low resolution and details cannot be seen.

    With Kohler illumination auxiliary lens of the lamp focuses the enlarged image on to the iris diaphragm of the sub-stage condenser. Different lenses corrected for chromatic aberrations are listed in Table 1. Light source is focused in the same plane as the object. Light passing through the periphery of the lens is refracted to a greater extent than through the central part.

    Kohler Illumination High intensity microscopes have a small light source that is insufficient to fill the whole field with light and are usually supplied by an auxiliary lens and iris which increases the apparent light source. The specimen is illuminated from the side and only scattered light enters the objective lens which results in bright objects against dark background. The resolving power of critical and Kohler are similar. Dark- field microscopy is especially useful for visualisation of small particles.

    Table 1. Illumination Critical Illumination This is used with simple equipment and a separate light source. Paraformaldehyde is a polymer of formaldehyde available as a white crystalline powder. The primary objective of fixation is that stained section of any tissue must maintain clear and consistent morphological features to almost that what was existing during life. It coagulates the tissue proteins and constituents. Tissue processing To visualise the microstructure of any tissue under a light microscope.

    Principle of tissue processing involves replacement of all extracellular water from the tissue and replacing it with a medium that provides sufficient rigidity to enable sectioning without any damage or distortion to the tissue. The sections are picked up in grids and are stained with uranyl acetate and lead citrate before viewing. To prepare such thin section one uses an ultramicrotome and instead of steel blades. In transmission electron microscopy the beam of electron passes through the tissue which is a thin section less than nm.

    This produces a bright image in a dark background. The confocal scanning optical microscope is designed to illuminate an object in a serial fashion. There are some natural fluorescence substances which fluoresce when ultraviolet light falls on them. Commonly tissue is obtained from. Transmission electron microscope is used for ultrastructural studies. Commonly used fixative is formaldehyde and glutaraldehyde Formaldehyde is a cross-linking fixative which acts by creating a covalent bond between proteins in the tissue.

    Instead of glass lenses here one uses electromagnetic lenses. The images are digitized and stored. The resolving power of an electron microscope is 0. Certain fluorescent dyes when added to the tissue lead to a secondary fluorescence which are visualised by a fluorescent microscope.

    In scanning electron microscope the beam of electrons are reflected back from the surface. A fluorochrome is excited with ultraviolet light and the resulting visible fluorescence is viewed.

    They are dried before staining. Making the tissue clear by removing the dehydrating agent e. The tissue blocks are ready for sectioning. Haematoxylin and eosin is routinely used for all teaching slides.

    Removal of water by a dehydrating agent. Paraffin wax is routinely used as an embedding media. The sections are cut and picked up on clean glass slides under a water bath. Commonly used dehydrating agent is alcohol in descending grades e. It has a melting point of degree. Haematoxylin is a basic dye and stains the nucleus blue while eosin is an acidic dye and stains the cytoplasm pink.

    Permeating the tissue with a support media. Other embedding media used are celloidin and resins. Morphological identification becomes easier. The tissue is embedded and orientated in the media and forms a solid block at room temperature. Once the sections are stained they are mounted and are ready for viewing under a light microscope.

    A cell is bounded by a cell membrane or plasma membrane within which is enclosed a complex material called protoplasm. The cytoplasm has a fluid base matrix which is referred to as the cytosol or hyaloplasm.

    Thus the membrane is metabolically very active. Many of them are in the form of membranes that enclose spaces. The cytosol contains a number of organelles which have distinctive structure and functions. Each phospholipid molecule consists of an enlarged head in which the phosphate portion is located.

    It has highly selective permeability properties so that the entry and exit of compounds are regulated. Chapter 2 Cell Structure introduCtion Cell is the fundamental structural and functional unit of the body.

    The cellular metabolism is in turn influenced and probably regulated by the membrane. The head end is called the polar end while the tail end is the non-polar end. Membranes are mainly made up of lipids. The contents of these compounds vary according to the nature of the membrane. These spaces are collectively referred to as the vacuoplasm. It consists of two densely stained layers separated by a lighter zone. The head end is soluble in water and is said to be hydrophilic.

