Cell Structure and Membranous Structures (PDF)

6 2. Freezing technique Tissues are frozen rapidly in liquid nitrogen, and then cut in a refrigerated cabinet (a cryostat) with a cold knife, t...

6 2. Freezing technique Tissues are frozen rapidly in liquid nitrogen, and then cut in a refrigerated cabinet (a cryostat) with a cold knife, then stained and observed in the microscope. This procedure is faster (takes only minutes) and can preserve tissue details that may be lost by the paraffin technique. Sections are 5 - 10 µm thick. Used for intra-operative consultation (needed in tumor surgery) as it allows rapid decision and report. The cell It is the structural and functional unit of all living tissues. Size: In human there is wide variation cell size. Shape: Varies according to the function of cells as in RBCs, WBCs, and nerve cell  7 Structure of the cell 1. Nucleus: The genetic control center of a eukaryotic cell 2. Cytoplasm: consists of cytosol, organelles, and inclusions. Cytosol: It is a jelly-like substance composed mainly of water and found between the cell membrane and nucleus. The cytoplasm makes up most of the "body" of a cell and is constantly streaming. Organelles: 1. Membranous organelles: Cell membrane, Mitochondria, Endoplasmic Reticulum, Golgi apparatus, Lysosome, Peroxisome and Vesicle. 2. Non membranous organelles: Ribosome, microtubules, and filament. Inclusions: Non-essential for vitality of the cells. Structure of the Cell Cytoplasm Nucleus Organelles Inclusions (Essential-for cell vitality) (Not essential-for cell vitality) A. Stored food Membranous Non-Membranous B. Pigments C. Crystals 1. Cell membrane 1. Ribosomes 2. Mitochondria 2. Microtubules 3. Endoplasmic reticulum 3. Filaments 4. Golgi apparatus 5. Lysosomes 6. Peroxisomes 7. Coated vesicles 1. Cell membrane  8 Definition: A living membrane forming the outermost cover of the cytoplasm (plasma membrane). It surrounds the membranous organelles (internal membranes) Structure: LM: Cannot be seen because it is very thin (8-10 nm) EM: the average cell membrane is seen to be about 7.5 nm thick. It consists of two densely stained layers separated by a lighter zone, thus creating a trilaminar appearance. Biochemical structure: Cell membranes are made up predominantly of lipids, Proteins and carbohydrates are also present. A. Lipids: 1. Phospholipids are the main constituents of cell membranes. Each phospholipid molecule consists of an enlarged head in which the phosphate portion is located; and of two thin tails. Arrangement of phospholipids in the cell membrane explains the Trilamellar EM structure The head end is the polar end, hydrophilic (soluble in water) and is directed peripherally forming the dark staining parts of the membrane seen by EM. The tail end is the non-polar, hydrophobic, and directed to the center forming the light staining intermediate zone  9 Outer layer Head Head Phospholipids Inner layer Outer layer Tail Cholesterol Tails Inner layer Head 2. Cholesterol provides stability to the membrane. 3. Glycolipids are present only over the outer surface of cell membranes. B. Proteins: Carbohydrates: Present at the external surface of the membrane. They are attached either to the proteins (forming glycoproteins) or to the lipids (forming glycolipids). This carbohydrate layer forms the cell coat or glycocalyx. Glycoprotein Glycolipid Cell coat Lipid bilayers Cholesterol Protein  10 The cell coats It is a layer of glycoproteins and glycolipids which are present on the external surface of cell membrane Functions of cell coat: Protection of the cell. It contains special adhesion molecules which enable the cell to adhere to specific types of cells, or to specific extracellular molecules. It contains antigens. As in erythrocytes, the glycocalyx contains blood group antigens. Most molecules in the glycocalyx are negatively charged causing adjoining cells to repel one another. This force of repulsion maintains the 20 nm interval between cells. Function of cell membrane: 1. Passive Transport Simple Diffusion: water, oxygen and other molecules move down a concentration gradient (from high to low concentration). Facilitation Diffusion assisted by proteins (channel or carrier) Osmosis - diffusion of water. 