Cell Structure PDF - Mansoura University 2024-2025

Prof. Dr. Shehab Hafez Cell Structure Prof. Dr. Shehab Hafez Professor of Histology & Cell Biology Head of Histology Department Faculty of Medicine, Mansoura University 2024-2025  Prof. Dr. Shehab Hafez...

Prof. Dr. Shehab Hafez Cell Structure Prof. Dr. Shehab Hafez Professor of Histology & Cell Biology Head of Histology Department Faculty of Medicine, Mansoura University 2024-2025  Prof. Dr. Shehab Hafez Cell structure 1. Cell membrane 2. Nucleus 3. Cytoplasm A. Cytoplasmic organelles ▪ Mitochondria. ▪ Endoplasmic reticulum. Membranous ▪ Golgi apparatus. organelles ▪ Lysosomes. Non-membranous ▪ Ribosomes organelles B. Cytoskeleton C. Cell inclusions  Prof. Dr. Shehab Hafez Cell Membrane ▪ It is the outer limiting membrane which surrounds the cell. ▪ Thickness: 7.5-10 nm & thus visible only by electron microscopy (E.M). Not visible by (L.M.) With Electron Microscopy (E.M). Molecular structure of cell membrane The cell membrane shows: Trilaminar appearance:- Two electron dense lines (black) separated by an electron I. Lipid molecules II. Protein III. Carbohydrate lucent one (white). molecules molecules  Prof. Dr. Shehab Hafez I. Lipid molecules 1. Phospholipids consist of: Head: Hydrophilic (water attracting). Tail: Hydrophobic (water-repelling). Arranged into a double layer (bilayer); hydrophobic tails are directed toward the center, away from water. 2. Cholesterol: Between the phospholipid 1. Integral proteins molecules. ▪ Firmly embedded in the lipid bilayer. Transmembrane proteins II. Protein molecules ▪ Large molecules Act as channels. 1. Integral proteins 2. Peripheral proteins: Loosely attached. Types III. Carbohydrate molecules 2. Peripheral proteins ▪ Glycolipids and glycoprotein. Project from external surface of the membrane, forming the Cell Coat.  Prof. Dr. Shehab Hafez Functions of cell membrane 1. Endocytosis C. Receptor-mediated endocytosis A. Phagocytosis Needs receptors (integral proteins). For solid particles: (Cell eating): Example: For large molecules: Example: hormones macrophages can engulf certain bacteria. and some drugs. 2. Exocytosis B. Pinocytosis Opposite to phagocytosis. The cell For fluids: (Cell drinking). expels any waste product.  Prof. Dr. Shehab Hafez The routine stain for light microscopy (LM) Hematoxylin & Eosin stain (H & E) Hematoxylin (H) Eosin (E) Nature Basic (alkaline). Acidic. Colour Blue. Red. E.M. Reacts with acidic structures inside Reacts with basic structures inside the cell: the cell: usually the cytoplasm ▪ DNA: (in the nucleus). which is rich in mitochondria. ▪ RNA: (in ribosomes & RER). The Cytoplasm which is The Cytoplasm which is rich in ribosomes & RER rich in mitochondria is is basophilic eg plasma acidophilic eg cells of cell. renal tubules.  Prof. Dr. Shehab Hafez Mitochondria Definition: membranous organelle containing enzymes specialized for production of energy (ATP). (Powerhouse of the cell). Size and shape: vary in size and shape. May be elongated, rod-shaped or spherical. Number: variable. depending on the activity of the cell: H.&E. Mitochondria are responsible Liver cells (active): contain for acidophilic cytoplasm. numerous mitochondria. Lymphocytes (less active): contain few mitochondria. Sites: Located at sites of maximum energy requirement e.g. Between myofibrils in cardiac muscle cells.  Prof. Dr. Shehab Hafez Electron microscopy (EM) The mitochondria is a double membranous organelle. 1. Outer membrane: Smooth, no folds. 2. Inner membrane Forms complex folds called cristae. The number of cristae is increased in active cells. 3. Mitochondrial matrix: Contains: Mitochondrial DNA (mitochondria can divide). Mitochondrial enzymes. Functions of mitochondria Provide the cell with ATP: Aerobic respiration takes place within the matrix & on the inner membrane to produce energy.  Prof. Dr. Shehab Hafez Ribosomes Definition: Non-membranous organelles. Concerned with Protein synthesis. Size: very small (20-30 nm in diameter). L.M. Aggregation of ribosomes leads to basophilia of the cytoplasm due to rRNA. E.M. Composed of small & large subunits. The two subunits are formed of rRNA and protein molecules.  Prof. Dr. Shehab Hafez Types A. Free ribosomes (polyribosomes or B. Attached ribosomes: ribosomes are attached to polysomes): Many ribosomes are bound to the surface of endoplasmic reticulum forming RER. a single mRNA molecule. Free ribosomes Synthesize proteins for the use of the cell: like cytoskeletal proteins. Functions Attached ribosomes synthesize proteins: To be secreted outside the cell OR Remain in the cytoplasm as primary lysosomes.  Prof. Dr. Shehab Hafez Rough Endoplasmic reticulum (RER) Definition: Membranous organelle, concerned with synthesis of protein, mostly to be secreted outside the cell. Sites:: Prominent in cells specialized for protein synthesis and secretion, such as: Fibroblasts (synthesize collagen). Plasma cells (synthesize antibodies). L.M. RER causes basophilia of the cytoplasm. Plasma cells  Prof. Dr. Shehab Hafez E.M. Parallel cisternae (long and flattened). Have attached ribosomes & polyribosomes. Functions Segregation of protein synthesized by the attached ribosomes → Golgi apparatus.  