Tissue Histology PDF

Introduction to Tissue Histology Mohammed Elsalanty, MBBS, PhD College of Osteopathic Medicine of the Pacific Western University of Health Sciences  Objectives 1. Contrast the definition of a tissue versus organ 2. Explain the histologic...

Introduction to Tissue Histology Mohammed Elsalanty, MBBS, PhD College of Osteopathic Medicine of the Pacific Western University of Health Sciences  Objectives 1. Contrast the definition of a tissue versus organ 2. Explain the histological characteristics of epithelial tissues 3. List the components of junctional complexes 4. Explain the function of gap junctions 5. Contrast the structural components of the basement membrane. 6. Contrast the different types of epithelium 7. Contrast the structure and function of microvilli versus cilia 8. Explain the histological characteristics of connective tissues 9. Contrast the major fiber types and major ground substance components in connective tissue matrix 10. Contrast the following types of connective tissue: loose, dense regular, dense irregular, and reticular, giving examples of each. 11. Contrast the structure and function of white versus brown adipose tissue  Cells, Tissues, and Organs Cells Tissues Tissues Make Organs PART I: Epithelial Tissue  Epithelium Surface cover Origin: Ectoderm or endoderm Mostly identical cells attach to each other to form one or more layers Basement membrane (or basal lamina) separating it from underlying connective tissue (lamina propria)  Epithelium Epithelial cells have polarity (direction): 1. Apical (free) surface, 2. Lateral surfaces (In between neighboring cells) 3. Basal surface (attached to the basal lamina)  Avascular: nourishment by diffusion through basal lamina Highly regenerative and continuously renewing Classified based on the number of layers (simple and stratified) & the cell shape (squamous, cuboidal, & columnar)  Functional Adaptation Cells strongly attach to as well as communicate with each other Basal layer attaches to the basement membrane and includes the stem cells JUNCTIONAL COMPLEXES Typically found near the cell apex 3 components: – Zonula Occludens or tight junctions – Zonula Adherens or anchoring junctions – Macula Adherens or desmosomes Gap junctions (small-molecule channels between neighboring epithelial cells) Formed by pairs of connexons, each composed of six protein subunits (connexins) that span the lipid bilayer of each cell membrane. Allows the free flow of electrolytes between attached cells Figure 4-7 Copyright © McGraw-Hill Companies  Basement Membrane Boundary between epithelium and underlying lamina propria. Consist of 3 layers: – Electron-lucent lamina lucida on the epidermal side (laminin) – Electron-dense lamina densa (Collagen IV) in the middle – Fibroreticular lamina (lamina reticularis; Collagen III, IV, and VII) with abundant fibronectin on the dermal side  laminin Basal lamina (BL) Reticular lamina (RL). Hemidesmosomes (H) Integrins bind the basal surface of the epithelial cell (C) to the basal lamina. X54,000. Figure 4-3 Copyright © McGraw-Hill Companies  Apical Surface The surface facing the environment (whether the internal, such as a cavity, or external) Epithelial cells may possess apical specializations such as – Microvilli – Cilia  Microvilli Finger-like projections (100 nm in diameter x 100 - 2,000 nm in length) Increase surface area for absorption (e.g. kidney tubules & small intestine) Glycocalyx (sticky sugar coat) traps enzymes and other substances Core: F-actin, myosin I, fimbrin, and formin Terminal Web: F-actin & myosin II  Cilia Actively motile processes (5-10 μm in length) à Push stuff up and down pipes Each grows from a basal body during ciliogenesis Produce energy-dependent movement  Core: longitudinally arranged microtubules bound together with various proteins (axoneme). The axoneme: Parallel arrangement of one central and nine peripheral microtubular doublets (pairs). Nexins, dynein, and other proteins link the peripheral doublets Continuous with a basal body inside apical cytoplasm: microtubular triplets linked together in a pinwheel-like arrangement. When activated by ATP, the dynein arms bend à microtubules slide past each other, restricted by nexin cross-links à axoneme bending. Figure 4-11 Copyright © McGraw-Hill Companies Types of Epithelium Simple (Single Layer) Stratified (Multiple Layers) Pseudostratified (Single layer that appear multiple) Transitional (Changes the number of layers)  Simple Squamous Epithelium Single layer of flattened cells, very little cytoplasm and organelles Typically, cells maintain tight adhesion to each other and to the basement membrane Molecule transport possible by simple diffusion across the cells Examples: Alveolar lining (lung), endothelium (lining of all blood vessels and heart) & inner corneal epithelium  Simple Cuboidal Epithelium Single layer of cubical cells with central nuclei More cytoplasm and organelles than SSE Examples: tubules in kidney, duct (drain) of lacrimal gland  Simple Columnar Epithelium Single layer of tall cells (height 2-3X width) Often have cilia or microvilli on their apical surface Example: lining of most of the gastrointestinal tract Functions include secretion, absorption, and protection  Stratified Epithelium Multiple layers Dividing (stem) cells in the basal layer Cells advance in differentiation as they are pushed towards the surface by newly formed cells. Surface cells eventually slough off Impermeable to water Highly protective (friction, elements, microorganisms, etc.) Can be keratinized or non-keratinized Examples: Oral mucosa, skin, conjunctiva, outer cornea Epithelium covering the lens: The lens capsule is a thick basement membrane secreted by subcapsular epithelium. Epithelium of the cornea: 1. External nonkeratinized stratified squamous epithelium (E): 5-6 cell-thick, densely supplied with sensory-free