2. Active Transport: occurs against the concentration gradient, and requires energy (ATP): Sodium-Potassium Pump: pumps out 3 sodium ions for every 2 potassium's taken in against gradient 3. Bulk transport: Endocytosis: taking substances into the cell (pinocytosis for water, phagocytosis for solids). The cell membrane first surrounds the molecule, invaginates and then separates to form an endocytic vesicle. Exocytosis: pushing substances out of the cell, such as the removal of waste molecules produced within the cytoplasm (e.g., secretions) may be enclosed in membranes to form vesicles that approach the cell membrane and fuse with its internal surface. The vesicle then ruptures releasing molecule to the exterior. 4. Support the cell and help maintain its shape.  11 2. Mitochondria (The powerhouse of the cell) Definition: membranous organelle, concerned with energy production. Their number varies from one thousand in liver cells (active) to few mitochondria in fat cells (inactive). L.M: Can be visualized only by using special stains. EM: Each mitochondrion has two membranes: 1- Outer smooth membrane. 2- Inner membrane which is folded into cristae. These cristae increase the surface area and possess the elementary particles, which carry the respiratory chain enzymes responsible for energy production. -The narrow space between the inner and outer membrane called the intermembrane space. The interior of the mitochondria is called the intercristae space (matrix space). This space is filled with a matrix rich in protein and contains mitochondrial RNA& circular DNA and dense granules rich in Ca2+. N.B: New mitochondria originate from preexisting mitochondria by growth and subsequent division (binary fission) of the organelle itself. Function: 1. The primary function of mitochondria is to convert organic materials into cellular energy in the form of ATP. 2. Mitochondria help the cells to maintain proper concentration of calcium ions within the compartments of the cell. 3. The mitochondria also play important role in the process of apoptosis. Mitochondrial Disease: Dysfunction in the mitochondria fails to produce energy that is needed for the sustainment of life and growth of an organism. The mitochondrial disease causes most of the damage to the cells of brain, heart, liver, muscles, and kidney. The symptoms may be loss of motor  12 control, muscle weakness and pain, gastro-intestinal disorders, poor growth, cardiac disease, and liver disease. Membranous organelles (cell factories and transport organelles): Rough Endoplasmic reticulum (rER) Smooth Endoplasmic reticulum (sER) Golgi apparatus Lysosomes Summary 3. Endoplasmic reticulum (rER) Rough endoplasmic reticulum Smooth endoplasmic reticulum Structure :Tubules + ribosomes Tubules Acidophilic H&E: Basophilic Acidophilic EM: Tubules + Ribosomes Tubules Function: Protein metabolism Lipid, Ca, HCl, drug metabolism  13 A. Rough Endoplasmic Reticulum (rER) Definition: RER is a series of connected parallel flattened tubules (cisternae) with ribosomes attached to its surface. Its density is higher near the nucleus and the Golgi apparatus. LM: It is basophilic due to the presence of ribosomes on its outer surface. EM: The RER appears as communicating flattened tubules called cisternae, covered with ribosomes. Function: It plays a central role in the synthesis of proteins. Proteins travel as transfer vesicles to Golgi apparatus. Manufacture of lysosomal enzymes. B. Smooth Endoplasmic Reticulum (rER) Definition: The SER appears as branching anastomosing tubules or flattened vesicles with smooth wall. There are no ribosomes on the surface. It is more tubular in shape than RER. Another type of endoplasmic reticulum is Sarcoplasmic Reticulum, which is a type of specialized SER that can be found in muscles. L.M: Cells with extensive SER may exhibit cytoplasmic acidophilia. EM: The SER appears as branching anastomosing tubules or flattened vesicles with smooth wall. There are no ribosomes on the surface. Functions: Lipid synthesis: Especially in the steroid-secreting cells such as cells of adrenal cortex. Glycogen metabolism: Enzymes involved in regulating glycogen metabolism are associated with the SER membrane for example in the liver cells. Regulation of mineral metabolism e.g. HCL production in the stomach. Calcium storage: in the skeletal and cardiac muscle fibers to control muscle contraction. Drug detoxification: Due to the presence cytochrome P450 enzyme in the SER membrane especially in the liver cells.  