Histology of Cells and Tissues PDF

# Cells: Structure and Function ## Cell Membrane - **Light microscope:** - Cannot be seen but can be inferred due to dyes' deposition. - Special stains: PAS or A97 (carbohydrate proteins) - **Electron microscope:** - Trilaminar membrane: two electron-dense bands separated by an electro...

# Cells: Structure and Function ## Cell Membrane - **Light microscope:** - Cannot be seen but can be inferred due to dyes' deposition. - Special stains: PAS or A97 (carbohydrate proteins) - **Electron microscope:** - Trilaminar membrane: two electron-dense bands separated by an electron-lucent one. - 8-10 nm thickness. - This appearance is due to osmium tetroxide deposition on hydrophilic heads of the lipid bilayer of the membrane. - Allows movement of other components of the membrane (fluid mosaic model). ### Molecular Structure of the Cell Membrane - **Lipid bilayer:** - 30% of cell membrane - Represented by phospholipid and cholesterol - One polar hydrophilic head and two non-polar hydrophobic tails. - Acts as a barrier. - Composed of: - Phospholipid - Cholesterol - **Proteins:** - 60% of cell membrane - Two types: integral and peripheral proteins - **Integral Proteins:** - Strongly embedded within the bilayer and difficult to be removed. - Completely span the bilayer - Form channel proteins and carrier proteins - Facilitate passage across cell membrane - Formation of many receptors - **Peripheral Proteins:** - Loosely attached to the cell membrane and can be extracted with changing of pH or mixing with salt solution. - Subunits known as "protiens," membrane receptors, membrane monotropic, bitopic, polytropic proteins. - Membrane proteins act as: - Recognition - Adhesion - Mechanical protection - Function - Maintenance & cellular integrity - Passage of materials - Cell junction & interaction with other cells ### Membrane Transport - **Simple Diffusion:** - Concentration from high to low - No energy needed - Small uncharged molecules or lipid molecules - O2, CO2, and most lipids - Osmosis: concentration from high to low, no energy needed, diffusion of water. - **Facilitated Diffusion:** - Concentration from high to low - Needs carrier - No energy needed - Sugars & amino acids - **Selective permeability:** allows entry of specific molecules for receptors. - **Active transport:** - Concentration from low to high - Energy needed - Through membrane proteins - Na+/K+ pump ## Mitochondria - **Light microscope:** - Granules, rods, or filaments. - Cannot be seen with H&E - Stains black with iron hematoxylin. - Stains green with Janus - Histochemical methods detect special enzymes within mitochondria (cytochrome oxidase). - **Electron microscope:** - Round or oval vesicle covered with a double membrane separated by intermembrane space. - Outer membrane is smooth - Freely permeable to small molecules - Contains special channels capable of transporting large molecules - Porins allow passage of small molecules - Involved in complex interactions regulating organelle and cellular metabolism - Inner membrane: less permeable as it allows only very small molecules - Form folds (cristae) - Sites of ATP production - Carry ATP synthase enzyme - Variable in size, shape, and number - Have a tubular appearance in steroid-secreting cells ### Function - Cell respiration (power house) - Contain oxidative enzymes - Produce energy (ATP) through the Krebs’ cycle - 50% stored as ATP - 50% dissipated as heat to keep normal body temperature. - Form their own proteins and can divide, as they contain DNA & RNA - Enzymes necessary to make protein ## Endoplasmic Reticulum ### Rough ER - **Light microscope:** - Not visible - Localized basophilic area with H&E - Prominent in pancreatic acinar cells, plasma cells & nerve cells (Nissl’s granules). - **Electron microscope:** - System of interconnecting lamellal (cisternae). - Covered with ribosomes - Continuous with SER - Connected to the outer membrane of the nuclear envelope #### Function - Protein synthesis, storage, transfer vesicle to Golgi, formation of lysosomes - Protein modification: glycosylation, phosphorylation - Prominent feature in protein secretory cells ### Smooth ER - **Light microscope:** - Not visible - Localized acidophilic area - **Electron microscope:** - System of anastomosing tubules & vesicles - Not covered with ribosomes - Lumen of SER is assumed to continue with that of RER #### Function - Synthesis of lipids (intracellular lipid, phospholipids, and cholesterol derivatives) - Prominent in all cells - Making steroids or lipids - Bile formation and detoxification of drugs & alcohol in hepatocytes - Lipid re-synthesis in absorptive cells - Synthesis & storage of glycogen in hepatocytes & muscles - Regulation of Ca++ in muscle cells - Regulation of HCl formation in gastric cells ## Golgi Apparatus - **Light Microscope:** - Not stained by H&E, but protein synthesizing cells contain 40 different proteins. - Seen as a negative Golgi image. - Silver stain and osmium tetroxide: appears as a dark fibrillar or granular network - **Electron microscope:** - It consists of saccules, or cisternae (3-10) called Golgi stacks - Saccules or cisternae are flattened & curved - Each stack consist of: - cis face (formation) (immature) - Convex, lies nearest to ER, related to small transport vesicles - trans face (secretory) (mature) - Concave, related to large secretory vesicle ### Function - Condensation, storing & packing of proteins transferred from RER - Protein modification - Formation of secretory vesicle - Package hydrolytic enzymes into lysosomes - Package secretory products - Turnover & maintenance of cell membrane & surface coat. ## Lysosomes - **Light microscope:** - Not visible by H&E. - They are visible by histochemical reaction for the enzyme catalase. - **Electron microscope:** - Single-membrane enclosed fine granular content. ### Function - Digestion of intracellular materials - Get off expired organelles, protecting the cell from inflammation. - Convert inactive prohormones to active hormones. - Needed for penetration of the ovum by sperm. - Play a role in bone resorption. ## Peroxisomes - **Light microscope:** - Not visible by H&E - They are visible by histochemical reaction for the enzyme catalase. - **Electron microscope:** - Single-membrane enclosed fine granular content. ### Function - Contain enzymes that use oxygen to detoxify a number of harmful substances. - They contain enzymes that assist in removal of free radicals. - They are involved in the oxidative processes. - Involved in lipid metabolism & beta oxidation of fatty acids - Involved in formation of bile acid & cholesterol derivatives. - They are involved in the metabolism of various toxic molecules & drugs in liver and kidney. ## Ribosomes - **Light microscope:** - Not seen, but in large amount give basophilia of the cytoplasm. - Basophilia may be localized: - Diffuse - Spotty - **Electron microscope:** - Small electron-dense particles formed of two subunits (large and small) between them is mRNA. ### Structure - Composed of large and small subunits. - Each subunit consists of protein parts synthesized within the cytoplasm. - DNA (ribonucleic acid) is synthesized within the nucleolus. ### Function - Protein synthesis ## Centrioles - **Light microscope:** - Stained with iron hematoxylin, they appear as paired small dark bodies in a pale area (centrosome). - **Electron microscope:** - Two short, hollow cylinders perpendicular to each other. - Each one is composed of a triplets of microtubules ### Structure - Non-membranous organelle - Formed of microtubules - Responsible for cell division - Absent in non-dividing cells - Present near to the nucleus (centrasome) ### Function - It is the microtubule organizing center (MTOC) - Consists of 2 centrioles perpendicular to each other - Has a thick filament (20 nm thickness) - Formed of myosin present in muscle cells. - Function: - Maintenance of cell shape - Anchorage of the nucleus and other organelles. - Connect organelles into a network. - Abundant in liver muscle. ## Cytoskeleton - **Light microscope:** - Not visible by H&E. - They are visible by histochemical reaction for the enzyme catalase. ### Microtubules - **Electron microscope:** - Very thin (5-7 nm diameter) - Formed of actin - Actin filaments are formed of G-actin. #### Function - Muscle contraction: formation of microvilli during cell division. - Adherent junctions between cells. - Maintenance of cell shape. - Anchorage of the nucleus & other organelles. - All inclusions. - Cell migration. #### Microtubules - **Electron microscope:** - Most rigid (20-25 nm). - Formed of tubulin protein. - Fixed length. - Fine tubules with a tense wall and a clear internal space. - The wall is made of 13 protofilaments. - Seen only by Immunofluorescence method. #### Function - Formation of mitotic spindle. - All-shape stability. - Mechanical support. - Formation of cilia, flagella, and centrioles. ### Intermediate Filaments - **Electron microscope:** - 8-10 nm diameter - Formed of different types of proteins. - Function: - Support the cell - Maintain all shape. - **Light microscope:** - Not visible by H&E. - They are visible by histochemical reaction for the enzyme Catalase. #### Function - Muscle contraction - Ensure equal distribution of the tensile forces throughout the cells. ### Microfilaments - **Electron microscope:** - Very thin (5-7 nm diameter) - Formed of actin - Function: - Muscle contraction - Formation of microvilli - Adherent junctions: between cells. ## Endogenous Pigments - **Light microscope :** - Not visible by H&E. - They are visible by histochemical reaction for the enzyme catalase. - **Electron microscope:** - They are seen only by immunofluorescence method. ### Exogenous Pigments - Taken into body from outside - Examples: - **Carotene** - **Carbon** - **Dust** - **Tatto** - **Hemosiderin** - Synthesized inside body - Examples: - Hemoglobin - Hemositrin - Myoglobin - Bilirubin - Lipofuscin - **Crystals** - Found in testes - Not visible by H&E. - They are visible by histochemical reaction for the enzyme catalase. - **Sertoli Cells:** - Interstitial cells - Not visible by H&E. - They are visible by histochemical reaction for the enzyme catalase. ## General Notes about Cells: - **H&E:** Hematoxylin and eosin. A common stain used to visualize tissues under a light microscope. Hematoxylin stains the nucleus blue, while eosin stains the cytoplasm pink. - **PAS:** Periodic acid-Schiff stain. It is a staining method that specifically stains carbohydrates, including glycogen and mucopolysaccharides. It is commonly used to visualize the basement membrane, mucus-secreting cells, and fungal cells. - **Osmium Tetroxide:** A heavy metal that is used as a fixative and stain in electron microscopy. It is particularly useful for staining lipids and cell membranes. - **Ribosomes are the sites of protein synthesis**, which is the process of creating proteins from amino acids. - **Golgi apparatus processes and packages proteins**, adding carbohydrates and lipids to them and then sends them to their specific destinations within the cell or out of the cell. - **Lysosomes are the "garbage disposals" of the cell**, breaking down and digesting old or damaged cell components (such as organelles), as well as materials brought in from outside the cell. They contain powerful digestive enzymes that can breakdown materials that are not normally found in the cell. - **Peroxisomes are small, membrane-bound organelles that contain mostly enzymes**, including catalase. They are involved in the degradation of fatty acids. - **The cytoskeleton is a network of protein fibers** that gives the cell its shape and structural support. - **Centrioles are involved in the formation of microtubules**, which are tiny, hollow tubes made of protein that form the structural support of the cell. - **Endogenous pigments are pigments that are made by the cell itself**, such as melanin. - **Exogenous pigments are pigments that are taken into the cell from the outside of the body**, such as carbon dust from pollution. - **Cells are the basic unit of life,** and all living organisms are made up of cells. - **There are many different types of cells**, each with its own specialized function.