Histology Epithelial Tissue PDF

HISTOLOGY EPITHELIAL TISSUE  1. Classification and function of Epithelial tissues 2. Classification and structure of CONTENTS lining epithelial tissues 3. Topography,characterization and function of single-layered and stratified e...

HISTOLOGY EPITHELIAL TISSUE  1. Classification and function of Epithelial tissues 2. Classification and structure of CONTENTS lining epithelial tissues 3. Topography,characterization and function of single-layered and stratified epithelia FUNCTIONS OF EPITHELIAL TISSUE CHARACTERISTICS OF EPITHELIAL CELLS  POLARITY OF EPITHELIAL CELLS ◦ 1. Apical domain ◦ Project towards the external surface or the lumen of an organ which epithelia cover, ◦ May also show apical membrane specializations which alter the shape of this surface ◦ 2. lateral domain ◦ They are the sites where adjacent cells interconnect, forming tightly packed contiguous cells ◦ 3. Basal domain ◦ Basal surfaces are in contact with the underlying basement membrane.  CHARACTERISTICS OF EPITHELIAL CELLS ◦ 1. SHAPE LAMINA PROPRIA BASEMENT MEMBRANE  CONSTITUENTS OF BASEMENT MEMBRANE ◦ Basement membrane has 2 ◦ Parts: ◦ 1. basal lamina (thin) ◦ 2. fibrous reticular lamina ◦ CONSTITUENTS: ◦ 1. Type IV collagen ◦ 2. laminin ◦ 3. Nidogen & Perlecan  FUNCTIONS OF BASAL LAMINA ◦ 1.Tissue scaffolding – serves as a guide during regeneration. Newly formed cells or growing processes of a cell use the basal lamina that remains after cell loss, thus helping to maintain the original tissue architecture; 2. Regulation and signaling – many molecules that reside in basal lamina interact with cell surface receptors, influencing epithelial cell behavior.  ◦ 1.Impermeable (occluding) – tight junctions: Zonula Lateral cell surface occludens specializations – ◦ 2. Adhering (anchoring) junctions- Zonula adherens and Macula adherens (desmosome) Intercellular junctions ◦ 3. Communicating junction: Nexus (gap junction)  Tight Junction/ zonulae occludens ◦ 1. They are the most apical of the junctions. ◦ They are multiprotein junctional complexes ◦ Function - prevent leakage of transported solutes and water and seals the paracellular pathway (transfer of substances across an epithelium by passing through the intercellular space between the cells) ◦ The seal between the membranes is due to interactions between the transmembrane protein claudin on each cell. ◦ Zonulae occludens in sheets of epithelial cells help form two functional compartments: ◦ 1. an apical compartment that is composed of an organ cavity (such as the lumen of a secretory unit or the gut) and ◦ 2. a basal compartment that begins at the junctions and surrounds the underlying tissue.  TIGHT JUNCTIONS ◦ -A tight junction restricts the movement of fluids between adjacent cells due to the presence of integral proteins ( claudin and occludin), that fuse together to form a firm seal. ◦ -The borders of two cells are fused together, often around the whole perimeter of each cell, forming a continuous belt like junction known as a tight junction or zonula occludens (zonula = latin for belt).  ADHERENT JUNCTION/ ZONULA ADHERENS Adherens junction/ zonula adherens, which also encircles the epithelial cell, and lies below the tight junction. Function- firmly anchors the cell to its neighbours, hence it is also called an adherens junction by connecting the cytoskeletons of adjacent cells.  ADHERENT JUNCTION/ ZONULA ADHERENS The cadherins ( cell membrane glycoprotein) from adjacent cells interact to 'zipper' up the two cells together in the presence of Ca2+. Inside the cell, Cadherins bind the protein catenin which is linked by means of actin- binding proteins to actin filaments, they produce electron-dense plaques of material on the cytoplasmic surfaces of adherent junctions  DESMOSOME/ MACULA ADHERENS The desmosome is a disk-shaped structure at the surface of one cell that is matched with an identical structure at the surface of an adjacent cell. They are members of the cadherin family. On the cytoplasmic side of each cell membrane these cadherin-type proteins insert into a dense attachment plaque of anchoring proteins (plakophilin, plakoglobin, and desmoplakin) which bind intermediate filaments rather than actin filaments. They are specialized for cell-to-cell adhesion They are localized spot-like adhesions randomly arranged on the lateral sides of plasma membranes. They form links between cells, and provide a connection between intermediate filaments of the cell cytoskeletons of adjacent cells. This structure gives strength to tissues.  