Cell Biology: Functions and Structure of Cells

Dr Dalia El Marakby The cell Defination: The basic structural & functional unit of the living body. Functions: absorption, respiration, secretion, exceretion, sensation, conduction, contraction, movement, growthand reproduction. Size: from 4 µm (granular cells of the cerebellum) to 150 µm (ovum). Shape: rounded, oval, flat, stellate, polygonal, cubical & columnar Structure: animal cell is eukaryotic formed of 1-cytoplasm 2-nucleus. Cytoplasm Cytosol Fluid containing: -proteins. -carohydrates. -lipids. -minerals. -ions&salts. -O2& CO2 -metabolites & RNA. Organelles Living Permenent Essential active Have vital functions Inclusions Non living Temprory Usually not essential inert Results from cell activity 1-Cytoplasmic Organelles They are classified according to presence of limiting membranes into:. 1 Dr Dalia El Marakby Membranous Organelles The Cell Membrane (Plasmalemma) Definition: limiting membrane that envelope all cells. LM: * Difficult to be seen because it is very thin (7.5-10 nm). *Can be seen when stained with silver (Ag) or PAS stains. E/M: • Two dark (electron-dense) layers separated by a light one (electron-lucent). i.e. 3 parallel lines "trilaminar membrane" or "unit membrane". • There is a fuzzy layer found on the outer surface only, which represents the cell coat or (glycocalyx). Molecular Structure: 1-Lipid component: Phospholipid molecules: arranged into 2 layers (lipid bilayer) phospholipid heads are directed outwards to aqueous solutions& tails inwards. Functions: It is permeable to certain substances (selective permeability) and impermeable to ions and polar molecules to perform a barrier between internal and external environment of the cell. Cholesterol molecules: present between hydrophobic fatty acids. Functions: 2 Dr Dalia El Marakby • Restrict movement of phospholipid molecules so stabilize cell membrane. • Modulate fluidity of membrane preventing it from being too fluid or too rigid. 2-Protein component: 50% of total membrane mass, present in two forms: Peripheral proteins: integral (transmembrane) proteins: *Small molecules forming a non-continuous * Embedded in the lipid bilayer. layer outside the lipid bilayer. *Loosely attached * Two types are present: -channels proteins for transport of ions & water. *On both surfaces of the cell membrane - carrier proteins for transport of small polar molecules as glucose and ions as Na K pump * Functions: specific transport of ions and polar molecules. 3-Carbohydrate (CHo) at the external surface formed of: Glycoproteins: Glycolipids: *oligosaccharide attached to protein molecules * oligosaccharide attached to phospholipid molecules. Cell coat (glycocalyx): • Formed of molecules of glycoproteins & glycolipids. • Present on the external surface of cell membrane. •It includes receptors for drugs, hormones, bacteria & viruses. Functions of cell coat: They are important in cell immunity, dentification (Receptor), adhesion & protection Functions of Cell Membrane: a) Endocytosis: bulk movement of substances into the cell by forming vesicles. Phagocytosis (cell eating): e.g. WBCs engulfing bacteria. *Pseudopodia extend from the cell to surround the particle e.g. bacterium. *Extensions meet & fuse enclosing bacteria in a vacuole called phagosome. * Phagosome fuses with lysosomes to digest its contents. Pinocytosis (cell drinking): e.g. pinocytosis of colloid in thyroid follicular cells *Invaginations of cell membrane are formed and surround the extracellular fluid. * Pinocytic vesicles detach off from the cell membrane. *The vesicle fuses with lysosomes to digest its contents. Receptor-mediated endocytosis (selective transport): e.g. uptake of protein hormones as growth hormone by the cell. *Receptors (integral proteins of the cell membrane) bind to specific substances. *These proteins accumulate at the coated pit. *They associate with other proteins on the cytoplasmic side called clathrin. *The coated pits invaginate and pinch off forming coated vesicles (contain specific substance+ receptors). 