    The nucleus is separated from the cytoplasm by a nuclear membrane. The protoplasm consists of a central more dense part called the nucleus. The tail end is insoluble and is said to be hydrophobic.

    Lipids in Cell Membrane The trilaminar structure of membranes is produced by the arrangement of lipid molecules predominantly phospholipids that constitute the basic framework of the membrane.

    These are called transmembrane proteins Fig. One glycolipid is galactocerebroside. Another category of glycolipids seen are ganglionosides. Most of them are embedded within the thickness of the membrane and partly project on one of its surfaces either outer or inner. The dark staining parts of the membrane seen by EM are formed by the heads of the molecules. Because of the manner of its formation.

    Structure of a phospholipid Fig. Fluid mosaic model of membrane molecule phosphatidyl choline seen in a Schematic representation cell membrane Schematic representation When such molecules are suspended in an aqueous medium. Chapter 2 Cell Structure Fig. They are of various types including phosphatidylcholine. For the same reasons. Proteins in Cell Membrane The proteins are present in the form of irregularly rounded masses.

    They do so by forming a bi-layer which forms the basis of fluid mosaic model of membrane Singer and Nicolson Fig.

    The carbohydrate layer is specially well developed on the external surface of the plasma membrane forming the cell boundary. They also help to provide adhesion between cells and extracellular materials. Some varieties of membrane proteins Schematic representation they form passive channels through which substances can diffuse through the membrane. They are attached either to the proteins Fig.

    These include major histocompatibility complexes MHC. This force of repulsion maintains the 20 nm interval between cells. Added Information The glycocalyx is made up of the carbohydrate portions or glycoproteins and glycolipids present in the cell membrane.

    Textbook of Human Histology The proteins of the membrane are of great significance as follows: Carbohydrates of Cell Membranes In addition to the phospholipids and proteins. Some functions attributed to the glycocalyx are as follows: This layer is referred to as the cell coat or glycocalyx. In erythrocytes. Ions get attached to the protein on one surface and move with the protein to the other surface.

    Glycolipid and glycoprotein molecules attached to the forming glycoproteins or to the outer aspect of cell membrane Schematic representation lipids forming glycolipids Fig. This vesicle can move through the cytosol to other parts of the cell..

    When suitably stimulated.. The term pinocytosis is applied to a process similar to endocytosis when the vesicles then called pinocytotic vesicles formed are used for absorption of fluids or other small molecules into the cell. Chapter 2 Cell Structure Functions of Cell Membrane The cell membrane is of great importance in regulating the following activities: Receptors also play an important role in absorption of specific molecules into the cell as described below.

    Molecules produced within the cytoplasm e. It results in contraction in the case of muscle. In a Nutshell: Large molecules enter the cell by the process of endocytosis Fig. The vesicles in question are called exocytic vesicles. Some cells use the process of endocytosis to engulf foreign matter e. The plasma membranes of such cells are normally polarised—the external surface bears a positive charge and the internal surface bears a negative charge. Role of Cell Membrane in Transport of Material into or out of the Cell It has already been discussed that some molecules can enter cells by passing through passive channels in the cell membrane..

    The process is then referred to as phagocytosis. In this process the molecule invaginates a part of the cell membrane. This is called depolarisation. The vesicle then ruptures releasing the molecule to the exterior. Stimulation of such receptors e. This function is most developed in nerve and muscle cells.

    Fusogenic proteins also help in exocytosis by facilitating fusion of membrane surrounding vesicles with the cell membrane. Coated pit as seen by electron microscope a scaffolding around the developing vesicle in cell membrane Schematic representation 12 contd. Three stages in the absorption Fig. The membrane lining the floor of the pits is thickened because of the presence of a protein called clathrin. Three stages in exocytosis. This protein forms Fig. ContaCts between adjoining Cells In tissues in which cells are closely packed.

