Epithelia Histology Chapter 4 PDF

H:: Epithelia Basic Tissue Types in the Human Body The human body's organs are composed of four basic tissue types: epithelial, connective, muscular, and nervous tissues. Each tissue type is a grouping of specialized cells performing specific functions, containing extra...

H:: Epithelia Basic Tissue Types in the Human Body The human body's organs are composed of four basic tissue types: epithelial, connective, muscular, and nervous tissues. Each tissue type is a grouping of specialized cells performing specific functions, containing extracellular matrix (ECM) as well as cells. These tissues associate in variable proportions and morphologies characteristic of each organ. Characteristics of Connective, Muscle, and Nervous Tissues Connective tissue is characterized by cells producing an abundant ECM. Muscle tissue consists of elongated cells specialized for contraction and movement. Nervous tissue comprises cells with long, fine processes specialized for receiving, generating, and transmitting nerve impulses. Parenchyma and Stroma in Organs Organs can be divided into parenchyma and stroma. Parenchyma consists of cells responsible for an organ's specialized functions. Stroma, mainly composed of connective tissue except in the brain and spinal cord, it supports the parenchyma. Composition and Function of Epithelial Tissues Epithelial tissues are composed of closely grouped polyhedral cells adhering strongly to one another and a thin layer of ECM. They form cellular sheets that line the cavities of organs and cover the body surface, lining all external and internal surfaces of the body. Epithelia are essential as all substances entering or leaving an organ must cross this tissue type. Principal Functions of Epithelial Tissues Epithelial tissues have principal functions including: o Covering, lining, and protecting surfaces (e.g., epidermis of skin). o Absorption (e.g., intestinal lining). o Secretion (e.g., parenchymal cells of glands). Certain epithelial cells may be contractile (myoepithelial cells) or specialized as sensory cells (e.g., in taste buds or olfactory epithelium).  Characteristic Features of Epithelial Cells Epithelial cells vary in shape and dimensions, ranging from tall columnar, to cuboidal, to low squamous cells. The size and morphology of epithelial cells are generally dictated by their function. The shape of epithelial cell nuclei corresponds to the cell shape: o Columnar cells have elongated nuclei. o Squamous cells have flattened nuclei. Do o Cuboidal or pyramidal cells have more spherical nuclei. Microscopic Identification of Epithelial Cells Due to lipid-rich membranes often being indistinguishable by light microscopy, the number and shape of stained nuclei are key indicators of cell shape and density. The nuclei help determine the number of cell layers in an epithelium, a crucial morphologic criterion for classifying epithelia. Epithelial Tissue Adjacency to Connective Tissue Most epithelia are adjacent to connective tissue containing blood vessels, providing nutrients and oxygen to epithelial cells. Even thick epithelia typically do not contain blood vessels themselves. The connective tissue under epithelia in the digestive, respiratory, and urinary systems is known as the lamina propria. Increased contact between epithelial and connective tissues is facilitated by papillae, small evaginations from connective tissue into the epithelium. Papillae are often found in epithelial tissues subject to friction, such as skin or tongue. Polarity and Structural Aspects of Epithelial Cells Epithelial cells show polarity, with organelles and membrane proteinsdistributed unevenly. The basal pole contacts the ECM and connective tissue, while the apical pole usually faces a space. The poles differ in structure and function. Cuboidal or columnar cells have lateral surfaces adjoining neighboring DO cells, often with folds in their membranes to increase surface area and functional capacity. Basement Membranes in Epithelial Tissues The basal surface of epithelia rests on a basement membrane, a thin Lmt extracellular sheet of macromolecules. The basement membrane acts as a semipermeable filter for substances reaching epithelial cells from below. It contains glycoproteins and other components, often visible with a light microscope. Basal Lamina and Reticular Lamina in Basement Membranes Under a transmission electron microscope (TEM), the basement membrane shows two distinct parts: o Basal Lamina: A thin, electron-dense, sheet-like layer of fine fibrils closest to the epithelial cells. o Reticular Lamina: A more diffuse and fibrous layer beneath the basal lamina. The terms "basement membrane" and "basal lamina" are sometimes used interchangeably. o "Basal Lamina" typically refers to the fine extracellular layer seen ultrastructurally. o "Basement Membrane" denotes the entire structure beneath epithelial cells visible with a light microscope. Components of the Basal Lamina The basal lamina's macromolecules are secreted from the basal sides of epithelial cells, forming a sheet-like array. Its components, important in extracellular matrix (ECM) structure, include: o Type IV Collagen: Forms a two-dimensional network of evenly spaced subunits, similar to a window screen's mesh. o Laminin: Large glycoproteins that attach to integrin proteins in the basal cell membrane, projecting through the type IV collagen mesh. o Nidogen and Perlecan: Nidogen is a rod-like protein and Perlecan is a proteoglycan. § Both cross-link laminins to the type IV collagen network, contributing to the basal lamina’s structure. § They bind the epithelium to the basal lamina, and determin its porosity and molecular filtration capacity. Basal Laminae and External Laminae Basal laminae, also known as external laminae, surround muscle cells, nerves, and