3 Dr Dalia El Marakby b) Exocytosis:* bulk movement of substance from inside to outside the cell by forming vesicles *The vesicles fuse with the cell membrane & discharge their contents into the extracellular space without affecting continuity of cell membrane. Medical application: despite normal level of growth hormone (GH) in blood, lack of GH receptors at the cell membrane of target cells, leads to a type of dwarfism. Mitochondria Definition: membranous organelles, containing enzymes of aerobic respiration and energy production. They are the powerhouse of the cell. Sites: area of most activity of the cell . Number:• More in active cells as liver cell 2000 mitochondria/ cell. • increase in number by simple division. LM: * cytoplasm acidophilic when abundant. * Special stain iron hematoxylin (dark blue). Janus green (green). *Appear as granules or rods EM: 1 *appear as rounded or oval membranous vesicles. 2*Surrounded with two-unit membranes ●The outer membrane: * Permeable to small molecules *Smooth *Contains porins (transmembrane protein). ●The inner membrane: * Less permeable (selective). * It projects into the matrix forming shelves called cristae. Increase number of cristae = more energy production. Cristae Increase surface area for elementary particles. Elementary particles: * are globular structures attach to the cristae with cylindrical stalk *Formed of protein complex+ ATP synthetase that form ATP in oxidative phosphorylation. The two membranes are separated by an inter-membranous space. ● Mitochondrial Matrix: composed of * Oxidative enzymes of citric acid (Krebs) cycle 4 Dr Dalia El Marakby * Electron dense granules rich in Ca++ act as a catalysts for mitochondrial enzymes. * DNA, mRNA, tRNA & rRNA. Functions of Mitochondria: responsible for energy production (Cell respiration): 1-Energy production (from metabolites in the cytoplasm by using Krebs cycle (citric acid cycle) enzymes. 2-most of the energy is stored as ATP molecules (by oxidative phosphorylation enzymes) Some of the energy is released in the form of heat to maintain body temperature. Medical application.: defect in Mitochondrial enzymes leads to failure to produce ATP which is needed for all vital activities e.g. in muscle cause muscular weakness Endoplasmic Reticulum (Endo = in, Rete = net) Definition: communicating membranous channels and sacs (cisternae). Extend from nucleus to cell membrane Types: according to presence or absence of electron dense particles (ribosomes) on surface there are two types: a) Rough b) Smooth. Rough Endoplasmic Reticulum (rER): • protein forming cells, e.g. plasma cells. Site LM localized or diffuse basophilia. EM *It appears as parallel flattened cisternae. *Covered with poly ribosomes (electron-dense particles). *Ribosomes are bounded by their large subunits to specific receptors (ribophorins) on the membranes. *Under the receptor is a pore to allow formed proteins to enter the rER cisternae. *Continuous with outer nuclear membrane. Functions 1. proteins formation by attached polyribosomes. 2. Segregation of formed proteins. 3. Protection of the cytoplasm from the hydrolytic enzymes formed inside its cisternae. 4. Initial glycosylation of some proteins by addition of monosaccharides. 5. Packing of formed proteins in vesicles that bud off from the cisternae (transfer vesicles] to reach Golgi apparatus. 6. Acts as an intracellular pathway for the formed substances. Smooth Endoplasmic Reticulum (sER): Lipid forming cells, e.g. liver cells. when abundant causes acidophilia. *Anastomosing cisternae of various shapes &size. *No ribosomes. * Continuous with rER. 1.Phospholipids molecules formation (in cell membrane). 2.Steroid hormones formation (as cortisone & testosterone). 3. Glycogen breakdown to glucose as in liver cells. 4. Detoxification of drugs, hormones, and alcohol in liver cells. 5. Calcium ions (Ca++) pump in muscle contraction. 