Prof. Dr. Shehab Hafez Smooth Endoplasmic reticulum (SER) Definition: Endoplasmic reticulum Cisternae with no attached ribosomes (they are not basophilic). E.M. The cisternae are in the form of tubules and vesicles. Smooth surface: no attached ribosomes. Sites and Functions of SER 1. In the liver cells Glycogen metabolism. Detoxification of drugs & toxic substances such as alcohol. 2. In the adrenal gland Lipid biosynthesis: synthesis of cortisone hormone. 3. In the muscle cell Calcium metabolism.  Prof. Dr. Shehab Hafez Golgi apparatus Definition: A membranous organelle, Concerned with secretion of protein synthesized by RER. L.M. H. & E. unstained area near the nucleus Silver: appears dark brown. (negative Golgi image).  Prof. Dr. Shehab Hafez E. M. 1. Transfer vesicles::Small, carry proteins from RER to the immature face of Golgi. 2. Golgi saccules 4-10 saccules forming a stack. Immature face (convex): receives transfer vesicles. Mature face (concave): releases secretory vesicles. 3. Secretory vesicles: ▪ Large , arise from the mature face. ▪ Either: Secreted outside the cell (exocytosis). OR Remain in the cytoplasm as primary lysosomes. Functions of Golgi apparatus Concentration and secretion of protein. Forms primary lysosomes.  Prof. Dr. Shehab Hafez Lysosomes Definition: Membranous organelles which contain about 40 hydrolytic enzymes. They are called suicide bags. Site: Present in the cytoplasm of all cells, but more abundant in cells with great phagocytic activity (eg, macrophages). E.M. Primary Lysosomes Secondary Lysosomes Freshly synthesized. Start intracytoplasmic digestion. Shape Spherical and regular. Irregular in shape. Size Small. Larger. E.M. Homogenous core. Heterogenous core. Functions: Digestion & Lysis of the phagocytosed particles into simple molecules. Digestion and Lysis of dead cell organelles.  Prof. Dr. Shehab Hafez The cytoskeleton General characters Non-membranous structures. Important for: Preserving the shape of the cell. Movement of the entire cell. Movement of the cell organelles. Diameter Includes 1. Microfilaments (Actin filaments) 5-9 nm 2. Intermediate filaments 10 nm 3. Microtubules 25 nm  Prof. Dr. Shehab Hafez Microfilaments (Actin filaments) Diameter: 5-9 nm. Actin filament is composed of actin protein. Synthesized by free ribosomes. Sites of actin Muscle cells. Microvilli. Contractile rings formed during cell division. Skeletal muscle Functions of actin Muscle contraction. Preservation of microvilli. Microvilli Contractile ring Cell division.  Prof. Dr. Shehab Hafez Microtubules Structure Hollow tubules. Diameter: 25 nm. Length: variable. Formed of tubulin protein synthesized by free ribosomes. Functions of microtubules 1. Supporting the shape of the cells. 2. Play a major role in cell division (formation of mitotic spindle). 3. They are the main structural component of: ▪ Centrioles. ▪ Cilia. ▪ Flagella.  Prof. Dr. Shehab Hafez Centrioles Non-membranous structures important for cell division. The 2 centriole are perpendicular. Each centriole is a short cylinder. The wall of each cylinder is composed of 27 microtubules arranged in 9 Triplets. Functions of centrioles: Cell division. Formation of the basal bodies of cilia.  Prof. Dr. Shehab Hafez Cilia Definition: Hair like processes projecting from the free surface of certain epithelial cells. Sites: in the respiratory system & female genital organs. E.M. of a cilium Shaft: Contains 9 peripheral doublets of microtubules & 2 central microtubules (Singlets). Basal body: Similar to centriole: 9 triplets of microtubules. Rootlets: formed of fibers. Functions of cilia: Movement of fluids or foreign bodies on the surface of the cells.  Prof. Dr. Shehab Hafez Cell inclusions Definition: Stored metabolites or other substances inside the cytoplasm. Types of cell inclusions 1. Glycogen granules: in the liver (stained 2. Lipid droplets: stained by Osmic acid & Sudan black. by Best’s carmine) and muscle fibers. 3. Pigments: Exogenous: like dust particles and carotene. Endogenous: like haemoglobin and melanin. L. M. The Nucleus 1. Number: Found in all cells EXCEPT RBCs & blood platelets. Cells are usually mononucleated. Binucleated cells eg some liver cells. Multinucleated cells eg skeletal muscle cells. 2. Size The nucleus is the largest structure in the cell. Liver cells Skeletal muscle Its size ranges from 3 to 14 m. 3. Shape Rounded. Oval. Kidney-shaped. Multilobed. 4. Staining & appearance Vesicular (open face) nucleus Condensed nucleus Lightly stained Deep basophilic Details of the nucleus could be seen No apparent details Nucleolus: appears clearly. Nucleolus: does not appear. Found in active cells eg liver cells Found in less active cells eg lymphocyte Vesicular (open-face) nucleus Condensed nucleus E.M. structure of the nucleus The nucleus is composed of 1. Nuclear membrane. 2. Nucleolus. 3. Chromatin. 4. Nuclear sap (karyoplasm). The Nuclear Membrane is a double membrane interrupted by nuclear pores. The Chromatin may be: It allows exchange of material between Extended chromatin (euchromatin): active. nucleus & cytoplasm through the pores. Condensed chromatin (heterochromatin): inactive. The Nucleolus: is the site of synthesis of ribosomal RNA.  Resources ▪ Junqueira LC, Carneiro J: Junqueira’s Basic Histology. Text and Atlas, thirteenth edition (2013), LANGE Mc Graw Hill. ▪ Gartner LP and Hiatt JL: Color Textbook of Histology. Third edition (2007). Saunders Elsevier. Prof. Dr. Shehab Hafez