Tissues and Epithelium PDF

TISSUE 1. CELLS 2. INTERCELLULAR SUBSTANCE 3. FLUID 1. CELLS Living component of a tissue. Vary in number, shape and arrangement depending on the type of tissue. 2. INTERCELLULAR SUBSTANCE Major component of connective tissue; minimal amount in musc...

TISSUE 1. CELLS 2. INTERCELLULAR SUBSTANCE 3. FLUID 1. CELLS Living component of a tissue. Vary in number, shape and arrangement depending on the type of tissue. 2. INTERCELLULAR SUBSTANCE Major component of connective tissue; minimal amount in muscle and nervous tissues; insignificant amount in epithelial tissue. Non-living material produced by cells, such as fibroblasts (connective tissue cells), chondroblasts (cartilage cells) and osteoblasts (bone cells). Fills interstices of tissues (between cells or group of cells). Varies in amount depending on the type of tissue. Two types of intercellular substance: o AMORPHOUS SUBSTANCE o Literally means “without form” and difficult to observe with routine paraffin technique o Viscous, gel-like; binds; binds large quantities of water. o Forms the ground substance or matrix of tissues; occupies areas not occupied by cells and formed intercellular substances. o Chemically composed of glycoproteins (mucopolysaccharides such as the sulfated or acidic mucins and non-sulfated or neutral mucins) and/or proteoglycans and hyaluronic acid). o Functions of amorphous substance: barrier to bacteria and shock absorber. o FORMED SUBSTANCES o Fibrous in nature; chemically composed of neutral mucosubstances (glycoproteins). o Classified into three types: collagen fibers, elastic fibers and reticular fibers. o COLLAGEN FIBER  The predominant fiber in most proper connective tissue  Likened to a “biological rope” because of its immense strength.  Provides high tensile strength and prevent over-distension.  Embryonic connective tissue: individual collagen fibers are fine filamentous materials stained glassy pink with H&E.  Adult connective tissue like tendon: individual fibers are wide glassy bands with smooth edges. 21   In stretch preparation of areolar connective tissue, the fibers appear as wavy thick bands of indefinite length and random arrangement  Composed of collagen fibrils; collagen fibril is in turn composed of overlapping tropocollagen molecules laid together in triplets like a braid of hair.  Overlapping collagen confers a “banded” pattern on collagen fibril in longitudinal section when viewed with the TEM.  Collagen is the most common protein in animlas.  Types of collagen:  TYPE I – banded; most abundant; present in skin, bone and tendon  TYPE II – UNBANDED; present in cartilage  TYPE III – make up reticular fibers  TYPE IV – unbanded; major component of basement membrane o ELASTIC FIBERS  Form strands or sheets  Difficult to observe in H&E sections.  Provide “springiness” and stretchability to the tissue.  In dense, regular connective tissue like ligamentum nuchae; individual fibers are long, narrow, straight and branched bands with rough edges. Under routine bright-field microscopy  In stretch preparation of loose collagen fibers appear red, along with (areolar) connective tissue, thin dark elastic fibers and cell nuclei. individual fibers are thinner than collagen fibers and may have a corkscrew appearance.  Each fiber consists of a core of protein elastic (amorphous substance) surrounded by microfibrils. o RETICULAR FIBERS  Chemically, made up of TYPE II collagen, a very fine collagen fiber.  Not visible with routine H&E but stain well with silver periodic acid-Schiff (PAS).  Silver stained individual fibers are short, black, wiry, branched structures that interconnect with each other to form the characteristic netlike arrangement  Invest the outside of small blood vessels and form part of the basal lamina of epithelium.  The principal role is to form delicate scaffolding to anchor blood vessels and nerve fibers to the more robust surrounding connective tissue (CT). 22  3. FLUID Liquid component of a tissue. Occurs in between cells where it is known as extracellular fluid (ECF) Occurs in blood where it is called plasma. Other examples of tissue fluids: lymph, cerebrospinal fluid and synovial fluid. FOUR BASIC TYPES OF TISSUE 1. EPITHELIAL TISSUE –covers external surfaces of the body, lines internal surfaces of the body and forms exocrine glands. 