6. Acts as an intracellular pathway 5 Dr Dalia El Marakby Golgi Apparatus Definition:. • Membranous organelles, responsible for secretion Sites: (well developed in secretory cells). L/M: *H & E-stained sections, (in deeply basophilic cytoplasm as in plasma cells), seen as unstained area near the nucleus called "negative Golgi image". *Silver (Ag), seen as a network of brown granules & fibrils. Apical: between the nucleus and the upper pole of the secretory cell, e.g. Secretory cells. Perinuclear; completely surrounds the nucleus, e.g. in nerve cells. E/M: *It is formed of membranous saccules. *Each saccule is flat, curved, has a narrow lumen & expanded ends, filled with low electron-dense material. *Saccules are inter-connected with each other. * 3-10 parallel saccules forming stacks. * Each stack has 2 faces: 1- Entry (Cis) Face: *They receive transfer vesicles, that arise from rER carrying proteins. 2 – Exit (Trans) Face: * vesicles that bud off from this face are either: a -secretory vesicle that discharge their contents b exocytosis b- lysosomes. Functions of Golgi Apparatus: 1- Packing, concentration & storage of protein received from rER. 2- Chemical modification of proteins & lipids by addition of carbohydrate. 3- Formation of Lysosomes, secretory vesicles. 4- Discharge of secretory products as hormones in secretory vesicles. 5- Renewal and maintenance of cell membrane & providing it with integral proteins from membrane of secretory vesicles Lysosomes Definition: • membranous organelle responsible for intracytoplasmic digestion. • Contain hydrolytic enzymes (protease, sulfatase). 6 Dr Dalia El Marakby Origin: • lysosomes is a secretory vesicle that arise from Golgi apparatus containing hydrolytic enzymes (lysosomal) enzymes. • Enzymes of lysosomes are proteins which are formed in rER transferred to the Golgi apparatus as transfer vesicle & come out Golgi apparatus as lysosomes. Number: abundant in phagocytic cells (macrophages, neutrophils) LM: Lysosomes are not detected by H&E stain. histochemical stain that detects the hydrolytic (lysosomal) enzymes. EM: 1- Primary Iysosomes 2- Secondary Iysosomes *They are newly released from Golgi apparatus. *Lysosome with variable contents. * EM homogeneous moderate electron-dense vesicles * EM heterogenous vesicles. Types of secondary Iysosomes according to their contents: 1- Heterolysosomes: primary lysosome + phagosome to digest solid particles as bacteria or viruses. 2-Multivesicular bodies: primary lysosome +pinocytic vesicle to digest fluid particles. 3-Autolysosomes: primary lysosome + autophagic vesicle containing destroyed substrate as mitochondria. Fate of secondary lysosomes: * Digested material diffuses back to the cytoplasm through lysosomal membrane. *Undigested materials remain in vesicles & called residual bodies. Fate of residual bodies: 1* discharge the undigested material by exocytosis (cytostool), 2 *accumulate over years in long-lived cells as nerve cells and heart muscles as lipofuscin granules (age pigments). Functions of Lysosomes: 1- Digest nutrients, bacteria & viruses. 2- Maintain cell health by removal of nonfunctional organelles. 3- postmortem autolysis by digestion of the whole cell after death due to rupture of lysosomes. 5- Fertilization by helping the head of sperm to penetrate the ovum. 6- Activation of thyroid hormones by hydrolysis of bond between the hormone and protein. Medical application: lack of lysosomal sulfatase→ ↑ sulfated compounds → interferes with normal function of the nerve cells. 7 Dr Dalia El Marakby Peroxisomes (Microbodies) Definition: membranous organelles, containing oxidases & catalase enzymes. Origin: -Membranous vesicles bud off from rER. -Enzymes are formed in free ribosomes. Number: * more in liver & kidney cells, • Increased in response to diet & drugs. • They divide by simple division (fission) LM: detected by histochemical method. EM: *small moderate electron density vesicles. * Spherical to ovoid organelle bound by a single membrane. Functions: 1* oxidases enzymes → β-oxidation of long chain fatty acid to produce: a) energy that comes out as heat and not stored as ATP. b) hydrogen peroxide (H2O2) which is a toxic product. 