    Caveolae also play a role in transport of extracellular molecules to the cytosol without formation of vesicles Fig. The process is called potocytosis Schematic representation and facilitates its separation from the cell membrane. In some regions the cell membranes of adjoining cells come into more intimate contact. These molecules occupy the entire thickness of the cell membrane i. Caveolae differ from coated pits in that they are not transformed into vesicles.

    This contact is sufficient to bind cells loosely together At such sites adjoining cell membranes are held together by some glycoprotein molecules. The transport takes place through invaginations of cell membrane called caveolae.

    A protein caveolin is associated with caveolae Fig. The other end of the CAM juts into the 20 nm intercellular space. In this way a path is established through which forces can be transmitted from the cytoskeleton of one cell to another Fig. How extracellular molecules can pass Fig. At its cytosolic end. The process is seen mainly in flat cells e. These areas can be classified as follows: Fibrous elements of the cytoskeleton are attached to this intermediate protein and thus indirectly to CAMs.

    How extracellular molecules enter the through the entire thickness of a cell transcytosis. Endocytic vesicles are Caveolae are involved Schematic representation not formed. Contacts between cells can be classified on the basis of the type of CAMs proteins present. The adhesion of some CAMs is dependent on the presence of calcium ions. Types of Specialised Junctions Fig. Intermediate proteins are also of various types catenins.

    Basic structure of an unspecialised contact The basic mode of intercellular contact in between two cells Schematic representation them is similar to that described above and involves CAMs. About 20 types of integrins. They can be of the following types: Various types of specialised junctions Schematic representation 14 Chapter Table 2.

    Leucocytes Integrins Between cells and intercellular substances. Macula Adherens These are the most common type of junctions between adjoining cells. We now know that the fibrils seen in the intercellular space represent CAMs Fig..

    Cytoskeletal filaments atta- ched to the thickened area are intermediate filaments. Such junctions allow direct transport of some substances from cell to cell. The region of the gap is rich in glycoproteins. Anchoring Junctions Adhesion Spots Desmosomes. The thickened area or plaque seen on the cytosolic aspect of the cell membrane is produced by the presence of intermediate link proteins. Electron microscope appearance of The thickened areas of the two sides are zonula adherens Schematic representation separated by a gap of 25 nm.

    CAMs seen in desmosomes are integrins desmogleins I. As seen by EM. Chapter 2 Cell Structure Modified anchoring junctions attach cells to extracellular material.

    At the site of a desmosome. Apart from holding cells together. Detailed structure of a desmosome in the epidermis epithelial cells. The thickened areas of the two membranes are held together by fibrils that appear to pass from one membrane to the other across the gap. Such junctions are seen as hemidesmosomes or as focal spots. Electron microscope appearance of a cytoplasm. The link proteins are desmoplakins.

    Adhesive Belts Zonula Adherens In some situations. Desmosomes are present where strong anchorage between cells is needed. Such junctions are of a transient nature e. In epithelial cells zona adherens are located immediately deep to occluding junctions Fig. Hemidesmosomes These are similar to desmosomes. Adhesive Strips Fascia Adherens These are similar to adhesive belts.

    The CAMs in focal spots are integrins. The cytoskeletal elements attached to intermediate proteins are keratin filaments as against intermediate filaments in desmosomes. This is similar to a desmosome in being marked by thickenings of the two plasma membranes. Zonula occludens as seen by electron Zonulae occludens are. They represent areas of local adhesion of a cell to extracellular matrix. Such contacts may send signals to the cell and initiate cytoskeletal formation. As in desmosomes.

    Gap junction as seen by electron tight junctions Fig. These are seen in relation to smooth muscle. They differ from the latter in that the areas of attachment are in the form of short strips and do not go all round the cell.

    At such junctions. Hemidesmosomes are common where basal epidermal cells lie against connective tissue. At such a junction the two plasma membranes are in actual contact Fig. Occluding Junctions Zonula Occludens Like the zonula adherens. Focal Spots These are also called focal adhesion plaques. The CAMs present are cadherins. The intermediate proteins that bind integrins to actin filaments are a-actinin.