2. CONNECTIVE TISSUE – connects and supports other tissues and organs of the body. 3. MUSCLE TISSUE – forms the muscular wall of tubular organs and the flesh of the body 4. NERVOUS TISSUE – makes up the organs of the nervous tissue. EPITHELIAL TISSUE Epithelial tissue or epithelium – a tissue consisting of closely aggregated cells apposed at most of their surfaces, having very little intercellular space and resting on a basement membrane Epitheloid – tissue similar to epithelium but lacks basal lamina. CHARACTERISTICS OF EPITHELIUM 1. Chief component is cells, which are generally similar in structure and function. Cells are closely joined together laterally and/or basally by junctional complexes. 2. Presents a free surface that faces a canal, a cavity or a lumen, and may present special modifications such as microvilli or cilia that enhance the function of the epithelium Microvilli increase the surface area for absorption or secretion. Cilia move secretions and other substances across the epithelium 3. Rests on a basement membrane that attaches it to the underlying connective tissue. 4. Polarity: epithelial cells have apex and base where different activities take place. 5. Avascular (no blood supply); nourished by diffusion of nutrients from capillaries in the underlying connective tissue. 6. Proliferation and high turn-over rate: epithelial cells are short-lived, typically days to weeks. Dead or damaged epithelial cells are regularly replaced by mitosis of younger epithelial cells. 7. Embryonic origin: mostly from ectoderm (skin epidermis and derivatives, like hair, gland, etc) and endoderm (lining epithelium of digestive tube); few are from mesoderm (lining epithelium of urinary organ (ureter) and reproductive tracts (vas deferens, fallopian tube, and uterus). FUNCTIONS OF EPITHELIUM 1. SECRETION – production of useful substances 2. EXCRETION - Disposal of nitrogenous waste by sweating and urine formation 3. INTERCHANGE WITH THE ENVIRONMENT – gas exchange; filtration; observation, etc. 23  4. PROTECTION – barrier to ultraviolet light, physical barrier to infection, provides camouflage. 5. STIMULUS RECEPTION – chemotactic sensation, olfactory and gustatory reception. TYPES OF EPTHELIUM A.SURFACE EPITHELIUM B.GLANDULAR EPITHELIUM C.SPECIAL EPITHELIUM SURFACE EPITHELIUM Covers the outer surfaces of the body and organs; lines lumen of tubular organs Classified according to two criteria: NUMBER OF CELL LAYERS AND SHAPE OF SUPERFICIAL CELLS. i. Number of cell layers: SIMPLE EPITHELIUM – one layer of cell that rests on the basal membrane. STRATIFIED EPITHELIUM – two or more cell layers: Basal layer rests on the basement membrane. One to several middle layer/s of polygonal cells. One to several layer/s of polygonal cells. One to several layer/s of superficial cells. ii. Shape of superficial cells (when viewed along their cut edges) Three basic shapes of cells: 1. COLUMNAR CELLS – tall prismatic cells with oval, basally located nucleus. 2. CUBOIDAL CELLS – cells with equal sides and with round to oval, centrally-located nucleus. 3. SQUAMOUS CELLS – thin flat cells with centrally located round to oval nucleus. All epithelial cells when viewed from the top are polygonal in shape. Squamous cells have a “fried egg” appearance. TYPES OF SIMPLE EPITHELIUM According to the shape of cells when viewed along the cut edges: simple squamous, simple cuboidal, simple columnar and pseudostratified columnar. 1. SIMPLE SQUAMOUS EPITHELIUM One layer of extremely thin flat cells with n almost visible cytoplasm and a clearly visible round to oval nucleus Highly suited to line structures with dialyzing or glomerular capsule, blood vessels (as endothelium) and body cavities (as mesothelium), where its “slick” surface facilitates flow of fluids. 24  Because of its delicate structure, the epithelium cannot withstand wear and tear e = SIMPLE SQUAMOUS EPITHELIUM Endothelium and mesothelium are nearly always simple squamous 2. SIMPLE CUBOIDAL EPITHELIUM One layer of cuboidal (square) cells Functions: o Mainly as a lining membrane, like in small ducts of exocrine glands, and collecting tubules of the kidney o May have a secretory function, like the endocrine cells of the thyroid follicles. 