nerve endings à blinking reflex 2. Innermost layer of simple squamous endothelium (EN). Conjunctiva: Non-keratinized stratified epithelium Some mucous-producing (Goblet) cells Pseudostratified Columnar Epithelium Columnar cells that appear to stack in multiple layers because of the crowding, but all rest on a basal lamina (single layer) Often ciliated May have Goblet cells (mucous- secretion) Examples: trachea (respiratory epithelium) Transitional Epithelium Line a rapidly expandable cavity The number of layers decrease as the cavity becomes more distended. EMPTY Cell membrane remodels to line up the cells next to each other instead of being on top of each other Example: Urinary bladder FULL Glandular Epithelium Consists of a secretory & ductal components that is separated from CT by a basal lamina Simple columnar epithelium Example: Lacrimal Gland  PART II: Connective Tissue  Connective Tissue Network of supportive cells that produce extracellular matrix à Fibers + Ground Substance (hydrated gel of glycosaminoglycans - GAGs). Common characteristics: Origin: mesoderm Supportive cells produce extracellular matrix in between them Matrix is composed of fibrillar proteins + ground substance. Cells adhere to extracellular matrix, rather than to each other. Cellular and structural diversity and high mobility Fibroblasts Spindle-shaped Large active nuclei and eosinophilic cytoplasm Cellular processes resemble the collagen bundles  Extracellular matrix has two major components: Ground substance: Gel-like Glycosaminoglycans (GAGs) Fibrillar proteins: for anchorage and mechanical integrity: 1. Collagen 2. Fibrillin 3. Elastin 4. Fibronectin. Type I collagen: 1. Most abundant collagen 2. Molecules packed into fibrils à fibers à bundled linked together by other collagen types. 3. Major component the sclera  Silver Stain or PAS X100 Type III collagen (reticular fibers): Form microscopic tissue chambers/compartments (e.g. in endocrine glands, bone marrow, and lymphoid organs) Copyright © McGraw-Hill Companies Elastic fibers or lamellae (sheets): Elastin core, surrounded by GAGs (heparan sulphate), and fibrillin. Location: Wherever stretching is needed: Skin, lung, blood vessels Copyright © McGraw-Hill Companies Collagen Subtypes  Fibronectin Most common glycoprotein of the ECM Essential for cell-to-ECM interactions Binds to cell membrane-spanning receptor proteins called integrins, as well as other extracellular matrix components (e.g. collagen, fibrin, and heparan sulfate) Regulates the deposition and orientation of collagen in the matrix Ground Substance: Glycosaminoglycans (GAGs): Large polysaccharides linked (by linker proteins) to core proteins to form proteoglycans. The large molecule is negatively charged and traps water and à hydrated gel Gives turgor and regulate the trafficking of molecules through matrix. Connective Tissue Proper Loose (areolar) CT Dense Regular CT Dense Irregular CT  Loose Connective Tissue Cellular Resident cells: components - fibroblasts - synthesize extracellular matrix - macrophages - Innate immune response - mast cells - adaptive immunity and wound healing; involved in allergic reactions - adipocytes - synthesize and store fat - mesenchymal stem cells - Back up cells Transient cells: - All other immune cells Extracellular Ground substance - Glycoproteins (fibronectin) matrix and proteoglycans Fibers: Col III, (some IV and VII) and elastic fibers (Elastin + Fibrillin) Example Lamina propria underneath epithelium Dense Regular CT: Densely packed, parallel, collagen fibers (Col I), lots of fibroblasts, & very little matrix Location: Sclera, Corneal Stroma (mixture of several collagen types), tendons, and ligaments (stress in only one direction). The sclera is a dense layer of connective tissue rich in collagen type I, arranged in parallel bundles.  Corneal stroma: – 90% of corneal thickness – ~ 200 layers of crisscrossing collagen type I fibers, each is about 1.5 to 2.5 microns, as well as flat fibroblasts – Light scatter by each fiber is cancelled out by destructive interference of another à transparency Dense Irregular CT: Collagen bundles (mainly type I, some III) packed but irregularly arranged and interwoven (sheets rather than cables) Location: Reticular layer of the dermis (skin)  Tenon’s Capsule (fascia bulbi) A dense membrane which envelops the eyeball from the optic nerve to the limbus, separating it from the orbital fat and forming a socket in which it plays Extraocular muscles pass through openings in the Tenon’s capsule to insert into the sclera White adipose tissue Space filling (e.g. subcutaneous, axilla, pelvis, and around viscera) Triglyceride storage and mobilization, thermal insulator, padding, hormonal function Lobules, each has lipid-filled cells and is surrounded by fibrous tissue Visceral fat (~10% of white fat) is located around viscera (internal organs) More linked to metabolic disorders, cancer, heart disease, stroke, and Alzheimer’s than subcutaneous fat. Brown adipose tissue Only small amounts in human adults, mainly around the adrenal glands. Involved in the regulation of body temperature and body weight. Highly metabolically active. Cells are packed with mitochondria, in between lipid-storing cells.  Bone Matrix: Type I collagen (structure), plus some proteoglycans and glycoproteins (regulation) Cells that make Type I collagen fibers then mineralize them with hydroxyapatite crystals [Ca10(PO4)6(OH)2]. Mineralization is responsible for bone hardness, while organic matrix provides flexibility (resilience).  Bone Cells Osteoblasts (Bone formation) Osteoclasts (Bone resorption) Osteocytes (Bone control centers?) References: Anthony L. Mescher: Junqueira’s Basic Histology, Text and Atlas, 14 Ed., 2012, McGraw Hill, Inc. Stevens and Lowe: Human Histology, 3rd Ed., 2005, Elsevier Mosby, Inc. Various online resources Copyright © McGraw-Hill Companies