    Textbook of Human Histology junction called the zonula adherens or adhesive belt Fig. These junctions not only bind the cells to each other but also act as barriers that prevent the movement of molecules into the intercellular spaces. Occluding junctions prevent lateral migration of such proteins. Chapter 2 Cell Structure Added Information Recent studies have provided a clearer view of the structure of tight junctions Fig.

    IB Singh - Textbook of Human Histology, 6th Edition [PDF] | Free Medical Books

    Other functions attributed to occluding junctions are as follows: Junctional Complex Near the apices of epithelial cells the three types of junctions described above. They collectively form a junctional complex. In transmission electronmicrographs this gap is seen to contain bead-like structures Fig.

    In some situations occlusion of the gaps between the adjoining cells may be incomplete and the junction may allow slow diffusion of molecules across it. Gap junctions are. Detailed structure of part of an occluding prevent back diffusion of transported sub.

    In some complexes. Apart from epithelial cells. Communicating Junctions Gap Junctions At these junctions. These are referred to as leaky tight junctions. A junctional complex schematic the cytoplasm of the two cells thus allowing the free representations passage of some substances sodium. They are widely distributed in the body. The cytosol also contains a cytoskeleton made up of microtubules. As seen under electron microscope.

    By allowing passing of ions they lower transcellular electrical resistance. They include the endoplasmic reticulum ER. Here they come in contact with and align perfectly with similar connexins projecting into the space from the cell membrane of the opposite cell to complete the channel. Centrioles are closely connected Fig. Gap junctions form electrical synapses between some neurons. A communicating junction gap junction. The cell organelles can be mem- brane bound or without membrane Table 2.

    Cell organelles The cytoplasm of a typical cell contains various structures that are referred to as organelles.

    To show the constitution of one channel of a communicating junction Schematic representation The wall of each channel is made up of six protein elements called nexins. Some features of a cell that can be seen with microtubules. The proteins. Various organelles found in a typical cell Schematic representation 19 Chapter Exocytic vesicles endoplasmic reticulum It is a network of interconnecting membranes enclosing channels or cisternae.

    Because of the presence of the endoplasmic reticulum ER the cytoplasm is divided into two components. ER is very prominent in cells actively synthesising proteins.

    The cytoplasm within the channels is called the vacuoplasm.

    Chapter 2 Cell Structure Table 2. Intermediate filaments. The mitochondrion is bounded by a smooth outer membrane within which there is an inner membrane.. Mitochondria are large in cells with a high oxidative metabolism. It is also continuous with the lumen of smooth ER.

    An interesting fact. It is also responsible for synthesis of lipids. Erythrocytes do not contain mitochondria. These are believed to carry information that enables mitochondria to duplicate themselves during cell division. It is a prominent feature of cells processing lipids. The space bounded by the inner membrane is filled by a granular material called the matrix. Most cells have very little smooth ER. Mitochondria vary in size.

    This matrix contains numerous enzymes. Smooth ER is responsible for further processing of proteins synthesised in rough ER. The ribosome is made up of two subunits small 40S and large 60S classified on the basis of their sedimentation rates. They may be present singly in which case they are called monosomes. The rough appearance of rough ER is due to ribosomes attached to cytoplasmic side of membrane which play an important role in protein synthesis.

    Each ribosome consists of proteins and RNA ribonucleic acid and is about 15 nm in diameter. Products synthesised by the ER are stored in the channels within the reticulum.

    Ribosomes play an essential role in protein synthesis. The inner membrane is highly folded on itself forming incomplete partitions called cristae. Mitochondria are of great functional importance. It is the power house of the cell. They may also lie free in the cytoplasm. The number of mitochondria varies from cell to cell being greatest in cells with high metabolic activity e. A schematic presentation of some details of the structure of a mitochondrion as seen by EM is shown in Figure 2.

    Mitochondria Mitochondria can be seen with the light microscope in specially stained preparations Fig. It is a prominent feature of cells synthesising proteins. The lumen of rough ER is continuous with the perinuclear space between the inner and outer nuclear membranes.