3. SIMPLE COLUMNAR EPITHELIUM (SCE) One layer of columnar cells Classified into 4 subtypes, namely: o SCE with typical columnar cells One layer of typical columnar cells Lines medium-sized ducts of exocrine glands o SCE with mucus secreting cells One layer of fan-shaped mucus-secreting columnar cells with wide vacuolated apex and narrow base containing oval nucleus 25   MUCUS-SECRETING COLUMNAR CELLS o Vacuolated apical cytoplasm in H&E section due to dissolution of mucin secretion by alcohol during dehydration. o Fan-shaped because the apical regions of adjacent cells expand uniformly with the accumulation of mucin granules exerting equal pressure against each other. Line the glandular stomach and the cervix of uterus o SCE with secretory and absorptive cells One layer of secretory (goblet) and absorptive (typical columnar) cells  GOBLET CELL  Like a wine goblet with an expanded theca (apical region) that contains mucin granules and a narrow stem (basal region) that contains a dense nucleus.  Interspersed between typical columnar cells n = cell nuclie, l = round nuclei, g = goblet cell  Shape results from expansion of the apical region, pushing against adjacent absorptive columnar cells.  ABSORPTIVE COLUMNAR CELLS  Lines the small and large intestines o SCE with secretory and ciliated cells One layer of secretory and ciliated columnr cells.  Secretory columnar cell presents apical bleb (bead-like cytoplasmic projection) in the apical border.  Ciliated columnar cell presents kinocilia (hair-like projections) in the apical border.  Lines the fallopian tube. 3. PSEUDOSTRATIFIED COLUMNAR EPITHELIUM One layer of tall and short cells Tall cells – typical columnar cells; extend from the basement membrane to the luminal surface o Nuclei are located at a higher level than the nuclei of short cells o Generally ciliated Short cells – pyramidal; rest on the basement membrane but do not extend to the luminal surface 26  o Nuclei are basally located Stereociliated in the epididymis PSEUDOSTRATIFIED COLUMNAR CILIATED WITH GOBLET CELLS Kinociliated columnar cells and goblet cells. Lines the respiratory tract. c= cilia, g = goblet cells, n blue = nuclie, n yellow = nerve, b = blood vessels TYPES OF STRATIFIED EPITHELIUM According to the shape of the cells in the superficial layer: stratified squamous epithelium, stratified cuboidal epithelium, stratified columnar epithelium and transitional epithelium. STRATIFIED SQUAMOUS EPITHELIUM  Two (bistratified) or more cell layers.  The multi-layered epithelium consists of the following:  BASAL LAYER – a layer of cuboidal to columnar cells that rest on a way or undulating basement membrane  MIDDLE LAYER/S – one to several layers of polyhedral or polygonal cells  SUPERFICIAL LAYER – one to several layers of squamous cells that lie in parallel to the luminal or external surface.  Two subtypes – keratinized or cornified and non-keratinized or noncornified. 27 o STRATIFIED SQUAMOUS KERATINIZED OR CORNIFIED o Superficial layer is covered by keratin (dead anucleated squamous cells filled with keratin granules). o Keratinization is an epithelial adaptation to wear and tear on abraded surfaces. o Keratinization normally occurs only in stratified squamous epithelium, NOT in other cell types. o STRATIFIED SQUAMOUS NONKERATINIZED OR NONCORNIFIED. o Superficial layer is not covered by keratin or cornified material. o Lines in the inner surfaces of the body that are exposed to considerable wear and tear such as the vagina, conj unctiva, and esophagus of man and carnivore. STRATIFIED CUBOIDAL EPITHELIUM o Two or more cell layers. o Two layers: basal cuboidal and superficial columnar epithelium o Three or more layers: basal cuboidal, middle polygonal, and superficial columnar. o Not commonly found in the body. o Lines the large excretory ducts of exocrine glands. m = mucous cells d = serous demilunes e = stratified cuboidal epithelium s = stroma TRANSITIONAL EPITHELIUM  Transitional epithelium – basal layer of cuboidal or columnar cells, 2 to 4 middle layers of polyhedral cells and 2 to 3 superficial layers of balloon- shaped cells with convex free surfaces.  Stretched epithelium – very similar to the stratified squamous nonkeratinized epithelium. 28  2. Lines urinary passages where great mechanical changes may occur due to contraction and distension 3. Water impermeable due to large numbers of zonula occludens between cells.  Impermeability protects the urinary tract from damage caused by the presence of hypertonic urine (in mammals and birds), and also prevents dilution of the urine by osmosis SURFACE SPECIALIZATION OF SURFACE EPITHELIUM Surface specializations refer to structural modifications along the lateral, apical, and basal surfaces of the epithelium. 