Introduction to Histology

Introduction to HistologyBYDr.P.Priya1CellHistology is the study of cells, tissues , and organsHuman body is made up of smallest units called cellsTissue is defined as the group of cells having similar functionAn organ is defined as the structure that is formed by 2 or more primary types of tissues, which execute the functions of the organEach cell is formed by a cell body and a membrane covering the cell body called the cell membrane

2CELL MEMBRANEThe cell membrane is a protective sheath,enveloping the cell bodyThe cell membrane is called a 3 layered membrane.central layer is a lipid layer,outer 2 layers are protein layersThe cell membrane is composed of 3 types of substances:1.Proteins 55%2.lipids 40%3.Carbohydrates5%The membrane is a fluid with mosaic of proteins that can readily reform when its continuity is disturbed

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Cell MembraneThis membrane separates the fluid outside the cell called extracellular fluid and the fluids inside the cell called intracellular fluidThe lipid layer of cell membrane forms a semipermeable membraneThere is free exchange of substances between the ECF and ICFCarbohydrate molecules are negatively charged and do not permit the negatively charged substances to move in and out of the cell5CytoplasamThe cytoplasam of the cell is the jelly like material formed by 80 percent of waterIt contains clear liquid portion and various particles are proteins ,carbohydrates,lipids or electrolytes in natureThe cytoplasam also contains many organelles with distinct structure and functionThe cytoplasmic organelles are the cellular structures embedded in the cytoplasam CELL JUNCTIONThe cell junction is the connection between the neighbouring cells or the contact between the cell and extracellular matrixThere are 3 main classes of cell junctions1.Occluding junctions which prevent intercellular exchange of substances 2.Communicating junctions which permit the intercellular exchange of substances3.Anchoring junctions which provide strength to the cells by acting like mechanical attachment7Occluding JunctionsTight JunctionTight junction is the intercellular occluding junction that prvents the passage of large molecules.It is also called Zonula occludens.The proteins involved in the formation of tight junctions are classified into 2 types.Integral membrane proteins,peripheral membrane proteinsFunctions1.Strength and stability to the tissues2.Blood brain barrier:Tight junction in the brain capillaries forms the blood brain barrierCommunicating Junctions

GAP JUNCTIONThe membranes of the 2 adjacent cells lie very close to each other and the intercellular space is rduced from the usual size of 2.5 to 3 nmThe cytoplasam of the 2 cells is connected by the channels formed by the membranes of both cells.The channel is surrounded by 6 subunits of proteins which are called connexonsFunctionsThe diameter of the channel in the gap junction is about 1 to 3 nm .So,the channel permits the passage of glucose,aminoacids,ions and other substances

ANCHORING JUNCTIONSInvolves cell adhesion moleculeThe firm attachment between 2 cells or between a cell and extracellular matrix is provided either by actin filaments or intermediate filamentsAnchoring junction is classified into 4 types1.Actin filament attachment(a)Adherens junction(b) Focal adhesion2.Intermediate filament attachment(a)Desmosome(b)Hemidesmosome11(a)Zonula adherensContinuous large area of attachment at apical part of epithelial cells.Gap between the cell surfaces is about 20nm .In adherens junction ,th membranes of the adjacent cells are held together by transmembrane proteins.The adherence junction present in the epidermis helps the skin to withstand the mechanical stress(b)Focal adhesionFocal adhesion is the cell to matrix junction ,which connects the actin filament of the cell to the extracellular matrix(c) Macula adherens(Desmosomes)Strong anchorage between cellsIt is a cell to cell junction ,where the intermediate filaments connect the adjacent cellsThey are plaque like strong contact which may be located anywhere on cell surface.Intercellular gap 25nm.filled with electron dense filamentous material

12(d)HemidesmosomesSingle sided desmosome which occur between basal layer of cells and basement membrane common at the dermis-epidermis JunctionDysfunction of adherens junction and focal junction in colon results in colon cancerDysfunction of desmosome causes bulbous pemphigoid(autoimmune disease with tense blistering eruptions of the skinDysfunction of hemidesmosome also causes bulbous pemphigoid

13CytoskeletonThe cytoskeleton is a complex network of structures present throughout the cytoplasam.The microtubules are formed by bundles of globular protein called tubulinThe intermediate filaments are made up of fibrous proteins.Microfilaments are made up of non tubular contractile proteins called actin and myosin

Cell organellesCytoplasam of typical cell contains various organelles which are visualized by the electron microscopeEndoplasmic ReticulumA system of membranes that form boundaries of flattened cisternae or tubules constituting endoplasmic reticulum.They form irregular network of brannching and anastomosing lamellar systems of parallel flat cavities17Rough endoplasmic reticulum-Endoplasmic reticulum are studded with ribosomes and are sites of protein synthesisSmooth endoplasmic reticulum-The membrane bound cisternae devoid of ribosomes.Here further processing of proteins,lipids and also in those which secrete steroidsRibosomesPresent in relation to the cytoplasmic surface of rough endoplasmic reticulumRNA-component of ribosomes.Ribosomes play an important role in protein synthesis18Golgi apparatusStacks of flattened membrane bound sacksClosely associate with vesicleAppear as a clear space surrounded partly by the well stained ER .To convert the inactive proteins to active formsFormation of condensing vacuoles that contain secretory product of the cell19LysosomesThese dense bodies contain acid hydrolase destroy unwanted material40 different lysosomal enzymes are identifiedMitochondriaThey are present in all cells except RBCS and terminal keratinocytesMitochondrion comprises 2 membranes separated by intermembranous spaceInner to inner membrane there is a granular matrix containing numerous enzymes,RNA,DNA.Site of ATP production 20CentriolesCentrioles embedded in a specialized zone of cytoplasam called centrosomeEach centriole consist of circularly arranged nine groups of microtubulesPlay an important role in cell divisionInclusion bodiesThe various membrane bound vesicles that serve to store materials are also included as cell organelles21NucleusThe nucleus takes up basophilic stainNuclei shows dark staining bodies called nucleoliBulk of nucleus formed by the chromatinChromatin forms irregular dark masses called heterochromatinLoose network of chromatin staining lightly is referred to as the euchromatinChromosomes are the rod like condensation of chromatin formed during the prophase of mitosis22NucleoliReddish tinge &rounded bodiesHigh content of RNASecondary constrictions of certain chromosomesRibosomal RNA synthesised in the nucleoli23Cell surface ProjectionsCiliaThe minute hair like projections from free surface of the cellCilium formed by extension of cell membrane with a core of microtubules arranged in a specific manner.These motile cilia move and cause movement of secretions or other substances lying on the surface in a particular direction.Trachea,bronchi move the secretions towards pharynx.Uterine tube-movement of ova.24FlagellaSimilar to cilia,larger exhibit movement different than ciliaMovement start at the base and then the next nearest segment bends in opposite direction to create a wave like motion that moves down flagellum. Ex:tail of spermatozoon25MicrovilliFinger like projections from cell surface.Core of cytoplasam contains numerous microfilaments.Microvilli help in absorption.Microvilli arranged parallel to one another give a straited appearance to small intestine.Irregularly arranged microvilli-brush border in gall bladder.Modified microvilli in epididymis,Internal ear 2627