    Adenosine triphosphate ATP and guanosine triphosphate GTP are formed in mitochondria from where they pass to other parts of the cell and provide energy for various cellular functions. When examined with the EM. Their lumen is not in communication with that of ER. In light microscopic preparations suitably treated with silver salts. The features which differ in intensity from patient to patient includes muscle weakness. Structure of a mitochondrion Schematic representation Added Information The enzymes of the TCA cycle are located in the matrix of mitochondria.

    The membranes form the walls of a number of flattened sacs that are stacked over one another. The condition can be diagnosed by EM examination of muscle biopsies. Enzymes for lipid synthesis and fatty acid metabolism are located in the outer membrane. This interferes with mitochondrial and cell functions. The cisternae of the Golgi complex form an independent system.

    Towards their margins the sacs are continuous with small rounded vesicles. The mitochondria show characteristic para-crystalline inclusions.

    Material from ER reaches the Golgi complex through vesicles. The role of the Golgi complex in formation of secretory vacuoles Schematic representation 22 Chapter The region nearest the nucleus is the cis face or cis Golgi. Here sugar residues are added to proteins to form protein-carbohydrate complexes glycoproteins. Textbook of Human Histology From a functional point of view the Golgi complex is divi- sible into three regions Fig. The opposite face nearest the cell membrane is the trans face also referred to as trans Golgi.

    From the cis face all these materials pass into the medial Golgi. Vesicles budding off from smooth ER transport this material to the cis face of the Golgi complex. Some proteins are phosphorylated here. The intermediate part between the cis face and the trans face is the medial Golgi. Material synthesised in rough ER travels through the Fig.

    Chapter 2 Cell Structure Finally. Exocytic Vesicles Just as material from outside the cell can be brought into the cytoplasm by phagocytosis or pinocytosis. Secretory Granules The cytoplasm of secretory cells frequently contains what are called secretory granules. Such membrane bound vesicles.

    Pinocytotic Vesicles Some fluid may also be taken into the cytoplasm by a process similar to phagocytosis. Such vesicles are called exocytic vesicles. In the case of fluids. Other Storage Vesicles Materials such as lipids. In this process. Details of the appearances of various types of vesicles will not be considered here.

    With the EM. These vesicles are derived from the Golgi complex. As proteins pass through successive sacs of Golgi they undergo a process of purification. Some vesicles serve to store material. Membrane bound Vesicles The cytoplasm of a cell may contain several types of vesicles. The contents of any such vesicle are separated from the rest of the cytoplasm by a membrane which forms the wall of the vesicle. The membranes of the Golgi complex contain appropriate enzymes for the functions performed by them.

    Others transport material into or out of the cell. Vesicles are formed by budding off from existing areas of membrane. This part of the cell membrane then separates from the rest of the plasma membrane and forms a free floating vesicle within the cytoplasm. These can be seen with the light microscope. Vesicles also allow exchange of membrane between different parts of the cell.

    The appearance. The stages in the formation of a lysosome are as follows: These endosomes possess the membrane proteins necessary for producing an acid medium. As many as 40 different lysosomal enzymes have been identified. Lysosomes belong to what has been described as the acid vesicle system. The enzymes in these vesicles are inactive because of the lack of an acid medium These are called primary lysosomes or Golgi hydrolase vesicles Flow chart 2.

    The enzymes present in lysosomes include amongst others proteases. Such material may have been taken into the cell from outside e. Schematic representation to show how lysosomes. The Flow chart Types of lysosomes 24 Chapter The enzymes are involved in oxidation of very long chain fatty acids. Examples of such disorders are lysosomal glycogen storage disease. As a result some molecules cannot be degraded. Lysosomal enzymes play an important role in the destruction of bacteria phagocytosed by the cell.

    This activates the enzymes and a mature lysosome is formed. A lysosome. Clinical Correlation Genetic defects can lead to absence of specific acid hydrolases that are normally present in lysosomes.