1. LATERAL SURFACE SPECIALIZATION – glycocalyx, junctional complexes, lateral interdigitations and gap junctions 2. BASAL SURFACE SPECIALIZATIONS – basal lamina or basement membrane, caveolae and hemidesmosome 3. APICAL SURFACE SPECIALIZATIONS – microvilli and cilia 1. LATERAL SURFACE SPECIALIZATION a. Glycocalyx – b. Junctional complexes o Structures that hold the lateral surfaces of epithelial cells together. o There are three kinds: zonula occludens or tight junction, zonula adhaerens or intermediate junction and macula adherens or desmosome  Zonula occludens (ZO) or tight junction o The most superficial; forms a belt that completely encircles cells near their apical borders. o TEM: ZO is a single dark line formed by the fusion of two adjacent cell membranes. o Provides adhesion between adjacent cells. o Controls intercellular passage of molecules (ion flux) and osmotic pressure  Zonula adherens (ZA) or intermediate junction o Located just below the zonula occludens; also encircles individual cells completely o TEM : adjacent cell membranes are slightly separated by a very narrow space, about 200 angstrom units filled with homogenous material.  Macula adhaerens (MA) or Desmosome o Most common junctional complex 29  o Little bodies that are distributed in a spotty way (macula=spot) around individual cells. o TEM: MA is composed of several dark and light parallel lines representing the layers of cell membranes of adjacent cells. o The innermost lines are associated with tonofibrils (bundled fine filaments) that extend into the cytoplasm and form part of the cytoskeleton that supports the cell. o Anchors cells to each other; gives the epithelium coherence and strength c. Lateral interdigitations o Cytoplasmic folds on the lateral surfaces of adjacent cells that interdigitate with each other d. Gap junction o Not for adhesion but for communication between adjacent cells. o 20 A in size. o Site of lowered resistance to passage ions. 2. BASAL SURFACE SPECIALIZATIONS a. BASEMENT MEMBRANE (BM) o Non-living amorphous substance produced by epithelial cells o Proteoglycans particularly heparin sulfate, laminin, fibronectin and Type IV collagen o LM: BM not identifiable in H&E sections because it is thin;  An unusually thick BM, as in the trachea, has a dense layer of collagen fibrils belonging to the underlying connective tissue. o TEM : BM consists of an electron dense basal lamina (lamina densa or lamina basalis) made up of fine filaments; and a low density lamina lucida composed of fine filaments and microfibrils that join the basal lamina to the cell membrane of epithelial cell. o Although the term basement membrane was originally used with light microscopy , and the term basal lamina referred only to the electron dense layer seen with the electron microscope, the two terms are now used interchangeably b. BASAL FOLDS (INFOLDINGS) OR CAVEOLA – o Bulb-like invaginations of the basal cell membrane o Reverse microvilli o May contain mitochondrion o Associated with active transport of finished products at slower rate 30  c. HEMIDESMOSOME - o A half desmosome facing the basement membrane. 3. APICAL SURFACE SPECIALIZATIONS a. Microvilli o Long, finger-like cytoplasmic projections from the apical or luminal border of epithelial cells, giving the epithelium a striated or brush border o Much smaller (10µm long and 1.0µm thick) than cilia (10- 100µm long and at least 10µm thick) and lack microtubule. o Roots originate from the terminal web (fine filaments in the apical cytoplasm) of the cell. o Found mainly in absorptive cells of the intestinal and renal epithelium where they increase the surface area for absorption without increasing the cell size o Microvilli are supported by actin filaments. b. Cilia o Are hair-like processes projecting from the apical border of the epithelial cell. o Two types of cilia: kinocilia and stereocilia  Kinocilia o Motile cilia that help in the movement of materials along a sheet of epithelium  Stereocilia o Structurally similar to microvilli except that the sterocilia are long and branched. o Like the microvilli, they increase the surface area of the epithelium but unlike the microvilli whose main function is absorption, the stereocilia’s function is secretion. 