Endoplasmic Reticulum, Golgi Apparatus & Lysosomes PDF

# Endoplasmic Reticulum ## Faculty of Veterinary Medicine Department of cytology and Histology General Program - 1^st Level Students ### What is the Endoplasmic Reticulum? * **Endo** = inside * **plasma** = cytoplasm * **Reticulum** = network It is an irregular network of branching and...

# Endoplasmic Reticulum ## Faculty of Veterinary Medicine Department of cytology and Histology General Program - 1^st Level Students ### What is the Endoplasmic Reticulum? * **Endo** = inside * **plasma** = cytoplasm * **Reticulum** = network It is an irregular network of branching and anastomosing tubules, cisternae and vesicles. ### Two Types of Endoplasmic Reticulum - **Rough ER** - **Smooth ER** # Rough Endoplasmic Reticulum (RER) ## Definition 1. The rough endoplasmic reticulum is a membranous organelle concerned principally with synthesis and secretion of proteins. 2. It is called rough due to the presence of a large number of ribosomes attached to its limiting membrane ## Appearance under a Light Microscope (LM) It appears as basophilic cytoplasmic areas that are referred to as the ergastoplasm or chromidial substances. ## Examples of Rough ER under a Light Microscope 1. **Diffuse:** - Example: plasma cells 2. **Localized:** - Example: pancreatic acinar cells 3. **Arranged into clumps:** - Example: Nissl granules in nerve cells ## Appearance under an Electron Microscope (EM) It consists of an anastomosing network of tubules, vesicles and flattened cisternae that ramifies throughout the cytoplasm. ## Function of RER - Synthesis of proteins for extracellular use (secretory proteins, lysosomal proteins and membrane proteins). - Glycosylation of proteins to form glycoproteins # Smooth Endoplasmic Reticulum (SER) ## Definition The smooth endoplasmic reticulum is a membranous organelle consisting primarily of a network of branching and anastomosing tubules and vesicles. It differs from the RER in that its limiting membrane is smooth and devoid of ribosomes. ## Appearance under a Light Microscope (LM) It does not appear. The cytoplasm of the cells contained abundant sER usually appears acidophilic ## Appearance under an Electron Microscope (EM) It appears as an irregular network of membranous tubules and vesicles devoid of ribosomes in contrast to flattened ribosome-studded cisternae of rER. ## Function of SER 1. Steroid hormone synthesis in the testicular interstitial cells, cells of the corpus luteum and adrenal cortex cell. 2. Drug detoxification in liver cells 3. Lipid synthesis in the intestinal absorptive cells. 4. Release and storage of Ca⁺⁺ ions in striated muscle cells. 5. Production of HCL in gastric parietal cells. # Golgi Apparatus (Golgi Complex) ## Definition The Golgi apparatus is a membranous organelle concerned principally with synthesis, concentration, packaging and release of the secretory products. ## Appearance under a Light Microscope (LM) It can be selectively stained with silver salts or osmium where it appears as a black network located near the nucleus. - **In H&E sections**, it may be visible as a lighter-stained region called **negative Golgi image**. - It is seen to great advantage in secretory cells such as osteoblasts. ## Appearance under an Electron Microscope (EM) - The main structure unit of the Golgi apparatus is a flattened membranous vesicle called a **Golgi saccule**. - The Golgi saccules are arranged in **Golgi stacks** that contain from 3 - 10 saccules. - **Each stack of saccules has:** 1. **A forming face or Cis face** that is convex in shape. 2. **A maturing face or trance face** that is concave. - The Cis face is usually associated with a number of small **transfer vesicles**. - The trance face is characterized by being associated with much larger **secretory granules**. - A diagram of the Golgi apparatus shows the Rough ER, Anterograde transport, Fusion, Transition vesicle, Cis face, Cisternae, Retrograde transport, Golgi apparatus, Trans face, Secretory vesicle, Budding, Exocytosis and Cell membrane. ## Function of Golgi Apparatus - Packaging and concentration of secretions - Modification of the secretory products such as glycosylation and sulfation of proteins to form glycoproteins and sulfated glycoproteins (mucus) - Production of primary lysosomes. - A diagram of Golgi apparatus functions shows ribosomes, synthesized protein, protein synthesis, Golgi complex, transport vesicle, cis cistern, medial cistern, trans cistern, lysosome, rough (granular) ER, protein packaging, secretory vesicle, plasma membrane, protein export from cell by exocytosis. ## Golgi Apparatus Functions: The Secretory Pathway - Model - A diagram of the secretory pathway includes RNA, rough ER, cis face, Golgi apparatus, trans face, plasma membrane. 1. Secreted proteins enter ER as they are being synthesized by ribosomes. 2. Protein exits ER in vesicles. 3. Protein travels through cisternae of Golgi apparatus. 4. Protein enters a secretory vesicle that fuses with cell membrane. 