2* catalase enzyme: breakdown H2O2 to H₂O + O₂. N.B. oxidases are important in liver cells to oxidize various organic substances to detoxify alcohol and drugs. Medical application: lack of peroxisomal enzymes affects the structure and function of some organs as liver. non membranous organelles 1- Ribosomes Definition: non membranous particles . formed of rRNA and protein. Origin: rRNA (formed in nucleolus) Protein (formed in cytoplasm) and passthrough nuclear pores both unite in the nucleolus into small & large subunits then pass into the cytoplasm through nuclear pores & join each other during protein formation Number: abundant in protein forming cells as plasma cells. LM: when abundant they cause cytoplasmic basophilia due to acidity of phosphate group of RNA. Basophilia may be • Diffuse: as in embryonic cells • Localized: as in pancreatic cells. • Focal: as in nerve cells (Nissl's granules). 8 Dr Dalia El Marakby EM: *small electron-dense granules. *Formed of 2 subunits small & large ones. *The 2 subunits unite by mRNA. *The large subunit contains a groove in its center which contains the polypeptide chain. * Ribosomes are found in 2 forms: 1- Free: a-scattered single b- Polysomes (polyribosomes) are linked together with mRNA and appear as rosettes or spiral chains 2-Attached: Ribosomes bind to the outer surface of rER by the large subunits at glycoprotein receptors (ribophorins) * mRNA carries the information of amino acids sequence. * tRNA picks up amino acids and transports them to ribosomes (rRNA) forming polypeptide chain. * Polypeptide chain passes through the groove and enter rER cisternae. Function of ribosomes: Protein synthesis. *Free ribosomes Form protein used in the cytoplasm as glycolytic enzymes *Attached ribosomes form proteins for secreted by the cells as digestive enzymes & protein hormones. 2- Cytoskeleton *Formed of microtubules, micro filaments & intermediate filaments together with some proteins to link them to each other and to cell membrane forming a micro trabecular lattice. Microtubules Thin filaments (Microfilaments) Intermediate filaments (Actin) * Hollow cylinder of *Fine strands *Formed by polymerization of Structure Diameter Protein subunits Diameter: fixed (wall formed of 13 protofilaments) Length: can be changed by polymerization of tubulin molecules This directed by the microtubular organizing center (MTOC) which contains gamma tubulin (24-25 nm). alpha and beta tubulin. * Formed of 2 chains of globular G actin, coiled around each other to form filamentous F actin. Tetrameric subunits that differ chemically. (5-7 nm). G actin (8-10 nm in diameter) Different proteins. 9 Dr Dalia El Marakby Location in the cell LM Functions: *Arise from MTOC to the *Under cell membrane cytoplasm. *Microvilli. *Cilia Difficult to be seen except by immunofluorescent technique. *Cytoplasm *Nuclear envelope. 1- Determination of cell shape & cell elongation. 2- intracellular transport of macromolecules, vesicles & organelles. 3- Formation of mitotic spindle during cell division. 4- Formation of centrioles, cilia & flagella. Support function: 1- cytokeratin filaments: in epithelial cells. 2- Vimentin: in connective tissue & muscle. 3- Desmin: in muscle cells. 4- Neurofilaments: in neurons. 5-Glial fibrillar acidic protein (GFAP): in glial cells of nervous tissue. 6-Lamins: in nuclear envelope. 1- cell shape changes as endocytosis, exocytosis & amoeboid movements. 2-- intracellular transport of organelles, vesicles, and granules. 3- cleavage of cells during cell division. 4- Muscle contraction. 5- formation of microvilli core to keep their shape and shortening and elongation. N.B.: Microtubules: 1- cancer chemotherapy is used to arrest cell proliferation in tumors by preventing microtubules formation. Intermediate filaments: identification of intermediate filaments protein by immunohistochemical methods is important in diagnosis & treatment of tumors. The cell origin of the tumor can be recognized. Structures Formed by Stable Microtubules 1) Centrioles Definition: Cylindrical structures composed of MT. LM: *appear with iron hematoxylin stain as two dark bodies near the nucleus. EM: * in non dividing cells: 2 cylinders at right angles to each other surrounded by a matrix of tubulin (centrosome). * Each cylinder is formed is formed of 27 microtubules arranged in 9 bundles each bundle is formed of 3 microtubules (triplets), i.e. (9 x 3 =27) 10 Dr Dalia El Marakby Functions of Centrioles: 1- Formation of mitotic spindle during cell division: *During S- phase of interphase the centrosome duplicate itself. *During mitosis it moves to the opposite poles of the cell. *The centrosomes become organizing centers for microtubules of the mitotic spindle (Microtubules Organizing Centers) (MTOC). 