31 GLANDULAR EPITHELIA OR EPITHELIAL GLANDS Derived from surface epithelium; highly specialized to perform secretory functions. Classified according to morphology, nature of secretory products CLASSIFICATION OF GLANDS BASED ON MORPHOLOGY OR STRUCTURE A. According to the presence of or absence of ducts i. Endocrine Gland  No ducts; secretions 9hormones) are transported by blood and lymph to the site of utilization  Secretory cells are arranged as cords or clumps that abut blood capillaries ii. Exocrine gland  Secretions are released into an external or internal surface  Secretory cells are arranged into a secretory unit with a lumen and enclosed by a basement membrane B. According to the number of secretory cells in a secretory unit (for exocrine glands) i. Unicellular gland – is a one-celled gland. Example is the goblet cell in the intestinal gland which is actually a single cell but is also considered a unicellular exocrine gland interspersed between nonsecretory epithelial cells ii. Multicellular gland – is composed of two (2) or more secretory cells that may form a cluster in the surface epithelium called intraepithelial gland or invaginate into the underlying connective tissue where they organize into secretory units enclosed by a basement membrane C. According to the shape of secretory units (for exocrine glands only) i. Tubular gland – secretory unit is like a hollow cylinder or a test tube. May be straight like the intestinal gland or coiled like the sweat gland. ii. Alveolar or acinar gland – secretory unit is pear-shaped or like a tear-drop as in the sebaceous gland 32  iii. Tubuloalveolar or tubuloacinar gland – secretory units are tubular and alveolar like those in the mammary gland and salivary gland D. According to the degree of division of ducts (for exocrine glands only) i. Simple gland – consists of branched and unbranched secretory units that all empty into a single undivided duct. The term simple refers to the undivided duct; “branched or unbranched” refers to the divided or undivided secretory units. There are six (6) types of simple gland:  Simple straight tubular gland Has a tubular secretory unit at the end of a main secretory duct. Not common in mammals A variant of this gland is the intestinal gland, which has the duct and secretory unit rolled into one.  Simple coiled tubular gland The secretory unit is tubular and coiled and empties into a straight duct. i.e sweat gland  Simple branched tubular gland Has two or more tubular secretory units that converge into a single duct Secretory units may be coiled as in the fundic gland of stomach  Simple branched tubuloalveolar or tubuloacinar gland Has two or more tubular secretory units that converge into a single duct. Secretory units may be coiled as in the fundic glad of stomach.  Simple alveolar or acinar gland Has a pear-shape secretory unit that connects with a single duct. This type is rare. Ex. Some sebaceous glands.  Simple branched alveolar or acinar gland Has two or more alveolar secretory units that converge into a single duct. Ex. Large sebaceous glands. Secretory unit at the end of a single duct. Ex. minor salivary gland like lingual gland. 33  ii. Compound gland – duct divides at least once and may form a duct system; secretory unit may or may not be branched. The term “compound” is used only for ductwork, not for secretory units.  The system consists of a main excretory duct that divides and subdivides; the smallest terminal duct connects with the secretory unit.  Divisions of the Main Excretory duct: Excretory duct=>interlobar duct=>intralobar duct=> interlobular duct=>intralobular(striated duct)=>intercalated duct. Excretory duct - main duct that opens to the surface epithelium of a tubular organ; usually lined by bistratified columnar epithelium. Interlobar duct - short branch that extends from the excretory duct to the lobe of the ezocrine gland; same lining epithelium as the excretory duct. Intralobar duct - short branch of the interlobar that enters the interlobular CT septa of the gland; same lining epithelium as the excretory duct. Interlobular duct - short branch of the intralobar duct that courses through the interlobular CT septa; lined by bistratified columnar or cuboidal epithelium. Intralobular or striated duct - small branch of the interlobular duct that penetrates the gland lobule and intermingles with the secretory units; l ined by simple columnar or simple cuboidal epithelium. Intercalated duct - very small, terminal branch of the intralobular duct(even smaller than the secretory units)that connects with a secretory unit; lined by simple cuboidal epithelium. There are (5) types of compound gland: a. Compound unbranched tubular gland – the terminal duct connects with a single tubular secretory unit. Ex. Some salivary glands. b. Compound branched tubular gland – presents two or more tubular secretory units at the end of each terminal duct. c. Compound unbranched alveolar gland – presents a single alveolar secretory units at the end of each terminal duct. Ex. Exocrine pancreas d. Compound branched alveolar gland – presents two or more alveolar secretory units at the end of each terminal duct. Ex. Mammary gland. e. Compound branched tubuloalveolar gland – presents tubular, alveolar and/or tubuloalveolar secretory units at the end of each terminal duct. Ex. Mammary gland. 