fat-storing cells as thin sleeves. They act as semipermeable barriers regulating macromolecular W exchange between these cells and connective tissue. Reticular Lamina and Collagen Types The reticular lamina contains type 3 collagen and is connected to the basal lamina by anchoring fibrils of type 7 collagen. The reticular lamina components are produced by cells of the connective tissue while the basal lamina components are produced by epithelial cells. Functions of Basement Membranes Basement membranes act as filters, provide structural support for epithelial cells, and attach epithelia to underlying connective tissue. Basal lamina components help organize integrins and other proteins in epithelial cell membranes, maintaining cell polarity, and localizing endocytosis, signal transduction, and other activities. Basement membrane proteins mediate cell-to-cell interactions, guide routes for cell migrations along epithelia, and serve as a scaffold for epithelial repair and regeneration. Intercellular Adhesion and Junctions in Epithelial Cells Epithelial cells adhere strongly to neighboring cells and basal laminae, especially in areas subject to friction or mechanical forces. Lateral surfaces of epithelial cells feature complexes of specialized intercellular junctions with distinct functions. Types of Intercellular Junctions The types of intercellular junctions include: o Tight or Occluding Junctions: Create a seal between adjacent cells, preventing the passage of molecules. o Adherent or Anchoring Junctions: Serve as sites of strong cell adhesion, linking cells together. o Gap Junctions: Act as channels for communication between adjacent cells, allowing the transfer of ions and small molecules. Order and Structure of Junctions in Epithelia In many epithelia, junctions are arranged in a definite order at the apical end of the cells. Tight junctions (also called zonulae occludens) are the most apical, forming a band encircling each cell, indicated by the term "zonula". Under a transmission electron microscope (TEM), the membranes at tight junctions appear fused or very tightly apposed. Tight junctions are formed by interactions between transmembrane proteins: claudin and occludin. After cryofracture of epithelia, tight junctions appear as a band of branching strands in the membrane around each cell’s apical end.  Function of Tight Junctions in Epithelial Transport Tight junctions ensure that molecules crossing an epithelium do so via a transcellular path (through cells) rather than a paracellular pathway(between cells). morp.me The permeability of an epithelium to water and solutes varies depending on the number of fused sealing strands in its tight junctions. o Epithelia with few strands, like the proximal renal tubule, are more permeable. o Epithelia with many strands, such as the urinary bladder lining, are less permeable. Function of Epithelial Tight Junctions as Membrane Fences Ereable Epithelial tight junctions act as fences within cell membranes, restricting the movement of lipids and proteins between the apical, lateral, and basalsurfaces. They maintain two distinct membrane domains (apical and basolateral), each with different components. This separation allows the two sides of the epithelium to display different receptors and proteins, and to function differently. The apical cell membranes are part of the luminal compartment, while the basolateral domains are part of a basal compartment* that includes the underlying connective tissue. Adherens Junctions (Zonula Adherens) The adherens junction or zonula adherens usually encircles the epithelial cell just below the tight junction. It is an adherent junction, strongly anchoring a cell to its neighbors. Cell adhesion in adherens junctions is mediated by cadherins, transmembrane glycoproteins that bind to each other in the presence of Ca²⁺. Cadherins and Catenins in Adherens Junctions In adherens junctions, cadherins, at their cytoplasmic ends, bind catenins that link to actin filaments via actin-binding proteins. The actin filaments linked to adherens junctions form the terminal web, a cytoskeletal feature at the apical pole in many epithelial cells. Tight and adherent junctions function collectively like plastic bands around a six-pack, encircling the apical ends of epithelial cells. Desmosomes (Macula Adherens) Desmosomes, or macula adherens, are disc- shaped anchoring junctions that resemble spot-welds rather than belts around the cell. Desmosomes consist of desmogleins and desmocollins, larger members of the cadherin family, which form connections with matching structures on adjacent cells. The cytoplasmic ends of these cadherin transmembrane proteins bind to plakoglobins (catenin-like proteins), which link to desmoplakins in an electron-dense plaque. Desmoplakins bind to intermediate filament proteins (e.g., cytokeratin or tonofilaments) rather than actins, providing strong cellular adhesionand strength in the epithelium. Gap Junctions and Intercellular Communication Gap junctions mediate intercellular communication, rather than adhesion or occlusion, between cells. They are abundant in many epithelia and functionally important in nearly all mammalian tissues. Cryofracture preparations reveal that gap junctions consist of aggregated n n transmembrane protein complexes forming circular patches in the plasma membrane. Structure and Function of Connexins in Gap Junctions The transmembrane proteins in gap junctions, called connexins, form hexameric complexes known as connexons. Each connexon has a central hydrophilic pore about 1.5 nm in diameter. When two cells attach, connexins in adjacent cell membranes align to produce connexons between the cells, with each junction containing dozens or hundreds of aligned connexon pairs. Gap junctions allow for the exchange of molecules with small (