5. Protein is secreted from cell. # Lysosomes ## Definition They are membrane-bounded vesicles (0.2 - 0.4µm) containing a number (more than 40) of hydrolytic enzymes that are active at acid pH (acid hydrolases) maintained within their interior. This group of enzymes is capable of destroying all the major macromolecules (e.g., proteins and lipids) of the cells. ## Appearance under a Light Microscope (LM) - It provides no direct evidence for the existence of lysosomes. - The lysosomes are resolved at the LM level when their enzyme contents (e.g., acid phosphatase) are stained by histochemical methods. ## Appearance under an Electron Microscope (EM) The lysosomes appear as spherical membrane-bounded vacuoles with contents showing varying degrees of electron density. ## Types of Lysosomes - **Primary** - **Secondary** - **Multivesicular bodies** - **Residual bodies** ## Primary Lysosomes * They are lysosomes freshly formed from the Golgi or rER. * They contain nothing but hydrolytic enzymes. A diagram of primary lysosomes shows: - **Primary Lysosomes:** These are just forming - **Secondary Lysosomes:** These are formed when a primary lysosome fuses with another vesicle. ## Secondary Lysosomes - Formed as the result of fusion of primary lysosomes with phagosomes. A **phagosome** is a membrane-bounded vesicle containing either exogenous material (e.g., bacteria) and is called heterophagosome or endogenous material (e.g., damaged organelle) and is called autophagosome. ## Secondary Lysosomes: Phagocytosis & Autophagy - This diagram shows how secondary lysosomes are formed by **Phagocytosis** and **Autophagy**. - **Phagocytosis** involves lysosomes digesting foreign matter, such as bacteria. - **Autophagy** involves lysosomes digesting organelles within the cell. ## Multivesicular Bodies These are spherical forms of heterophagosomes. They are membrane-bounded vesicles containing a number of smaller vesicles. - Diagram depicting the formation of a multivesicular body; - **Phagosomes:** These are vesicles that enclose foreign matter. - **Pinocytotic vesicles:** These bring in smaller amounts of material to form endocytotic vesicles. - **Endosomes:** These contain special membrane proteins. - **Primary Lysosomes:** These are formed by the Golgi Apparatus. - **Secondary Lysosomes or Endolysosomes:** These are formed by the fusion of primary lysosomes with other existing vesicles. ## Residual Bodies - Residual bodies are debris containing vacuoles representing the terminal stage of lysosomal activities. - Their contents may either be extruded from the cell by exocytosis or accumulate in the cytoplasm as lipofuscin pigments. -Diagram depicting the formation of a residual body; - **Rough Endoplasmic Reticulum:** This is where synthesis of proteins begin. - **Mitochondrion:** These are the powerhouses of the cell. - **Golgi apparatus:** Proteins and lipids are packaged here. - **Primary lysosome:** One of the stages in the process of lysosomal formation. - **Phagocytosis:** The process of engulfing foreign materials into the cell. - **Phagosome:** This is the cell membrane wrapped around the material being ingested. - **Autophagic vacuole:** This is a vesicle containing material from within the same cell. - **Residual body:** This is the last stage of lysosomal action, forming a waste product for elimination. ## Lysosomes - Functions - Degradation of any exogenous macromolecules (phagocytosis and pinocytosis) - Disposition of any organelles or cell constituents that are no longer useful to the cell (autophagy) # Peroxisomes ## Definition Peroxisomes are spherical, membrane-bounded organelles containing peroxide-forming enzymes and catalase that are involved in the formation and degradation of intracellular hydrogen peroxide. ## Appearance under a Light Microscope (LM) It does not appear. ## Appearance under an Electron Microscope (EM) They are membrane-bounded vacuoles, vary in size and appearance depending on species and cell types. ## Peroxisomes - Appearance under the Electron Microscope - In human cells, they contain a finely granular matrix of moderate density. - In many other species, they have a crystalline core called a nucleoid. ## Peroxisomes - Functions - Peroxisomes contain at least three oxidase (D-amino acid oxidase, urate oxidase and catalase). - The D-amino acid oxidase, urate oxidases are responsible for the production of hydrogen peroxide (H2O2). - The catalase then utilizes the H2O2 in oxidation (and therefore, detoxification) of various toxic substances such as phenol, alcohol and fatty acids. A diagram shows: - **H2 + o2**, **D-amino acid oxidase**, **catalase**, **H2O2**, **H2o + o**, **Urate oxidase (0)** - **Toxic substance**, **oxidation**,**Nontoxic Substance** - The toxic substance is acted on by D-amino acid oxidase and urate oxidase to produce hydrogen peroxide. - The catalase enzyme then uses the H2O2 to detoxify the toxic substance, turning it into a nontoxic substance. # References - Color Textbook of Histology, Gartner, 2006 - Netter's Essential Histology, Ovalle, 2013 - Functional Histology, Kerr, 2009 - Histology and Cell Biology: An Introduction to Pathology, Kierszenbaum, 2015 - Medical Cell Biology Made Memorable, Norman, 1999 - Internet sources.