2- Share in the formation of cilia and flagella. 2) Cilia Definition: * Motile processes with microtubular core covered by cell membrane. Origin: * Centrioles duplicate thousands of times to form basal bodies of cilia that migrate to the apex of the cytoplasm. * Shaft grows up from each basal body. LM: hair-like striations. EM: a cilium is formed of 3 parts: a basal body, shaft & rootlets. 1- Basal body: * a single centriole formed of 27 microtubules in 9 triplets. * It is embedded in the cytoplasm. 2- Shaft (axoneme): * Finger-like cytoplasmic projection from the surface of the cell covered by cell membrane. * formed of 9 doublets (9 x 2) at its periphery 2 singlet (2 microtubules in the center From each triplet, the inner 2 microtubules (A & B) (doublets)] Formed by polymerization push the cell membrane in front of them Thus each shaft is formed of 20 microtubule (9 x 2) + 2 3-Rootlets: *They are formed by growth of the microtubule (C) of each triplet into the cytoplasm (9 MT). * Fix the basal body and shaft to the cytoplasm. Functions of Cilia: 11 Dr Dalia El Marakby 1- Repeated beating motion by bending adjacent doubles →Move secretions or particles in one direction e.g.in the respiratory system & female genital system. 2- Cilia Modify to act as receptors e.g. rods & cones of the retina. 3) Flagella Definition: *They are motile projections from the cell (move the cell itself). * Have the same structure as the axoneme of the cilium (9 peripheral doublets+ 2 central singlets). * Extremely longer than cilia. Function: *forms the tail of the sperm, which helps its movements. Medical application: inability of the cilia to move results in bacterial infection on top of accumulated secretion causing chronic respiratory infections. immotile flagella cause male infertility. Structures Formed by Microfilaments 1-Microvilli •Finger like projections (shorter than cilia) from cell membrane of some cells. •L.M.: Apical straited brush border. •E.M.: A core of actin filaments (to maintain its shape), is covered by cell membrane and inserted into a terminal web. •Function: Increase surface area for more absorption e.g. small intestine. • 2-Stereocilia: Non motile solid cilia Not true cilia but long microvilli. •L.M.: hair like processes from free surface of some cells. •E.M.: Their core has actin filaments (no microtubules). Function: help absorption in male genital system e.g. epididymis II- Cytoplasmic Inclusions*They include: A- Stored Food 1- Carbohydrates B- Pigments Definition: colored particles produced by the cell or Site: stored as glycogen granules in liver & muscle cells. LM: H & E glycogen dissolve during heating & appear as vacuoles. Best's carmine red granules PAS Magenta red granules. EM: Glycogen granules appear as - single granules. -or as rosette-shaped aggregations. *They are concentrated in areas of cytoplasm rich in sER. taken from outside. They possess color of their own nature. Types: 1- Endogenous: a- Hemoglobin (Hb): in RBCs, carry gasses (o2 & Co2). b- Melanin: in the skin, to give its color & protects from ultraviolet rays. c- Lipofuscin: in cardiac muscle & nerve cells. 12 Dr Dalia El Marakby 2- Fats Site: fat cells as large globules or in liver cells as small droplets In H & E appear as vacuoles as fat dissolves during preparation. Sudan III stain: fat appear as orange globules in fat cells. 