14 Medical application: Jaundice is caused by accumulation of bilirubin which are normally metabolized by SER enzymes in cells of the liver and excreted as bile. A frequent cause of jaundice in newborn infants is an underdeveloped state of SER in liver cells. 4. Golgi complex (Apparatus) Definition It is a membranous organelle that processes, packages, and sorts of macromolecules such as proteins and lipids after their synthesis. L.M It could be visualized only by using special stains. In the deeply basophilic cytoplasm of the protein secreting cells as plasma cell, its position appears as non-stained area called Negative Golgi Image. E.M The Golgi complex is formed of: 1- Golgi stack: It is composed of a variable number, typically 3-6, of flattened sacs called cisternae. Each stack has a convex immature surface facing the nucleus called (Cis Face) and a concave mature surface towards the cell membrane called (Trans Face). 2-Transfer (microvesicles): They derived from rough endoplasmic reticulum and fuse with the convex surface. 3- Secretory (macrovesicles): They are formed by budding from the mature surface of Golgi. It remains within the cytoplasm as lysosomes or exocytosed out-side the cell. Function: 1. Share in the formation of lysosomes. 2. Modifies the proteins by the addition of carbohydrates and phosphate to proteins 3. Keeps cell membrane and cell coat intact through the process of exocytosis.  15 5. Lysosomes Definition They are a membrane bound spherical vesicles containing hydrolytic enzymes, concerned as the rubbish disposal unit of the cell. Lysosomes are abundant in cells with phagocytic activity such as macrophage. LM: They may be demonstrated in cell sections by immunohistochemical techniques. EM: Primary lysosomes appear spherical homogenous electron-dense vesicles. Secondary lysosomes appear spherical heterogenous electron-dense vesicles as they contain digested elements. Formation of lysosomes Lysosomal enzymes are synthesized and segregated in the RER. Then they transferred to the Golgi complex as transfer vesicles. In the Golgi complex enzymes are modified and packed as lysosomes. Types of lysosomes: 1. Primary Lysosomes: Lysosomes that have not entered into a digestive process appear as homogenous vesicle. 2. Secondary Lysosomes: Lysosomes that have entered into a digestive process include different subtypes: a) Heterophagosome: result when the secondary lysosome fuse with solid particles (phagosome). b) Multi-vesicular body: result when the secondary lysosome fuse with pinocytotic vesicle. c) Autophagosome: if the digested material is one of cytoplasmic organelles. Fate of the digested material 1. After digestion, nutrients diffuse through the lysosomal membrane and enter the cytoplasm for reuse.  16 2. Indigestible compounds are retained within the vacuoles, which are now called residual bodies extruded outside the cell by exocytosis. Functions of lysosome: 1. Release enzymes outside the cell for destroying material around the cell such as in osteoclast. 2. Digestion of material from inside the cell such as old mitochondria. 3. Digestion of material engulfed from outside the cell example bacteria. 4. Digestion of completely breakdown cells that have died (autolysis). N.B: In some long-lived cells (e.g. neurons, heart muscle), large quantities of residual bodies accumulate and are referred to as lipofuscin, or age pigment. In some diseases, a specific lysosomal enzyme is absent or inactive, and certain molecules (e.g. glycogen) are not digested. As a result, these substances accumulate in the cells, interfering with their normal functions. 6. Peroxisomes (microbodies) Definition: Small (0.2-1.0 μm), membrane-bound organelles that contain oxidative enzymes. Functions: