Cytology Lecture 2, 2024 PDF

 Ribosomes  Non-membranous cell organelles.  Are small electron-dense particles,  about 20 x 30 nm in size. Types:  Free ribosomes: scattered in the cytoplasm.  Attached ribosomes: attached to ER membranes forming the rough endoplasmic reticulum (RER). Structure:  Has two subunits o...

 Ribosomes  Non-membranous cell organelles.  Are small electron-dense particles,  about 20 x 30 nm in size. Types:  Free ribosomes: scattered in the cytoplasm.  Attached ribosomes: attached to ER membranes forming the rough endoplasmic reticulum (RER). Structure:  Has two subunits of different sizes bound to a strand of mRNA.  Small (light), composed of four segments of  Cause basophilia in ribosomal RNA (rRNA) and approximately 80 secretory cells. different proteins.  Group of ribosomes are  Large (heavy), has two binding sites for transfere present on an mRNA RNA (tRNA): called polyribosomes (or P [peptide] site. And A [Amino acid] site. polysomes).  Ribosomes Functions: Protein synthesis with aid of m-RNA and t-RNA. Free ribosomes for synthesis of structural proteins as cytoskeleton, glycolytic enzymes. Attached ribosomes for synthesis of secretory proteins, as lysosomal enzymes and plasmalemmal proteins.  Endoplasmic Reticulum (ER) Network of interconnected membranes (cisternae ). Two types 1) Rough endoplasmic reticulum ( RER ), with ribosomes. Basophilic due to presence of ribosomes. Functions of RER: Synthesis of proteins, by ribosomes on its outer surface. Transfer vesicles, transfer the formed protein to Golgi. 2) Smooth endoplasmic reticulum ( SER ), no ribosomes. Functions of SER: Synthesis of lipids & cholesterol. Synthesis of steroid hormones, e.g. cortisone. Helps muscle contraction, (sarcoplasmic reticulum). Detoxification of drugs & toxins. Abundant in liver cells.  Endoplasmic reticulum Rough Endoplasmic Reticulum (RER) Abundant in cells that secrete proteins Smooth Endoplasmic Reticulum involved in steroid and lipid metabolism Mitochondria (red) Cytoplasm (green)  Typical protein secreting cell Chief cells of the stomach secrete digestive enzymes (pepsinogen and gastric lipase). These enzymes are produced by the large amounts of rough endoplasmic reticulum in the base of the cell. The Golgi packages these proteins into large secretory granules. Nucleus (blue) Secretory Granules (purple) Golgi Apparatus (yellow) Mitochondria (red) Rough endoplasmic reticulum (RER, cyan) Cytoplasm (green)  Golgi Apparatus or Golgi complex Named for histologist Camillo Golgi who discovered it in 1898. Is a highly plastic, the secretory organelle of the cell. Structure: Are membranous organelle. Complex traffic of vesicles, each vesicle has two faces: Convex (forming) face (cis face ), receives transfer vesicles from RER. Concave (mature) face (exit or trans face ), also named (condensing vacuoles ), forms secretory vesicles. Functions: Sorting, modification & packaging of proteins. Secretory vesicles formation. Formation of lysosomes. Transport vesicles from RER move toward and fuse at the forming face of the Golgi. Golgi Apparatus or Golgi complex  Golgi complex Golgi Apparatus (yellow) - stack of flattened cisterna. Transfer Vesicles (purple) - transfer newly synthesized proteins from the endoplasmic reticulum to the Golgi apparatus. Secretion Granules (gray) - Nucleus (blue) Nuclear Envelope (purple) Mitochondria (red) Endoplasmic Reticulum (cyan) Cytoplasm (green) Golgi complex  Secretory Vesicles or Granules Originating from Golgi apparatus. found in those cells that store a product until its release by exocytosis. Zymogen granules , with dense contents of inactive digestive enzymes. Intensely eosinophilic structures.  Lysosomes Structure: Spherical, diameter from 0.05 to 0.5 um. Single membrane sacs containing enzymes. Contain 40 different hydrolytic enzymes which formed in the RER. Originate from mature surface of the Golgi apparatus. Electron-dense in EM. Found in most cells, but abundant in macrophages and neutrophils, are larger and visible with L/M. Functions: * Intracellular digestion of ingested material or old organelles (autophagy). * Turnover of cellular components (recycling center).  Proteasomes Cytoplasmic protein complexes not associated with membrane. Is a cylindrical structure made of four rings, each composed of seven proteins (Ubiquitin proteins). At each end is a regulatory particle contains ATPase. Function to degrade unneeded or damaged proteins by proteolysis. Present in all eukaryotic cells.  Peroxisomes or Microbodies  Are spherical membranous organelles.  Approximately 0.5 m in diameter.  Are vesicles containing enzymes (oxidases).  They utilize oxygen by removing hydrogen atoms from fatty acids, then transferred to molecular oxygen (O2).  Accumulation of these fatty acids in body fluids can disrupt the myelin sheaths in nerve tissue, causing severe neurologic symptoms. Medical application Deficiency of peroxisomal enzymes cause Zellweger syndrome that affects the structure and functions of several organ systems (is an inherited disorder).  The Cytoskeleton Is a complex network of : (1) Microtubules. 1. Microtubules (2) Microfilaments (actin filaments).  The thickest component of cytoskeleton. (3) Intermediate filaments.  Found in the cytoplasm as cilia, flagella Functions: or centeriole.  determine the shape of cells,  outer diameter of 24 nm and a hollow  helps transport of material within lumen 14 nm wide. the cell. composed of tubulin molecules are arranged  allow the movement of entire cells. to form 13 protofilaments The Cytoskeleton  The Cytoskeleton, Microtubules I. Centrioles  Pair of cenerioles, forming a centrosome, are at right angles to each other.  Wall is made of 9 triplets of microtubules, i.e. 27microtubules. Functions:  Essential for cell division. Formation of cilia and flagella.  The Cytoskeleton, Microtubules II. Cilia  Are motile processes, covered by cell membrane.  Hair-like striations on the free surface of some cells.  With basal bodies.  Cross section named axoneme.  Each axoneme composed of 9 doublets and 2 central singlets of microtubules, i.e. 20 microtubules.  doublets are linked together by proteins nexins and dynin.  a radial spokes projecting toward the center. Function: movement of particles or fluids on the free surface of the cell in one direction.  The Cytoskeleton, Microtubules III. Flagella  Longer and larger than cilia.  Same structure as cilia (axoneme).  Form the tails of sperms for movement. Axoneme  2. Microfilaments (Actin Filaments)  The thinnest component of cytoskeleton.  Contraction in cells results from an interaction between actin and its associated protein, myosin.  actin filaments (F-actin) composed of globulin protein  forming globular actin subunits (G-actin)  arranged as a double-stranded helix.  Intermediate Filaments  Includes filaments intermediate in size between the other two, with a diameter averaging size 10–12 nm.  Increased mechanical stability to cell structure Forms of intermediate filaments in different cells: Filament Type Cell Type Examples Keratins or Epithelium Both keratinizing and nonkeratinizing Cytokeratins epithelia Vimentin Mesenchymal chondroblasts, macrophages, endothelial Fibroblasts, cells cells, vascular smooth muscle Desmin Muscle Striated and smooth muscle (except vascular smooth muscle) Glial fibrillary Glial cells Astrocytes acidic proteins Neurofilaments Neurons Nerve cell body and processes  Glycogen granules 3. Inclusions  Are not considered organelles.  Are not surrounded by a plasma membrane.  Composed of accumulated metabolites or other substances.  Serve as temporary storage for these molecules.  Non-motile and with little or no metabolic activity. Types: Fat droplets, accumulations of lipid molecules, in adipocytes (fat cells), in adrenal cortex cells and liver cells. Glycogen granules, aggregates of a carbohydrates, in several cell types, mainly liver cells. Lipofuscin granules, pigmented, coloured bodies accumulate with age, as: Melanin (dark brown) in skin, Lipofuscin (pale brown) in neurons & cardiac muscle. Hemosiderin, a dense brown in phagocytic cells of the liver and spleen.