2- Exogenous a- Carbon particles as in dust cells of the lung. Dust particles b- Carotene pigments, in carrots. d- Tattoo marks: dyes are injected under the skin & taken by phagocytic cells. NUCLEUS Definition: • The largest component of the cell. • Present in all cells, except RBCs and platelets (not true cells). Number: usually one nucleus in the cell (mononucleated) some cells have two nuclei (binucleated) as liver cells some have more than two nuclei (multinucleated) as osteoclasts & skeletal muscles. Position: central, eccentric, basal or peripheral. Shape: rounded, flattened, oval, kidney-shaped, segmented, bilobed or multilobed. Staining: .* basophilic in H&E stained section due its content of DNA & RNA. Appearance: • Lightly stained (vesicular): • Deeply stained (condensed): -pale stained -dark stained. -in active cells. - in inactive cells. -e.g. nerve cells nucleus. -e.g. small lymphocytes. Structure of the nucleus: 4 components 1- Nuclear membrane (Envelope). 2- Chromatin material. 3- Nucleolus. 4- Nuclear sap. 13 Dr Dalia El Marakby Structure of the Nucleus 1- Nuclear Membrane (Nuclear Envelope) 2- Chromatin LM: blue line (basophilic) due to chromatin on its inner side ribosomes on its outer side EM: *Formed of two parallel unit membranes *Separated by a perinuclear space (30-50 nm). * And interrupted by nuclear pores Outer nuclear membrane *It is rough & granular due to ribosornes on it *Continuous with rER cisternae. Inner nuclear membrane *It is fibrillar due to attached chromatin threads (peripheral chromatin). *.it is associated with nuclear lamina formed mainly of lamins to stabilize nuclear envelope Nuclear pore complex: *Are circular openings at intervals. where the inner & the outer membranes fuse. * They formed of 30 nucleoporin proteins which form octagonal ring. *The nucleoporin filaments extend into the cytoplasm and nucleus. * Function: *Transport of proteins to the nucleus and transport of RNA and ribosomal subunits to the cytoplasm through receptor protein. Definition: * uncoiled chromosomal material (in non dividing nuclei) *formed of nucleoproteins (DNA + Histone & non histone protein) *Have 2 forms according to cell activity: 1- Euchromatin Extended (uncoiled) Contains active genes. Predominates in active cells as in protein forming cells LM fine threads correspond to pale basophilic nucleus with clear nucleolus EM it appears as electron lucent 2- Heterochromatin Coiled (inactive) Contains inactive genes. Predominates in inactive cells e.g. small lymphocytes LM coarse clumps → to dark basophilic nucleus with unclear nucleolus. EM: it appears as electron dense. Sites of heterochromatin: a) Peripheral chromatin: attached to the inner surface of nuclear membrane. b) Chromatin islands: clumps in the nuclear sap. c) Nucleolus-associated chromatin: condensed around the nucleolus. Functions of Chromatin: 1- Carries genetic information. 2- Control protein formation. 3- Formation of RNA (mRNA, rRNA& tRNA). 14 Dr Dalia El Marakby Medical application: Chromosomal alterations are associated with tumors and genetic diseases. 3-Nucleolus LM: *Rounded, deeply basophilic mass, rich in nucleic acids & surrounded with chromatin. *One or two in each nucleus. EM: spongy, not limited by membrane. It has dark and light areas. • The dark areas are formed of: - Pars amorpha (nucleolar organizer): filaments of DNA, which carrying the genes encoding rRNA. - Pars fibrosa (fibrillar component): strands of newly formed rRNA. - Pars granulosa (granular component): granules of mature rRNA. Both are called "nucleolonema" • The light areas: formed of the nucleolar sap Functions: formation of ribosomal RNA & ribosomal subunits (rRNA+ protein), which pass through nuclear pores to the cytoplasm. Medical application: large nucleoli are found in rapidly growing malignant cells. 4- Nuclear Sap Definition: • A colloidal solution that fills the spaces between chromatin & nucleolus. Constituents: Formed of nucleoproteins, enzymes, sugars, calcium, potassium & phosphorous ions. Functions of Nuclear Sap: Provides a medium for transport of RNA through nuclear pores to cytoplasm for protein synthesis Functions of the Nucleus 1- Carries all the genetic information and hereditary factors. 2- Controls all the cell functions including protein synthesis. 3-Responsible for the formation of RNA. 4- Directs cell division. 15