    Lysosomes are present in all cells except mature erythrocytes. Peroxisomes These are similar to lysosomes in that they are membrane bound vesicles containing enzymes. The enzymes in most of them react with other substances to form hydrogen peroxide.

    The structures formed by such fusion often appear to have numerous small vesicles within them and are. Some of it is thrown out of the cell by exocytosis. Chapter 2 Cell Structure product formed by fusion of the two vesicles is an endolysosome or secondary lysosome Flow chart 2. They are a prominent feature in neutrophil leucocytes. Peroxisomes are most prominent in cells of the liver and in cells of renal tubules. These bodies consisting of fused phagosomes and lysosomes are referred to as phagolysosomes Fig.

    Some peroxisomes contain the enzyme catalase which converts the toxic hydrogen peroxide to water. In a similar manner lysosomes may also fuse with pinocytotic vesicles. Lysosomal enzymes may also be discharged out of the cell and may influence adjoining structures.

    Hydrogen peroxide resulting from the reactions is bactericidal. The filaments can separate under the influence of actin severing proteins. They prevent tubules of ER from collapsing. Such transport is specially important in transport along axons. The roles played by microtubules are as follows: The cell cortex helps to maintain the shape of the cell.

    Apart from maintaining cellular architecture the cytoskeleton facilitates cell motility e. Microfilaments These are about 5 nm in diameter. The wall of a microtubule is made up of thirteen protofilaments that run longitudinally Fig. The elements that constitute the cytoskeleton consist of the following: Filaments also extend into other protrusions from the cell surface. This network is called the cytoskeleton.

    The tubu- lin protofilaments are stabilised by microtubule associated proteins MAPs. The meshwork of the cell cortex is labile. Schematic representation to show how a Microtubules are formed in microtubule is constituted centrioles see below which constitute a microtubule organising centre.

    They are made up of the protein actin. Some proteins dynein. These join together polymerise to form long chains called F-actin. The basic constituent of microtubules is the protein tubulin composed of subunits a and b. Individual molecules of actin are globular G-actin.

    Chains of tubu- lin form protofilaments. That is how the shape of a cell is altered. Microtubules Microtubules are about 25 nm in diameter Fig. Actin filaments form a meshwork just subjacent to the cell membrane.

    It also facilitates transport of some constituents through the cytosol. Microvilli contain bundles of actin filaments. This meshwork is called the cell cortex The filaments forming the meshwork are held together by a protein called filamin. Textbook of Human Histology the Cytoskeleton The cytoplasm is permeated by a number of fibrillar elements that collectively form a supporting network. Laminin in the nuclear lamina of cells.

    Neurofibrils help to maintain the cylindrical shape of axons. They do so as they are attached to transmembrane proteins at desmosomes. The filaments also facilitate cell attachment to extracellular elements at hemidesmosomes. The proteins constituting these filaments vary in different types of cells.

    The roles played by intermediate filaments are as follows: They include cytokeratin in epithelial cells. They also form the main constituent of hair and nails. In some cells the nuclei are relatively large 28 Chapter The intracellular deposition of masses of filaments may cause disruption of cytoskeleton architecture and subsequent neuronal cell death leading to dementia memory loss.

    Note nine groups various cellular structures that are made up of microtubules. Defect in the proper assembly of intermediate filaments leads to various diseases. When we examine a transverse section across a centriole by EM it is seen to consist essentially of a series of microtubules arranged in a circle. All cells in the body contain nucleus. There are nine groups of tubules. It is characterised by defect in the organisation of microtubules that can immobilise the cilia of respiratory epithelium resulting in the inability of the respiratory system to clear accumulated secretions.

    Chromatin In usual class-room slides stained with haematoxylin and eosin. It is of interest to note that cilia. Transverse section across a Centrioles play an important role in the formation of centriole near its base. The changes in the neurofilaments within the brain lead to Alzheimer's disease. The disease is characterised by accumulation of tangles inside the neurons. Centrioles All cells capable of division and even some which do not divide contain a pair of structures called centrioles.

    With the light microscope. It is usually rounded or ellipsoid. Occasionally it may be elongated.

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