HEMIDESMOSOMES They are found at the basal domain of an epithelilal cell and connect to the underlying basal lamina. 2. These adhesive structures resemble a half- desmosome and bind the cell to the basal lamina. 3. Hemidesmosomes the plaques contain abundant integrins, transmembrane proteins that are receptor sites for the extracellular macromolecules laminin and collagen type IV.  GAP JUNCTIONS ◦ Gap junctions passageways between adjacent cells that allow the transfer of small molecules between adjacent cells. The communicating channels are formed by pairs of abutting particles (connexons), which are in turn each composed of six protein subunits (connexins) that span the lipid bilayer of each cell membrane. The channel formed by paired connexons is about 1.5 nm in diameter, limiting the size of transmitted molecules.  SPECIALIZATIONS OF APICAL CELL SURFACE MICROVILLI, STEREOCILIA, CILIA MICROVILLI Microvilli - These are small finger like projections. The microvilli are shorter and narrower than cilia. Microvilli are present on the luminal surface of many epithelia. Form a structure called the brush border that is found on the apical surface of some epithelial cells, such as the small intestines.  MICROVILLI They have a core of actin filaments that are cross- linked by several actin- bundling proteins, such as fascin , espin , and fimbrin. Their barbed (plus) ends are anchored to villin. The actin bundle extends down into the apical cytoplasm. & interacts with a horizontal network of actin filaments, the terminal web. The core of actin filaments is associated with myosin I Presence of myosin II and tropomyosin in the -terminal web give it contractile ability.  CLINICAL RELEVANCE ◦ 1. Celiac Disease/ Gluten Sensitive Enteropathy ◦ Loss of microvilli brush border of absorptive cells. ◦ Immune reaction against gluten. ◦ PATHOLIGCIAL  STEREOCILIA -Stereocilia - a type of microvilli, but these are immotile and longer than microvilli. LOCATION: epididymis, the proximal part of the ductus deferens of the male reproductive system, and the sensory (hair) cells of the inner ear. FUNCTION-Absorption, mechanic sensory reception. Stereocilia of the sensory epithelium of the ear- uniform in diameter and organized into ridged bundles of increasing heights, forming characteristic staircase patterns  CILIA.Are long, highly motile , hairlike extensions of apical plasma membrane, larger than microvilli. They are made up of microtubules. Each cilium contains nine pairs of microtubules forming the outside of a ring, and two central microtubules(‘9+2’)  CILIA Cilia are classified into three basic categories: 1. Motile cilia- found on the apical domain of many epithelial cells, typical 9+2 axonemal organization , Protein- DYNEIN is responsible for generating force required for movement 2. Primary cilia (monocilia) - solitary projections found on almost all eukaryotic cells.It is non motile and functions as sensory organ 3. Nodal cilia -found in the embryo on the bilaminar embryonic disc at the time of gastrulation.They play an important role in early embryonic development.  CLINICALS ◦ PRIMARY CILIARY DYSKINESIA/IMMOTILE CILIA SYNDROME (PCD) ◦ 1. Kartagener’s syndrome -absence of dynein arms ◦ 2. Young’s syndrome- malfunction of radial spokes & dynein arms  TYPES OF EPITHELIA ◦ 1. COVERING/LINING EPITHELIA ◦ 2. GLANDULAR EPITHELIA COVERING OR LINING EPITHELIA SIMPLE SQUAMOUS EPITHELIA  SIMPLE CUBOIDAL EPITHELIUM  SIMPLE COLUMNAR EPITHELIUM  STRATIFIED EPITHELIUM TRANSITIONAL EPITHELIUM PSEUDOSTRATIFIED EPITHELIUM REFERENCES 2015 Junqueira's Basic Histology Text and Atlas, 15th Edition, pages 71-84