34 Classification of Glands Based on the Nature of Secretory Products a. Serous gland Secretory a clear watery substance. Secretory unit presents a small lumen and triangular serous secretory cells with dense acidophilic cytoplasm and round, centrally located nuclei. Ex. Pancreatic acinus. b. Mucous gland Secretes a viscous substance. Secretory unit presents a large lumen and triangular mucous secretory cells with light cytoplasm and flattened or round nuclei generally located at the extreme base of the cells Ex. Sebaceous gland. c. Mixed gland Secretes serous and mucous substances. Presents serous, mucous and/or mixed secretory units. Mixed secretory unit-mucous cells partly covered by half- moon shaped multinucleated serous cells called serous demilunes. Ex. Sublingual gland. Classification of Glands Based on the Manner of Elaboration of Secretory Products a. Eccrine (merocrine)gland Secretory cells release their secretions without loss of any components. Most exocrine and ALL endocrine glands are eccrine. 35  b. Apocrine gland Secretory cells release their secretions by pinching off of their protruding apical regions (apical blebs). c. Holocrine gland Whole secretory cells die and are discharged together with their secretions. Extruded cells are replaced by mitosis of stem cells at the base of the secretory unit.Ex. Sebaceous gland. d. Cytocrine gland Secretory cells transfer their secretory products to other cells.Ex. melanocytes C. Special Epithelia Epithelial tissues modified to perform highly specialized functions. There are five(5) types of special epithelia: ciliated epithelium, neuroepithelium, pigmented epithelium, myoepithelium and other special epithelia. 1. Ciliated epithelium L ining epithelium with ciliated(kinocilia or stereocilia) cells. kinociliated epithelium lines respiratory tract and fallopian tube. Stereociliated epithelium lines the epididymis. 2. Neuroepithelium Cells are modified to receive and transmit sensory information. Associated with special sensory organs. Examples: olfactory cells of the olfactory organs, gustatory cells of the taste duds, hair cells of the organ of corti, and the rods and cones of the retina. 3. Pigmented epithelium Composed of pigmented epithelial cells. Examples: lining epithelium of ciliary process and iris. 4. Myoepithelium Composed of cells with contractile myofibrils in the cytoplasm. Associated with the secretory units of most exocrine glands where they are commonly known as myoepithelial basket cells. 5. Other Special Epithelia a. Endothelium – simple squamous epithelium modified for transport of materials between the vascular and connective tissue spaces. It lines the walls of blood vessels, heart and lymph vessels. b. Mesothelium – simple squamous epithelium that lines body cavities (e.g. peritoneal, pericardial and pleural cavities) and covers the visceral organs. c. Reticuloendothhelium – also known as the reticuloendothelial(RE) system. Example is the reticuloepithelium of thymus that produces T lymphocytes. d. Endocrine epithelium – consists of hormone-secreting cells in the gastrointestinal tract and endocrine glands. Examples are the enteroendocrine cells in the stomach and intestine and the alpha and beta cells in the pancreatic islets and pituitary gland. 36 Regeneration and Repair of Surface and Glandular Epithelia A. Surface Epithelium Subjected to a certain amount of wear and tear because of its location. Epithelial cells die and are replaced regularly. In simple epithelium, regeneration occurs by mitosis of the remaining cells. In stratified epithelium regeneration is by mitosis of cells at the basal layer of the epithelium. As the daughter cells mature, they move to the superficial layer, eventually die and sloughed off to be replaced by succeeding generations of cells. B.Glandular Epithelium Regenerate by mitosis of the remaining cells in the secretory units. In the intestinal gland secretory cells are replaced by mitosis of undifferentiated cells at the base of the gland. As daughter cells differentiate, they migrate to the surface lining ephithelium where they eventually die, slough off and replaced. 37